Lilliput (Toxolasma parvum): COSEWIC assessment and status report 2024

Official title: COSEWIC Assessment and Status Report on the Lilliput (Toxolasma parvum) in Canada

Committee on the status of Endangered Wildlife in Canada (COSEWIC)

Special Concern

2024

COSEWIC assessment summary

Assessment summary – November 2024

Common name

Lilliput

Scientific name

Toxolasma parvum

Status

Special Concern

Reason for designation

This small freshwater mussel has a Canadian distribution restricted to Ontario within the drainages of Lake St. Clair/Detroit River, Lake Erie, and Lake Ontario. Status has been revised from the previous assessment of Endangered because criteria are no longer met: 18 additional subpopulations were discovered during increased sampling effort, there is no evidence of continuing decline in abundance, and numbers appear stable or increasing. However, the species occurs in a very small area where habitats are highly degraded with continuing decline from urban and agricultural pollution, agricultural dredging, water management activities, and droughts.

Occurrence

Ontario

Status history

Designated Endangered in May 2013. Status re-examined and designated Special Concern in November 2024.

COSEWIC executive summary

Lilliput

Toxolasma parvum

Wildlife species description and significance

Lilliput (Toxolasma parvum) is a small freshwater mussel generally reaching 25 mm in adult length (maximum length: 58 mm). The shell is elliptical to ovate in shape with a periostracum (outside of shell) that is dull, lacks sculpture (no ridges or bumps), and appears cloth-like. Freshwater mussels provide a number of ecosystem services in the aquatic environment where they are found. Lilliput is the only member of the genus Toxolasma occurring in Canada.

Aboriginal (Indigenous) knowledge

All species are significant and are interconnected and interrelated. There is no species-specific Aboriginal traditional knowledge in the report.

Distribution

The global range of Lilliput is limited to central North America where it is widely distributed from the Gulf of Mexico to the Great Lakes basin. In Canada, this species occurs only in southwestern Ontario.

Habitat

Lilliput is found in a variety of habitats including small to large rivers, wetlands, shallows of lakes, ponds, and reservoirs. It is common in soft substrates such as mud, sand, and silt.

Biology

Lilliput is a short-lived (maximum age of 16 years), benthic, burrowing filter-feeder and believed to be predominantly hermaphroditic. Age at first maturity was calculated to be in the first year of life; generation time was estimated to be approximately three years. Like all unionids, it is an obligate parasite on vertebrate hosts during the transition from larva to juvenile. Although not confirmed for Canadian subpopulations, host fishes for Lilliput in the U.S. are Johnny Darter, Green Sunfish, Bluegill, White Crappie, Warmouth, and Orangespotted Sunfish.

Population size and trends

There is no size estimate for the Canadian population because no quantitative sampling has been undertaken. It is also difficult to assess population trends given the limited historical records and more recent single sampling events. Some Lilliput subpopulations included in the 2013 COSEWIC assessment appear to be either stable or potentially increasing; however, this is based on a limited number of records and a semi-quantitative survey method (timed searches). Similar inferences can also be made using trends from extent of occurrence and index of area of occupancy. Lilliput was detected alive in an additional 18 waterbodies since the last assessment, likely due to targeted sampling efforts in the species’ preferred habitat.

Threats

Medium-low level impact threats include Natural System Modifications (IUCN threat 7), Pollution (IUCN 9), Climate Change and Severe Weather (IUCN 11). Low level impacts include Human Intrusions and Disturbance (IUCN 6) and Invasive and Other Problematic Species, Genes and Diseases (IUCN 8). The overall assigned threat impact was Medium.

Protection, status, and recovery activities

Lilliput was assessed as Endangered by COSEWIC in 2013 and listed as Endangered in Schedule 1 of the federal Species at Risk Act in 2019. In 2024 COSEWIC reassessed this species as Special Concern. Freshwater mussels and their habitat are also protected under the federal Fisheries Act. Lilliput was listed as Threatened in Schedule 3 of the Ontario Endangered Species Act in 2013.

Lilliput is considered globally secure (G5), and nationally secure (N5) in the United States. In Canada, it is considered Critically Imperilled at both the national level (N1) and the provincial level in Ontario (S1). Lilliput has been assessed as Least Concern by the IUCN (International Union for Conservation of Nature).

The majority of the land adjacent to the rivers where Lilliput is found is privately owned and unprotected; however, the river bottom is generally owned by the provincial Crown. Some protection is afforded to the subpopulations that occur in conservation areas and within the Royal Botanical Gardens property.

Recovery activities include:

A federal recovery potential assessment was completed in 2014.

A federal recovery strategy and action plan, including the identification of Critical Habitat, was completed in 2022.

A provincial recovery strategy was completed in 2023.

Targeted surveys within the Royal Botanical Gardens property, Canard River, Lake St. Clair, and Lake Erie tributaries have been undertaken.

Age and growth have been assessed using thin shell sections allowing the estimation of age-at-first maturity and generation time.

Technical summary

Toxolasma parvum

Lilliput

Toxolasme nain

Range of occurrence in Canada: Ontario

Demographic information:

Requested data

Supporting explanation

Generation time (usually average age of parents in the population)

Most likely 3 years (mean range of 2.18 to 5.12)

Coarse estimate derived from length-at-age relationship (from ageing of thin shell sections collected within Cootes Paradise Marsh) and from Haag’s (2012) equation for calculating age at maturity. Generation time estimated as average age of reproducing member of population.

Is there an [observed, estimated, inferred, or projected] continuing decline in number of mature individuals?

n/a

No evidence of continuing decline.

[Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 3 years [or 1 generation; whichever is longer up to a maximum of 100 years]

n/a

No evidence of continuing decline.

Observed, estimated, or projected] percent of continuing decline in total number of mature individuals within 5 years [or 2 generations; whichever is longer up to a maximum of 100 years]

n/a

No evidence of continuing decline.

[Observed, estimated, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the last 10 years [or 3 generations; whichever is longer]

Unknown

There is no estimate of the number of mature individuals, hence trends cannot be determined.

[Projected, inferred, or suspected] percent [reduction or increase] in total number of mature individuals over the next [10 years, or 3 generations, up to a maximum of 100 years]

Unknown

There is no estimate of the number of mature individuals; hence, trends cannot be determined.

[Observed, estimated, inferred, projected, or suspected] percent [reduction or increase] in total number of mature individuals over any period of 10 years [or 3 generations; whichever is longer, up to a maximum of 100 years], including both the past and future (up to a maximum of 100 years in future)

Unknown

There is no estimate of the number of mature individuals; hence, trends cannot be determined.

Are the causes of the decline clearly reversible?

n/a

No evidence of decline.

Are the causes of the decline clearly understood?

n/a

No evidence of decline.

Have the causes of the decline clearly ceased?

n/a

No evidence of decline.

Are there extreme fluctuations in number of mature individuals

No

Unionids do not tend to experience extreme fluctuations.

Extent and occupancy information:

Requested data

Supporting explanation

Estimated extent of occurrence (EOO)

20,622 km²

Calculated based on minimum convex polygon around known occurrences of live specimens and fresh shells from 2013 to August 2024, as well as occupied sites outside that timeframe that have not been resampled.

Historical EOO: 26,616 km² (based on all shell and live specimen records since 1943).

Index of area of occupancy (IAO), reported as 2x2 km grid value

596 km²

Includes live specimen and fresh shell records from 2013 to August 2024 as well as records from occupied sites prior to 2013 that have not been resampled. When IAO was calculated, most areas were determined to be continuous as opposed to discrete.

Historical IAO: 780 km² (based on all shell and live specimen records since 1943).

Is the population “severely fragmented”, that is, is >50% of individuals or >50% of the total area “occupied” (as a proxy for number of individuals) in habitat patches that are both (a) smaller than required to support a viable subpopulation, and (b) separated from other habitat patches by a distance larger than the species can be expected to disperse?

1. Unknown
   • neither the minimum viable subpopulation nor the habitat area required to support such a subpopulation have been calculated.
2. Yes
   • Most subpopulations are likely isolated from the others (the mussels move very little as adults and their host fishes are not known to move large distances).

Number of “locations” (use plausible range to reflect uncertainty if appropriate)

Likely 20 (range of 20 to 32) due to the threats of waterborne pollution, natural system modifications, and climate change and severe weather acting at drainage, watershed, or subwatershed level

Subwatershed/Waterbody (n=20) - Black Creek/North Sydenham River, Long Creek/Sydenham River (east branch), Thames River/Tilbury/Baptiste/Jeannettes creeks, Pike Creek, Puce River, Belle River, Duck Creek, Moison Creek, Ruscom River, Little Creek, Canard River, Big Marsh Drain No. 2/Drain No. 1 (Pelee Island), Lebo Creek, Grand River/Sulphur Creek, Welland River, Oswego Creek, Binbrook Stormwater Management Pond, Jordan Harbour, Cootes Paradise Marsh/Grindstone Creek/Spencer Creek/Lake Jojo/Long Pond/Sunfish Pond, and Delsey Pond

Waterbody (n=32) – if every waterbody is considered separately

Is there an [observed, inferred, or projected] continuing decline in extent of occurrence?

No

There could be an inferred past decline of ~23% (loss in Detroit River, Lower Ausable River); however, the majority of this decrease occurred outside of the 10-year assessment window.

Is there an [observed, inferred, or projected] continuing decline in index of area of occupancy?

No

There could be an inferred past decline of ~24%; however, the majority of this decrease occurred outside of the 10-year assessment window (for example, Detroit River, Rondeau Bay).

Is there an [observed, inferred, or projected] continuing decline in number of subpopulations?

No

The number of subpopulations has increased since the original assessment; however, it is unlikely that these are recently established. This increase is likely a result of targeted sampling.

Is there an [observed, inferred, or projected] continuing decline in number of “locations”?

No

The number of locations has increased because of additional search effort since the last assessment, and no continuing decline is expected in the future.

Is there an [observed, inferred, or projected] continuing decline in [area, extent and/or quality] of habitat?

Yes

Observed decline in quality of habitat based on current water quality conditions and continuing threats.

Are there extreme fluctuations in number of subpopulations?

No

Are there extreme fluctuations in number of “locations”?

No

Are there extreme fluctuations in extent of occurrence?

No

Are there extreme fluctuations in index of area of occupancy?

No

Number of mature individuals (by subpopulation):

Quantitative analysis:

Threats:

Key threats were identified as:

1. Natural System Modifications (IUCN 7), Pollution (IUCN 9), Climate Change and Severe Weather (IUCN 11) – medium-low impact
2. Human Intrusions and Disturbance (IUCN 6) and Invasive and Other Problematic Species, Genes and Diseases (IUCN 8) – low impact

What limiting factors are relevant?

• Obligate parasitic life stage requiring attachment to a host fish
• High mortality rate associated with early life stages
• Limited ability of adults to disperse

Rescue effect (from outside Canada):

Wildlife species with sensitive occurrence data (general caution for consideration):

Status history:

COSEWIC:

Designated Endangered in May 2013. Status re-examined and designated Special Concern in November 2024.

Status and reasons for designation:

Status

Special Concern

Alpha-numeric codes

Not applicable

Numeric code for change in status

IV.i,vi,vii; V.ii

Reasons for designation

This small freshwater mussel has a Canadian distribution restricted to Ontario within the drainages of Lake St. Clair/Detroit River, Lake Erie, and Lake Ontario. Status has been revised from the previous assessment of Endangered because criteria are no longer met: 18 additional subpopulations were discovered during increased sampling effort, there is no evidence of continuing decline in abundance, and numbers appear stable or increasing. However, the species occurs in a very small area where habitats are highly degraded with continuing decline from urban and agricultural pollution, agricultural dredging, water management activities, and droughts.

Applicability of criteria:

A: Decline in total number of mature individuals:

Not applicable. Insufficient data to reliably infer, project, or suspect possible population trends. Observed declines in EOO and IAO were below thresholds for Threatened, A2c, may not have resulted in inferred or suspected declines in number of mature individuals, and most likely occurred over 10 years ago.

B: Small range and decline or fluctuation

Not applicable. IAO (596 km²) is below the threshold for Threatened and there is an observed continuing decline in the quality of habitat, but the population is not severely fragmented, occurs at more than 10 locations, and does not experience extreme fluctuations.

C: Small and declining number of mature individuals

Not applicable. The number of mature individuals is unknown and there is no evidence of continuing decline or extreme fluctuations.

D: Very small or restricted population

Not applicable. The number of mature individuals is unknown and the IAO and number of locations are above the typical thresholds for Threatened, D2.

E: Quantitative analysis

Not applicable. Quantitative analysis was not conducted

Reasons for Special Concern

1. the species is particularly sensitive to human activities or natural events but is not Endangered or Threatened.
2. the Wildlife Species may become Threatened if factors suspected of negatively influencing its persistence are neither reversed nor managed with demonstrable effectiveness.
3. the Wildlife Species nearly qualifies as Threatened under B2 given its IAO of 620 km² and the continuing decline in habitat quality.

Preface

Since the original COSEWIC status and assessment report (COSEWIC 2013a), additional targeted surveys, incidental catches, iNaturalist records verified by species experts, and relocations (that is, mussels moved because of anticipated in-stream work) have occurred in preferred habitat of the Lilliput. This has resulted in the detection of Lilliput in 18 additional waterbodies, including on Pelee Island, as well as expanded distributions in previously known waterbodies. Evidence of recruitment was also observed in some of these subpopulations. In addition, Fisheries and Oceans Canada (DFO) aged 52 shells collected from the Cootes Paradise Marsh (collected by P. Smith and Royal Botanical Gardens staff) and estimated that the species has a lifespan of up to 16 years of age. DFO also fitted a von Bertalanffy growth function (VBGF) to the data obtained. Using a VBGF and calculations developed by Haag (2012), an age at maturity was calculated (~1 year of age) along with an estimate of the generation time (~3 years of age) specific to Lilliput in Ontario.

Due to a historical lack of sampling effort, it is impossible to determine if these 18 more recent detections represent recent range expansions or if these subpopulations existed during the original COSEWIC assessment but were not sampled. Two of these recent detections were made in stormwater management ponds: Brinbrook, which has been in operation since 2000, and Delsey Pond, which was converted from a natural wetland in 2005. While these 18 recent detections raise the possibility of a recent range expansion, it is likely that the recently discovered Lilliput occurrences were present at the time of the original assessment, but were not found due to the limited sampling conducted in the species’ preferred habitat.

The species was listed as Threatened in Schedule 3 of Ontario’s Endangered Species Act in 2013 (Government of Ontario 2023c) and as Endangered in Schedule 1 of the federal Species at Risk Act (SARA) in 2019 (DFO 2022). The listing process under SARA included the identification of Critical Habitat in the Belle, Ruscom, East Sydenham, and Grand rivers, Hamilton Harbour (Cootes Paradise Marsh and Grindstone Creek estuary), Jordan Harbour, Welland River, and Oswego Creek.

In August 2024, four live individuals were found in Lebo Creek (drains into Lake Erie), representing an additional subpopulation and potential location. This record has been included in Distribution and Population size and trends but was not specifically discussed under Threats due to time limitations. Threats acting on this subpopulation would be similar to those already discussed for other subpopulations.

COSEWIC history

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2024)

Wildlife Species

A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)

A wildlife species that no longer exists.

Extirpated (XT)

A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)

A wildlife species facing imminent extirpation or extinction.

Threatened (T)

A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)^*

A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)^**

A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD)^***

A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.

^* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.
^** Formerly described as “Not In Any Category”, or “No Designation Required.”
^*** Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994. Definition of the (DD) category revised in 2006.

The Canadian Wildlife Service, Environment and Climate Change Canada, provides full administrative and financial support to the COSEWIC Secretariat.

Wildlife species description and significance

Name and classification

Current classification:

Class: Bivalvia

Order: Unionida

Family: Unionidae

Genus: Toxolasma

Species: Toxolasma parvum

The scientific name is Toxolasma parvum (Barnes, 1823) (MolluscaBase eds. 2022).

Taxonomic changes since previous report (for reassessments):

Since the COSEWIC status report (2013a) the Order has been updated from Unionoida (Turgeon et al. 1998) to Unionida (Williams et al. 2017).

Common names:

English: Lilliput

French: Toxolasme nain (Martel et al. 2007)

Indigenous: not available

Description of wildlife species

The following description is modified from Clarke (1981), Metcalfe-Smith et al. (2005), and Watters et al. (2009). Lilliput is a small freshwater mussel generally reaching 25 mm in adult length (maximum length of 58 mm; P. Smith unpubl. data as cited in Bouvier et al. 2014). The shell is elliptical to ovate in shape with a periostracum (outside of shell) that is dull, lacks sculpture (no ridges or bumps), and appears cloth-like. The anterior end (the part of the shell that is generally burrowed in the sediment) is rounded and the posterior end (the part of the shell that is above the sediment/water interface) is either rounded (males) or squared (females), although this difference is subtle. The beak is slightly elevated above the hinge line and sculpture consists of 4 to 6 heavy concentric ridges. Juvenile shells are thinner, more pointed in the posterior region, more compressed, and the beak sculpture is more obvious (Utterback 1916). The shell is generally brown to brownish-black (especially in older individuals) or green. Green rays may be present and, if so, are mostly seen on the posterior slope. The hinge teeth are found on the inside, top of the shell, and aid in preventing shell shearing. There are two sets of fully developed teeth: the pseudocardinal teeth, located towards the anterior end of the shell, are thin and serrated, and the lateral teeth, located towards the posterior portion of the shell, are long, thin, and straight. Species that are similar in appearance include Rayed Bean (Paetulunio fabalis) and Salamander Mussel (Simpsonaias ambigua). Rayed Bean is distinguished by its prominent rays and thick hinge line. Salamander Mussel has a thinner shell, more elongate shell shape, and lacks obvious internal hinge teeth.

Designatable units

All Canadian subpopulations of Lilliput are found within the Great Lakes–Upper St. Lawrence National Freshwater Biogeographic Zone. No subpopulation-specific genetic work has been conducted (see Population structure). Given the lack of data to examine criteria for both discreteness and evolutionary significance, the species is considered to be a single designatable unit.

Recognized subspecies or varieties in Canada:

There are no recognized subspecies or varieties in Canada.

Designatable units (dus):

N/A, one DU in Canada.

Special significance

Freshwater mussels play an integral role in the functioning of aquatic ecosystems. Unionids are responsible for numerous water column (for example, size-selective filter-feeding, species-specific phytoplankton selection, nutrient cycling) and sediment processes. Invertebrates and algae also colonize mussel shells and benthic invertebrate densities have been shown to correlate with mussel densities (Vaughn and Hakenkamp 2001; Spooner and Vaughn 2006; Vaughn and Spooner 2006; Newton et al. 2011). Mussels also play a role in the transfer of energy to the terrestrial environment through predation by Muskrat (Ondatra zibethicus) and Raccoon (Procyon lotor) (Neves and Odom 1989; Owen et al. 2011; Edelman et al. 2015). However, given that Lilliput appears to have always been a minor component of the freshwater mussel community in Canada, its relative contribution to all of these processes is likely minor. Lilliput is the only member of the genus Toxolasma found in Canada.

Aboriginal (Indigenous) knowledge

Aboriginal traditional knowledge (ATK) is relationship-based. It involves information on ecological relationships between humans and their environment, including characteristics of species, habitats, and locations. Laws and protocols for human relationships with the environment are passed on through teachings and stories, and Indigenous languages, and can be based on long-term observations. Place names provide information about harvesting areas, ecological processes, spiritual significance or the products of harvest. ATK can identify life history characteristics of a species or distinct differences between similar species.

Cultural significance to Indigenous peoples

There is no species-specific ATK in the report. However, Lilliput is important to Indigenous Peoples who recognize the interrelationships of all species within the ecosystem.

Distribution

Global range

The global range (Figure 1) of Lilliput is limited to central North America where it is widely distributed, occurring in 26 American states and one Canadian province (NatureServe 2024). In the United States, it occurs in the Great Lakes drainage and Mississippi River system, including its western drainages. In Canada, this species occurs only in southwestern Ontario.

Figure 1. Global distribution of Lilliput (Toxolasma parvum). Map created by T.J. Morris (DFO).

Canadian range

Overall, less than 5% of Lilliput’s global distribution occurs in Canada. However, up to 25% of the Great Lakes drainage occurs in Canada. The species’ distribution is restricted to southwestern Ontario, in COSEWIC’s Great Lakes–Upper St. Lawrence Biogeographic Zone (COSEWIC 2021a). Currently, the species can be found in the Lake St. Clair/Detroit River drainage, Lake Erie drainage (including Pelee Island), and in the westernmost portion of the Lake Ontario drainage (Figures 2, 3; Lower Great Lakes Unionid Database [LGLUD] 2024; see Abundance for details).

Lilliput is an obligate parasite (larval stage only) requiring an interaction with a host species to complete its complex life cycle (see Life cycle and reproduction for more detail); therefore, the host’s range is an important determinant of the mussel species’ own range. Although host species have not been confirmed for the Canadian Lilliput population, six fishes have been identified as hosts in the U.S.: (1) Johnny Darter (Etheostoma nigrum), (2) Green Sunfish (Lepomis cyanellus), (3) White Crappie (Pomoxis annularis), (4) Bluegill (Lepomis macrochirus), (5) Warmouth (Lepomis gulosus), and (6) Orangespotted Sunfish (Lepomis humilis; Watters et al. 2009). All of these species are found in Ontario, and there is distributional overlap between the fish species listed above (Holm et al. 2009; Eakins 2022) and Lilliput.

Since the original Lilliput status report, 910 sites have been surveyed for freshwater mussels in southwestern Ontario by different organizations using various methods or through incidental observations (LGLUD 2024). Evidence of Lilliput (live animals or shells) has been observed at 184 (~20%) of these sites. Although the sampling methods used are generally effective at capturing Lilliput when present, the habitat in most of these sites would not be considered preferred habitat for the species (see Habitat requirements for further details). Therefore, there is an under-representation of sampling conducted in deep, soft substrates, which is where Lilliput is likely to be found. This suggests that the species may be more widespread than this report indicates given the paucity of surveys conducted in preferred habitat. Three targeted Lilliput surveys have been completed since the original status report: the Canard River (Morris et al. 2020), Pelee Island (Natural Resources Solutions Inc. 2020), and field sampling conducted in 2022 by Fisheries and Oceans Canada in Lilliput preferred habitat in tributaries along Lake St. Clair and Lake Erie (Gibson et al. 2023). Of the 46 sites surveyed, evidence of Lilliput was found at 26 sites (18 sites with live individuals and eight with shells).

Population structure

There is no information available on the genetic structure of the Canadian population of Lilliput. However, some subpopulations (see Figure 2) are separated by distances of 50 to 200 km, and genetic isolation in freshwater mussels (including in Canada) is possible at these spatial scales (Zanatta et al. 2007; Zieritz et al. 2010).

Figure 2.  The current extant distribution of Lilliput (Toxolasma parvum) in Canada. Records (circles) obtained from the LGLUD (2024) include live animals and fresh shells/valves observed between 2013 and 2022, previously occupied sites outside of this timeframe that were not resampled during the current period, and one live record from August 2024. Map created by J. Colm (DFO).

Figure 3. Close-up view of the current extant distribution of Lilliput (Toxolasma parvum) in Canada with (a) sites on the western side of its distribution and (b) on the eastern side. Records (circles) obtained from the LGLUD (2024) include live animals and fresh shells/valves observed between 2013 and 2022, previously occupied sites outside of this timeframe that have not been resampled during the current period, and one live record from August 2024. Map created by J. Colm (DFO).

Extent of occurrence and area of occupancy

Since the original assessment and status report (COSEWIC 2013a), Lilliput has been detected in 18 additional waterbodies. Due to a historical lack of sampling effort, it is impossible to determine if these detections represent recent range expansions or are simply a result of limited sampling in these areas in the past. Interpretation is complicated given the detection of Lilliput in two stormwater management ponds in recently modified habitat: Binbrook stormwater pond, which has been in operation since 2000, and Delsey Pond, which was converted from a natural wetland in 2005. Because these stormwater management pond subpopulations have likely existed for at least three generations and because the other 16 newly discovered waterbodies are all natural, it is most likely that all previously undiscovered Lilliput occurrences were present at the time of the original assessment, but were not found due to a lack of sampling in these habitats.

Current EOO:

The current extent of occurrence (EOO) within Canada is 20,622 km², calculated using a minimum convex polygon that encompassed live specimen and fresh shell records from 2013 to August 2024 along with previously occupied sites that were not resampled (LGLUD 2024; see Collections examined for details; Figure 4).

Figure 4. Current extent of occurrence and index of area of occupancy for Lilliput (Toxolasma parvum) within Canada. Map created by A. Saini (ECCC).

Current IAO:

The current index of area of occupancy (IAO) within Canada is 596 km², calculated using a 2 km x 2 km grid drawn over live specimen and fresh shell records from the LGLUD (2024; see Collections examined for details) from 2013 to August 2024 along with previously occupied sites outside of this timeframe that have not been resampled; Figure 4). In the Sydenham and Grand rivers, and the main stem of the Welland River, continuous IAO was calculated from the most upstream record to the most downstream record. A discontinuous IAO was used in the Welland River watershed to reflect the separation between the most upstream site, which occurs in a stormwater management pond, and the main stem of the Welland River because: (1) it is uncertain if or how often this pond would overflow into the river; and, (2) two sites were formally surveyed for mussels in the main stem of the Welland River downstream of the pond but no Lilliput were found. A discontinuous IAO was also used for Lebo Creek, given its August 2024 discovery. Records from Tilbury, Baptiste, and Jeannettes creeks extend continuously from the most upstream record to the outflow at the Thames River and continue to the mouth at Lake St. Clair. For the remainder of the Lake St. Clair tributaries, IAO was calculated from the most upstream record to the mouth of the river. Given the habitat similarities of these river sections, it is likely that the species’ distribution is continuous throughout them.

Fluctuations and trends in distribution

There are insufficient quantitative data to assess fluctuations given the number of new detections in Ontario; however, freshwater mussel populations do not usually demonstrate extreme fluctuations as defined by COSEWIC (that is, rapid, frequent, and greater than an order of magnitude).

The historical EOO of 26,616 km² was calculated using all known records (live specimens and fresh/weathered shells from 1943 to August 2024); the new detections likely result from increased survey effort and not range expansion (Figure 5). Thus, there is an inferred decline of 22.5% but it is uncertain if this decline occurred within the last three generations. A continuing decline is not expected.

The decline in EOO is mostly because of the loss of Lilliput occurrences at Port Franks, along the Lower Ausable River which flows into Lake Huron. This occurrence is from iNaturalist observations. All three shells were weathered; at least one was a dead shell that was likely there in 2011 but was not found until 2023 and so was not in the 2013 COSEWIC status report. These shells are not indicative of an extant subpopulation; thus, they are considered historical records. The Ausable-Bayfield Conservation Authority is very active and knowledgeable about mussel surveys within the Ausable River and there is no recent indication of Lilliput in that river.

Figure 5. Historical extent of occurrence and index of area of occupancy for Lilliput (Toxolasma parvum) based on all available records (including live specimens and shell records) within Canada. Private coordinates of one iNaturalist (2022) record from near Long Point Bay were not available. The large circle represents the obscured buffer. Map created by A. Saini (ECCC).

The historical IAO of 780 km² was calculated using all known records (live specimens and fresh/weathered shells from 1943 to August 2024); new detections likely result from increased survey effort and not range expansion (Figure 5). There is an inferred decline of ~23.6% in IAO between the historical and current time period; however, much of this decline likely occurred outside of the 10-year/3 generation COSEWIC assessment window. For example, the only Lilliput record in the Detroit River is from 1943 and the record from a Thames River tributary near Chatham, Ontario is from 1963. Taking into account the continued presence of the species in waterbodies included in the original COSEWIC status report and increased knowledge of the species’ distribution in Ontario, the data show no decline in IAO since 2013. A continuing decline is not expected over the next assessment window.

In the original status and assessment report (COSEWIC 2013a), EOO was 10,221 km² and IAO was 60 km². The differences between EOO and IAO in the 2013 assessment and this update report are due to an increase in the knowledge of the species’ distribution because of targeted surveys and do not likely reflect range expansions. For example, Lilliput was most likely present on Pelee Island during the original assessment; however, no mussel surveys were completed on the island at that time.

Biology and habitat use

Life cycle and reproduction

Lilliput is a small and relatively short-lived species. According to Haag (2012) and Moore et al. (2021), this species does not fit neatly into any of the opportunistic, periodic, or equilibrium life history strategies. Nonetheless, it seems to lean towards the opportunistic strategy in that it is short-lived and has an early age at maturity and high growth rates. Recent information (based on ageing of 52 shells) from the Hamilton Harbour subpopulation (Lake Ontario watershed) indicates that this species has a lifespan of up to 16 years of age (DFO unpubl. data). Other studies have found maximum ages to be from four to five years old with lengths up to a maximum of 26.7 mm (Haag and Rypel 2011) or up to 12 years old with a maximum length of 50 mm (Watters et al. 2009). The aged shells from Hamilton Harbour ranged in length from 16.39 to 54.80 mm (DFO unpubl. data). Growth was assessed by fitting a von Bertalanffy growth function (VBGF) to length-at-age data.

Haag (2012) observed mature Lilliput in Davis Lake, Mississippi (MS) within their first year of life. Unfortunately, age at first reproduction has not yet been observed in Ontario subpopulations. Haag (2012) developed a relationship to estimate age at maturity using the VBGF coefficient, k:

$$T_{mat}=0.69k^{-1.031}-1$$

Using this relationship, the age at maturity for Lilliput in Hamilton Harbour was estimated to be one year of age (that is, matured within the first year of life, and individuals are expected to be mature by the age of one; DFO unpubl. data), which is consistent with observations in Davis Lake. The size at maturity was calculated to be less than 16.6 mm in Hamilton Harbour (DFO unpubl. data). There are no fecundity estimates for Ontario; however, Haag (2012) reported a mean annual fecundity of 11,000 for this species in Davis Lake, MS. Other species within this genus in the U.S. (for example, Texas Lilliput, Toxolasma texasiense) have reported mean annual fecundities of 20,089 and 33,500 in Quarterliah Creek, Mississippi and St. Francis River, Arkansas, respectively (Haag 2013).

Using length data from live Lilliput collected in Ontario from 2013 to 2022 coarse ages were estimated using a VBGF. Using the calculated age at maturity and the age estimates from live Lilliput records from 2012 to 2022 in the LGLUD (LGLUD 2024), an overall generation time (± standard error) of 3.22 ± 0.16 was calculated for Lilliput in Ontario. Although it would be ideal to collect length-at-age information and calculate growth curves to inform generation times for each Ontario subpopulation, this was not possible with current resources. Therefore, these data should be used cautiously. Nonetheless, this is the best available information at this time and is more appropriate than the estimated generation time of 6 years used in the last COSEWIC status report (2013a).

Lilliput is believed to be predominantly hermaphroditic (that is, presence of both male and female gonads; Utterback 1916; Tepe 1943; van der Schalie 1970; Haag 2012); however, some studies suggest that dioecious populations do occur (Utterback 1916; Ortmann 1919; Baker 1928; van der Schalie 1966). Whether this species is wholly hermaphroditic in Canada is unknown.

Unionid mussels have a complex life cycle that involves a period of obligate parasitism on a vertebrate host. During spawning, male mussels release sperm into the water and females living downstream filter the sperm out of the water using their gills . Lilliput is thought to be bradytictic (long-term brooder), with the eggs typically fertilized in late summer, and glochidia (mussel larvae) held by the female mussels over winter and then released the following spring/early summer. Details of Lilliput spawning remain unknown in Ontario; however, there is some information available for populations in the United States. Eggs have been observed in various U.S. states from April through August (Texas, Arkansas, Indiana; summary in Williams et al. 2008). In addition, Ortmann (1912) noted the presence of eggs in Lilliput during June in Pennsylvania. Female mussels brood their young from the egg to the glochidial stage in specialized regions of their gills known as marsupia. Glochidia develop within the marsupial gills and are released by the female mussel into the water column, where they undergo a period of parasitism on a suitable host fish species. Gravid females were observed in Ontario in (1) the lower Grand River in July 2014; 7 out of 9 individuals ranging in length from 18.3 to 42.4 mm were gravid (the other two were too small to check for gravidity); (2) the Sydenham River in July 2018, where one individual was found to be gravid (no length recorded); and, (3) the Puce River in June 2022, where two individuals, 30.0 and 33.0 mm in length, were observed to be gravid (DFO unpubl. data). Based on the VBGF results mentioned above, these individuals ranged from one to three years old. The presence of glochidia in June and July in Ontario is similar to the timing observed in the United States: May/June in Arkansas (Ortmann 1915) and July in Minnesota (Hove 1995). Mortality is difficult to estimate and mortality data are not available for Lilliput specifically. Some studies have suggested that early life stages of various mussel species have survival rates of 0.001% or less (Bauer 2001; Jansen et al. 2001).

Lilliput has evolved a complex host attraction strategy to increase the probability of encountering a suitable host (Zanatta and Murphy 2006). Worm-like caruncles (on the posterior mantle margin) and mantle flaps, which likely serve as active lures, have been observed in different mussel populations (Zanatta and Murphy 2006; Barnhart et al. 2008; Watters et al. 2009). Lilliput forms and releases conglutinates (bundles of glochidia bound in mucus). These conglutinates are white and club-shaped and contain glochidia embedded among unfertilized eggs (Williams et al. 2008; Watters et al. 2009). When the conglutinate is released by the mussel, it elicits a predatory response from its host species, causing the host to ingest the conglutinate. Once ruptured, the glochidia will be released and attach to the host and undergo metamorphosis.

Host fishes have not been confirmed for Lilliput subpopulations in Ontario; however, six fish species have been identified as hosts in the U.S.: Johnny Darter, Green Sunfish, Bluegill, White Crappie, Warmouth, and Orangespotted Sunfish (Hove 1995; Watters et al. 2005, 2009). Although there is potential for distributional overlap between the current Lilliput distribution and all these host fishes, the most likely hosts in Ontario (based on visual distributional overlap and preferred habitats) are Bluegill, Green Sunfish, White Crappie, and Johnny Darter. The first three species are typical of slow-moving lakes, streams, and wetlands, and are present throughout most of the Lilliput range (Scott and Crossman 1998; Holm et al. 2009; Eakins 2022). Johnny Darter prefers a variety of habitat bottoms in streams and lakes, particularly those with moderate or no current, and sand, gravel, and/or silt bottoms (Scott and Crossman 1998; Holm et al. 2009).

The duration of encystment on their host is unknown in Ontario. However, during host fish experiments in the USA, juvenile Lilliput metamorphosis and excystment occurred between 12 and 30 to 35 days post-infestation on Johnny Darter and Green Sunfish, respectively, at approximately 20ºC (Hove 1995; Watters et al. 2005). After excystment from the host, juveniles settle to the river bottom and begin life as free-living mussels. Juvenile mussels usually remain burrowed in the sediment where their food source is suspected to be a combination of detritus, algae, and bacteria obtained from the interstitial pore water or through pedal feeding (Yeager et al. 1994; Balfour and Smock 1995; Gatenby et al. 1997; Watters et al. 2001; Schwalb and Pusch 2007). Once they reach sexual maturity, they migrate to the substrate surface and begin the cycle again (Watters et al. 2001).

Habitat requirements

Lilliput is found in a variety of habitats including small to large rivers, wetlands, shallows of lakes, ponds, and reservoirs. It is common in soft substrates such as mud, sand, and fine gravel (Parmalee and Bogan 1998; Metcalfe-Smith et al. 2005; Watters et al. 2009; Reid et al. 2018). At sites where Lilliput was found during DFO timed-search surveys in 2013 to 2022, substrate type was visually estimated to consist of over 70% soft substrates (sand/silt/clay or muck = loosely consolidated, organic-rich sediment which is common in wetlands), which is consistent with the results of Reid et al. (2018). Boulder, cobble, and gravel made up less than 10% of the substrate and the remaining ~15% was detritus (DFO unpubl. data). Reid et al. (2018) also noted that Lilliput was found in areas with low (1 to 25%) aquatic macrophyte coverage. Although specific habitat details are unknown, juveniles can be expected to occur in habitat similar to that used by adults; however, there may be some microhabitat differences.

To date, the largest Lilliput subpopulations identified (based on total numbers found) occur in Hamilton Harbour, the Grand River, and agricultural drains on Pelee Island (LGLUD 2024). There is no quantitative information that can be used to determine which particular habitats would be considered optimal or marginal, or that could serve as reproductive sinks; however, given the ability of this species to successfully colonize new habitats (see Physiological, behavioural, and other adaptations), reproductive sinks are not expected.

Five of the six host fish species (see Life cycle and reproduction) are from the Centrachidae family (Green Sunfish, Warmouth, Bluegill, White Crappie, and Orangespotted Sunfish); these fish prefer warm water areas of lakes, wetlands, or slow-moving streams with some aquatic vegetation. Johnny Darter, a member of the Percidae family, is widespread throughout Ontario, preferring a variety of habitat bottoms in streams and lakes, particularly those with moderate or no current, and sand, gravel, and/or silt bottoms (Scott and Crossman 1998; Holm et al. 2009).

Movements, migration, and dispersal

In general, adult mussels have very limited dispersal and movement abilities; however, Lilliput is considered one of the more active species owing to its rapid heart rate (Utterback 1916). The species is also found in soft substrates, which innately allows for more movement. Although adult mussels can move upstream or downstream in a river, studies have found net downstream movement through time (Balfour and Smock 1995; Villella et al. 2004). Small-scale vertical and horizontal movement can be influenced by mussel diversity, reproduction, current velocity, substrate type, temperature, and invasive dreissenid mussel infestations (Kat 1982; Lewis and Riebel 1984; Amyot and Downing 1998; Watters et al. 2001; Schwalb and Pusch 2007; Allen and Vaughn 2009; Sullivan and Woolnough 2021). However, the primary opportunity for large-scale dispersal, upstream movement, and movement into novel habitat occurs during the encysted glochidial stage on the host fish.

Suspected host fishes for Lilliput include members of the Centrarchidae and Percidae families (see Life cycle and reproduction), which are not known for large-distance movement. Most centrarchids tend to have a home pool (remain in the same area) or exhibit homing behaviour (Matthews 1998; Irmscher and Vaughn 2015). Paukert et al. (2004) estimated that Bluegill’s core home range was 0.01 to 27.2 ha in Pelican Lake, Nebraska, and Thompson (1933) found that these fish moved approximately 23 km in one year in Illinois streams. Green Sunfish and Warmouth also have a restricted home range and most of these fish tend to remain in certain areas or pools, moving very little (approximately 20 to 100 m; Thompson 1933; Gerking 1953; Gatz and Adams 1994; Smithson and Johnston 1999; Gatz 2007). Funk (1957) observed movement greater than 40 km for a few individual Green Sunfish; however, this was rare. White Crappie movement has been found to vary seasonally, ranging from less than 1 km/week (with a median home range of 30.65 ha) (Garavaglia 2019) to ~36 km over one year (Thompson 1933) and up to 42 km in three months (Funk 1957). Mundahl and Ingersoll (1983) found that Johnny Darter moved very little (mean = 55 m) from the initial capture point in early autumn; however, movement may expand during the reproductive season.

Interspecific interactions

Predators and competitors:

Freshwater mussels are known to transfer energy from aquatic to terrestrial ecosystems by serving as prey for various species (see Special significance). However, since Lilliput is a small mussel and constitutes a small component of the freshwater mussel community, its contribution to energy transfer is likely minor.

Host/parasite/disease interactions:

Lilliput, like all members of the Unionidae, is an obligate parasite that requires a vertebrate host. Six species of fish have been identified as host species for Lilliput in the United States (see Life cycle and reproduction).

Other interactions:

See Threats section for negative impacts with other species.

Physiological, behavioural, and other adaptations

The presence of freshwater mussels of the family Unionidae is generally considered indicative of the overall good health of an aquatic ecosystem. This family (especially the early life stages) has been shown to be sensitive to heavy metals (Keller and Zam 1991), ammonia (Goudreau et al. 1993; Mummert et al. 2003), acidity (Huebner and Pynnonen 1992), salinity (Liquori and Insler 1985; Gillis 2011), and copper (Gillis et al. 2008). Nonetheless, specific information on Lilliput is lacking.

Lilliput is considered to be hermaphroditic (see Life cycle and reproduction). Hermaphroditism may be an adaptation to low-flow environments in which gamete dispersal may be hindered. Hermaphrodism also increases the chances of fertilization in low-density populations, possibly allowing for colonization of new habitats by very small numbers of initial colonizers (Williams et al. 2008; Bogan et al. 2011; Haag 2012).

Lilliput is adapted to habitats not favoured by most other unionids, including areas with soft substrates, high turbidity, and low flow (see Habitat requirements). These habitat conditions are often associated with areas of human disturbance and enable Lilliput to persist in areas with highly modified habitats, including agricultural drains (for example, Pelee Island) and stormwater ponds (for example, Delsey Pond). In addition, in a study of individuals collected in Bayou Manchac, Louisiana, Stern (1976) found that Lilliput could survive in areas with low dissolved oxygen levels (mean of 2.6 mg/L). During DFO sampling campaigns between 2012 and 2022, the species was observed alive at sites where oxygen levels were as low as 2.4 mg/L and as high as 9.84 mg/L (median of 6.25 mg/L) (DFO unpubl. data).

Limiting factors

Limiting factors are generally not human-induced and include intrinsic characteristics that make the species less likely to respond to conservation efforts. Limiting factors may become threats if they result in a population’s decline.

The main limiting factors for Lilliput are the obligate parasitic life stage requiring attachment to a host fish, the high mortality rate associated with the early stages of the life cycle (see Life cycle and reproduction), and the limited ability of adults to disperse into new habitats given their sedentary nature (see Movements, migration, and dispersal).

Population size and trends

Data sources, methodologies, and uncertainties

See Collections examined for details on data sources. Quantitative data were not available for Lilliput; therefore, abundances are only reported as the total number of individuals found during the survey. No subpopulation size estimates could be derived with the information currently available.

Abundance

Because of a lack of quantitative density estimates, it is not possible to estimate the size of the Canadian Lilliput population. All the information below was collected using semi-quantitative timed searches (catch-per-unit-effort) or incidental catches, neither of which is conducive to estimating subpopulation sizes. The total observed abundance of Lilliput in each known waterbody is provided to offer insight into the subpopulations; however, these numbers do not represent an estimate of the total number of mature individuals, the subpopulation, or overall population abundance. The highest numbers of Lilliput have been found on Pelee Island and in the Grand River (Lake Erie drainage), as well as in Hamilton Harbour (Lake Ontario drainage).

Known extant occurrences in Ontario are restricted to the:

• Lake St. Clair/Detroit River drainage (Sydenham River [Black Creek, north branch, east branch, and north branch of Long Creek (previously called Coke Drain in COSEWIC 2013a)], Thames River [Tilbury, Baptiste, and Jeannettes creeks], Pike Creek, Puce River, Belle River, Duck Creek, Moison Creek, Ruscom River, Little Creek, and Canard River),
• Lake Erie drainage (Pelee Island, Lebo Creek, Grand River), and
• Lake Ontario drainage (Welland River, Oswego Creek, Jordan Harbour, Hamilton Harbour [Delsey Pond, Lake Jojo, Cootes Paradise Marsh, Grindstone Creek]).

Lake St. Clair/Detroit river drainage

Lilliput was first recorded in the Sydenham River watershed in the North Sydenham River in 1967 but no details on the condition of the individual found were included (for example, live, shell). Between 1991 and 2011, eight individuals were recorded from the East Sydenham River (COSEWIC 2013a). Since the last COSEWIC assessment, an additional five live individuals have been found: two in the north branch and three in the east branch. Two live Lilliput were found at two of 60 sites surveyed in the North Sydenham River watershed between 2017 and 2018 by the St. Clair Region Conservation Authority (LGLUD 2024). These individuals were found in Black Creek and were the first recent records of live Lilliput in the North Sydenham River since the 1967 detection by H.D. Athearn. Weathered shells/valves were found at an additional five sites in Black Creek (LGLUD 2024). One additional fresh Lilliput shell was detected in the North Sydenham River in 2020 downstream of the confluence of Bear Creek and Black Creek (LGLUD 2024). One live Lilliput was found during one of the 65 sampling events in the East Sydenham River between 2013 and 2022 (LGLUD 2043). This record represents the furthest upstream observation of an individual reported in the East Sydenham River. Two live Lilliput were found in 2017 at a single site surveyed in the north branch of Long Creek, a tributary of the East Sydenham River. This was the first record of Lilliput in Long Creek.

The presence of Lilliput in the Thames River watershed as mentioned in the COSEWIC status report (2013a) was limited to a historical record in McGregor Creek (near Chatham), found by H.D. Athearn in 1963, and one live individual found at a single site in Baptiste Creek in 2010. Since the original COSEWIC status report, eight additional Lilliput have been observed in the Thames River watershed using semi-quantitative timed searches. The species is now known to occur in the main stem of the river as well as in three tributaries of the lower watershed: Tilbury, Baptiste, and Jeannettes creeks. In 2022, Lebaron et al. surveyed 34 sites using a brail and found one Lilliput at a single site in the main stem of the Thames River during Ponar™ sampling. Tilbury, Baptiste, and Jeannettes creeks were sampled by DFO in 2022 (Gibson et al. 2023). One individual was found at one of two sites searched in Tilbury Creek, with shells found at the other site. The site where Lilliput was found in Baptiste Creek in 2010 was resampled in 2022 but no evidence of the species was observed. However, an additional site was sampled downstream of the original record and three live Lilliput were found, indicating that the species is still present in Baptiste Creek. Two sites were sampled in Jeannettes Creek in 2022 and Lilliput was found at both of them (three at one site and one at the other).

Fifty-two live Lilliput have been found in seven Lake St. Clair tributaries: Pike Creek, Puce River, Belle River, Duck Creek, Moison Creek, Ruscom River, and Little Creek since 2010, with 47 of them found since the last COSEWIC assessment. For details on the number of sites surveyed, see Table 1. A total of four (at one site) and nine (three sites) Lilliput were observed during 2022 surveys in Pike Creek and Puce River, respectively. Lilliput was first observed in the Belle River (weathered shell) during surveys conducted in 1999 by D. Zanatta (LGLUD 2024). Three sites in the Belle River were surveyed in 2010 with two live individuals found at a single site. These three sites were resampled in 2022, and again three Lilliput were found at the same site as in 2010. One additional site was also sampled in 2022, and three live Lilliput were found. A single site was sampled in both Duck Creek and Boda’s Lake: two live individuals were found in Duck Creek and two weathered shell fragments in Boda’s Lake. The site sampled on Moison Creek in 2022 had the highest abundance of all Lake St. Clair tributaries, with 24 juveniles (15.5 mm or less in length) observed. Six sites were surveyed in the Ruscom River in 2010 and three live Lilliput were found at one site and shells at another. Four of those sites were resampled in 2022 and one live Lilliput was found at the same site as in 2010; fresh shells were also found at one other site (although a different site than that of 2010). Three additional sites were also surveyed in the Ruscom River in 2022 but no evidence of Lilliput was found. One Lilliput was found at one site sampled in Little Creek.

In 2019, a total of 14 live Lilliput were found in the Canard River, a tributary of the Detroit River, representing the first detection of this species in the waterbody (Morris et al. 2020). The individuals were found at three of the nine sites surveyed; these three sites were all in the main branch of the Canard River. Three weathered Lilliput valves were found at two additional sites, one in the main branch and one in the south branch (Morris et al. 2020). Lilliput most likely no longer occurs in the Detroit River (Schloesser et al. 2006; Keretz et al. 2021). Unionids were considered extirpated from the main channel of the Detroit River in 1998 when it was last surveyed comprehensively (Schloesser et al. 2006). In 2019, Keretz et al. (2021) returned to the Detroit River to quantify potential remnant unionid populations. A total of 56 sites were surveyed and, while evidence of unionids was found, no live Lilliput or shells were observed at any of the sites (Keretz et al. 2021). However, the continued persistence of Lilliput in the Canard River could provide a source of mussels to aid in the natural recovery of this species in the Detroit River (Morris et al. 2020).

Lake Erie drainage

The first observation of Lilliput in Lake Erie was recorded by Walker (1913). This was followed by a study published by La Rocque and Oughton (1937), who confirmed the identity of specimens from the National Museum of Canada and the Royal Ontario Museum. Unfortunately, no information is available regarding the specific collection sites of these specimens (that is, Canadian or U.S. side); therefore, they are not included in the distribution maps. Crail et al. (2011) found live Lilliput on the U.S. side of Lake Erie during surveys between 2007 and 2009. Very few surveys have focused on the Canadian side of Lake Erie. One such study in 2014 to 2015 found five weathered Lilliput valves at four different sites surveyed in Rondeau Bay (Reid et al. 2016) but no evidence of live individuals. These represent the only recent records for the species in the area and suggest that there is no reproducing subpopulation there.

Since the previous COSEWIC status report (2013a), Lilliput has been found in agricultural drains on Pelee Island in the western basin of Lake Erie (Natural Resources Solution Inc. 2020; LGLUD 2024). A total of 61 live Lilliput were found in two drains on Pelee Island. Two live individuals were found in Drain Number 1 in 2016 by DFO, and 59 live individuals were found in Big Marsh Drain Number 2 in 2020 by Natural Resource Solutions Inc. These are the first known records of Lilliput on Pelee Island.

Lilliput was first recorded in Lebo Creek (drains into Hillman Marsh and then into Lake Erie) in August 2024, with the incidental observation of four live individuals (Stammler pers. comm. 2024).

Lilliput was first recorded in the Grand River sometime between 1934 and 1946, with the first confirmed live specimen found in 1997 to 1998. This individual, plus an additional 14 found at five sites in 2010 to 2011, represented the known extent of the species in the Grand River at the time of the last COSEWIC assessment (McNichols-O’Rourke et al. 2012; COSEWIC 2013a; Minke-Martin et al. 2015). Since then, an additional 34 live Lilliput, including 10 individuals classified as juveniles (16.5 mm or less), were found in the Grand River across seven sites, including three sites where Lilliput had not previously been detected (Goguen et al. 2023; LGLUD 2024; DFO unpubl. data). DFO completed a total of 62 freshwater mussel surveys throughout the Grand River watershed between 2014 and 2021, and live Lilliput were found during six surveys at sites in the lower reaches of the main channel and one site in Sulphur Creek (Hoffman et al. 2017a; Wright et al. 2020; Goguen et al. 2023; DFO unpubl. data).

Lake Ontario drainage

The presence of Lilliput in the Welland River watershed reported in the last COSEWIC status report (2013a) was limited to a single individual found at one of eight sites sampled in the Welland River. DFO surveyed 19 sites across five waterbodies in the Welland River watershed in 2015. A total of 18 Lilliput were found across six sites in two waterbodies: the Welland River and Oswego Creek (Wright et al. 2017). Twelve live Lilliput were found across five of the 10 sites surveyed in the Welland River. These individuals were found both upstream and downstream of the live individual detected in 2008, extending the known distribution in the river. Six live Lilliput were found during two surveys at a single site in Oswego Creek, where Lilliput had not previously been known to occur. A weathered Lilliput shell and valve were found at an additional site in the Welland River in 2016 and 2017, respectively (LGLUD 2024). In 2022, a single Lilliput was found in the Binbrook Stormwater Management Pond (LGLUD 2024), which was created in 2000 (City of Hamilton 2001).

As reported in the COSEWIC status report (2013a), seven individuals were found across five sites sampled in Jordan Harbour (20 Mile Creek) in 2012 (Reid et al. 2014). Since then, two live individuals were found at one site in 2014 (Hoffman et al. 2017b; DFO unpubl. data). No surveys have been completed in Jordan Harbour since 2014.

Many Lilliput shells have been collected from several sites in Lake Ontario’s Hamilton Harbour since 2000, with live individuals found in Sunfish Pond (part of Grindstone Marsh or Grindstone Creek estuary) in 2011 (COSEWIC 2013a; Minke-Martin et al. 2015; LGLUD 2024). In 2015, DFO and Royal Botanical Gardens (RBG) surveyed 10 sites within the Cootes Paradise Marsh and Spencer Creek and detected 27 live Lilliput across seven of the sites (Wright et al. 2020). One shell was found at an additional site in the Cootes Paradise Marsh. Between 2016 and 2022, an additional 148 live Lilliput were found in Cootes Paradise across 14 sites, and over 350 shells/valves across 35 sites(Richer and Theijsemeijer 2017; P. Smith pers. comm. 2022; LGLUD 2024). In addition, over 110 Lilliput were found at one site in Grindstone Creek marsh in 2022 (P. Smith pers. comm. 2022). These surveys confirmed the presence of large numbers of Lilliput that are reproducing within Cootes Paradise Marsh and Grindstone Creek Marsh. In 2021, fresh shells were detected in Delsey Pond, a stormwater management pond just outside the western boundary of Cootes Paradise that was converted from a natural wetland around 2005 (Campbell 2022a). In 2022, 64 live Lilliput were found in Delsey Pond (Campbell 2022b), confirming the existence of large numbers of reproducing Lilliput. Delsey Pond is connected to Lake Ontario through a series of waterbodies that ultimately drain into the western end of Cootes Paradise. Two live Lilliput were also recorded from nearby Lake JoJo in 2022 (LGLUD 2024).

Fluctuations and trends

It is not possible to evaluate Lilliput subpopulation fluctuations because no quantitative data, indices, or monitoring information have been collected. However, unionid mussel populations do not tend to fluctuate according to the definition used by COSEWIC (that is, rapid, frequent, and greater than an order of magnitude). There are currently 184 Lilliput records in the LGLUD (2024) representing 596 live individuals (Table 2; Figure 6). Seventy percent of the records have been collected since the last COSEWIC status report (2013a) and represent over 90% of the live individuals found. This is the result of increased sampling effort in Lilliput preferred habitat. Over 900 sites have been surveyed using various methods since 2013 (Figure 7), most of which did not occur in Lilliput preferred habitat.

Figure 6. All Lilliput (Toxolasma parvum) records (n = 184) drawn from the LGLUD (2024), including live specimens, fresh, and weathered shells/valves within Canada. Private coordinates of one iNaturalist (2022) record near Long Point Bay were not available. The large circle represents the obscured buffer. Not all waterbody names are included due to limited space. Map created by J. Colm (DFO).

Figure 7. All sites surveyed for unionids in Ontario using various methods or from incidental observations from 2013 to 2022, as well as the six sites from 2024 (n = 910; LGLUD 2024). Private coordinates of one iNaturalist (2022) record near Long Point Bay were not available. The large circle represents the obscured buffer. Map created by J. Colm (DFO).

Since 2013, Lilliput has been found in Black Creek and the North Sydenham River as well as further upstream in the east branch of the Sydenham River and in the north branch of Long Creek. These detections likely do not represent an expansion of the distribution of Lilliput within the watershed, but rather an increase in search effort. Based on the continued presence of the species and observations of Lilliput at additional sites within the watershed, the Sydenham River subpopulation appears to be stable to increasing (Table 3).

Lilliput is now known from three tributaries in the lower Thames River watershed, with two specimens detected in 2022. Again, the species was most likely present in these tributaries during the last assessment but was not found due to lack of survey effort in its preferred habitat. It is not possible to discuss trends in these waterbodies at this time due to the species’ recent detection. Lilliput was found at one site in Baptiste Creek in 2010 . No evidence of the species was found during the 2022 survey at the same site; however, the species was found further downstream within the known distribution. It is suspected that the species is still present but that it was not detected due to differences in sampling effort. Taking into account the continued presence of the species and observations of Lilliput at additional sites within the watershed, this subpopulation appears to be stable to increasing (Table 3).

Lilliput is now known from seven tributaries of Lake St. Clair, with specimens detected in five of them in 2022. The detections of Lilliput in Pike Creek, Puce River, Duck Creek, Moison Creek, and Little Creek in 2022 likely do not represent a range expansion in the Lake St. Clair drainage. They are most likely attributable to insufficient survey effort in the past in these waterbodies. Given the recent detections, it is not possible to determine trends in these waterbodies. At the time of the last COSEWIC status report (2013a), the species was known to occur in the Belle and Ruscom rivers. Although Lilliput was found at one additional site in the Belle River, it is within the previously known distribution. In 2022, fresh shells were observed in the Ruscom River upstream of the previously known distribution. This discovery is likely attributable to an increase in survey effort. Given the continued presence of Lilliput at low numbers and the detections at additional sites in the Belle and Ruscom rivers, these subpopulations appear to be stable (Table 3).

Lilliput was first detected in the Canard River in 2019 (Morris et al. 2020); it was not found during surveys in 1993 (Morris and Di Maio 1998 to 1999). Owing to differences in sampling effort, it is not possible to conclusively determine whether the detections in 2019 represent a recent range expansion or are a consequence of increased effort. No trends can be determined given the subpopulation’s recent discovery; however, it may be increasing since it was not observed during the 1993 surveys (Morris and Di Maio 1998 to 1999; Table 3).

The Pelee Island subpopulation, one of the largest based on the total number of Lilliput found, was first detected in 2016, with more individuals found in 2020 (Natural Resource Solutions Inc. 2020). Given the large number of adults, including smaller individuals (Figure 8), it is likely that the subpopulation is reproducing. The Pelee Island subpopulation likely does not represent an expansion of the range as formal surveys were not completed there until 2016 and 2020. No inferences can be made regarding trends on Pelee Island given the species’ recent discovery there (Table 3).

Figure 8. Length (mm) distribution of Lilliput (Toxolasma parvum) observed on Pelee Island (n = 59) in 2020 (Natural Resource Solutions Inc. 2020). The dotted line represents the delineation between juvenile and adult.

Given the August 2024 observation of the Lebo Creek subpopulation, no information is available on trends or distribution in this waterbody.

Historically, Lilliput was known from six shell records collected in the Grand River. Since the last COSEWIC assessment (COSEWIC 2013a), more individuals have been found at the previously occupied sites and some have been found at additional sites, including sites upstream of the previously known distribution (McNichols-O’Rourke et al. 2012; Minke-Martin et al. 2015; Hoffman et al. 2017a; Wright et al. 2020; Goguen et al. 2023; LGLUD 2024). These discoveries make Grand River one of the largest subpopulations in Ontario. Recruitment appears to be occurring taking into account the presence of juveniles (Figure 9). A weathered valve found in Caledonia in 2020 (Natural Resources Solutions Inc. 2020) represents the furthest upstream record of the species in the Grand River. This record is not included in the IAO calculation as it did not consist of a live specimen or a fresh shell. Given the signs of recruitment, the species’ continued presence at previously occupied sites, and the discovery of a live specimen upstream of the previously known distribution in the Grand River, this subpopulation appears to be stable and possibly increasing (Table 3).

Figure 9. Size distribution of Lilliput (Toxolasma parvum) in the Grand River (LGLUD 2024). Lengths (mm) are separated into individuals observed prior to 2013 (n = 16) and those observed between 2013 and 2021 (n = 28). The dotted line represents the delineation between juvenile and adult.

Since the last COSEWIC status report (2013a), additional live individuals were found in the Welland River both upstream and downstream of the initial 2008 record; this increase in detections in the watershed is likely due to increased search effort rather than an extension of distribution. Lilliput has only been known to inhabit Oswego Creek since 2015 and the Binbrook Stormwater Management Pond since 2022; therefore, no comparison can be made to determine historical trends. These recent detections are likely the result of increased search effort. Given the continued presence of Lilliput and its detection at additional sites within the Welland River, as well as its detection in Oswego Creek and Binbrook Stormwater Management Pond, the Welland River subpopulation appears to be stable to increasing (Table 3). Due to the lack of historical records in Jordan Harbour, there is not enough information to determine trends for this subpopulation.

The Hamilton Harbour subpopulation appears to be the largest reproducing Lilliput subpopulation in Ontario. Although shells of the species have been observed in the area since 2001, live individuals were not observed until 2011 (Minke-Martin et al. 2015). Since then, numerous live individuals have been detected throughout Cootes Paradise Marsh, Spencer Creek delta, and Grindstone Creek marsh. Considering the continued presence of Lilliput in this area and the large numbers of live animals collected recently (P. Smith pers. comm. 2022), the subpopulation appears to be large and stable (Table 3). Given the recent discovery of Lilliput in Delsey Pond and Lake JoJo, subpopulation trends cannot be determined there.

Continuing decline^{Footnote 1} in number of mature individuals:

There is no evidence of a continuing decline in number of mature individuals of Lilliput in Canada.

Evidence for past decline (3 generations or 10 years, whichever is longer) that has either ceased or is continuing (specify):

There is no evidence for a past decline over the last 3 generations (that is, 10 years).

Evidence for projected or suspected future decline (next 3 generations or 10 years, whichever is longer, up to a maximum of 100 years):

According to currently available information, a projected or suspected decline within the next 3 generations is not expected.

Long-term trends:

There are no long-term monitoring programs in place for this species. Going back more than three generations, the presence of dreissenid mussels (Dreissena polymorpha, Zebra Mussel and D. rostriformis, Quagga Mussel) was likely the cause of the loss of Lilliput in Rondeau Bay. Their presence has been one of the most significant threats to unionid mussels in the Great Lakes.

Population fluctuations, including extreme fluctuations:

Due to a lack of repeated quantitative sampling over an appropriate time-scale, it is not possible to assess population fluctuations; however, fluctuations (as defined by COSEWIC) do not commonly occur in unionids.

Severe fragmentation

Neither the minimum viable subpopulation size nor the habitat area required to support a viable subpopulation have been identified for Lilliput. Its hermaphroditic nature (see Life cycle and reproduction) nonetheless suggests that the minimum viable subpopulation size is likely small in Lilliput relative to other unionids. Given both the short generation time and limited lifespan observed for Lilliput in Canada, its continued persistence in each of the subpopulations is likely indicative of their continued viability.

Each of the Lilliput subpopulations is likely isolated from most of the other ones. Although Lilliput is considered an active species (see Movements, migration, and dispersal), movement by adults is considered to be on the order of 10s to 100s of metres and insufficient to provide connectivity between subpopulations. Movement during the encysted glochidial stage is the primary opportunity for dispersal and connection. While connections between some subpopulations (for example, Pike Creek and Puce River) may be possible, most occurrences are separated by distances that exceed the annual movements of host species (see Movements, migration, and dispersal).

Rescue effect

Although suitable habitat is available for immigrants, Lilliput subpopulations in Canada are currently isolated from one another and from U.S. populations by large areas of unsuitable habitat. This makes the likelihood of extirpated subpopulations becoming re-established through immigration small. The suspected hosts of Lilliput are generally not capable of large-scale movements (see Life cycle and reproduction and Movements, migration, and dispersal). In Ohio, the species is considered Secure (S5; NatureServe 2024) given that surveys conducted throughout the northern part of the state in Lilliput preferred habitat resulted in the successful collection of the species within the western basin of Lake Erie. This includes drainage ditches as well as the Maumee River where over 1,200 individuals were found across 152 cells (Benshoff pers. comm. 2023). Although Lilliput habitat remains under-sampled in northern Ohio, the distribution of the species is state-wide (Shoobs pers. comm. 2023). Therefore, it could act as a source population for Ontario. Other Lilliput subpopulations in adjacent U.S. states, however, are unlikely to act as sources given that the species’ status is Critically Imperilled to Imperilled in Michigan, Pennsylvania, and – York (NatureServe 2024; see Non-legal status and ranks). Given the large amount of unsuitable habitat, the small home range/movement in potential host fishes, and the species’ status in U.S. states, rescue from outside Canada is unlikely to lead to a change in status.

Threats

Historical, long-term, and continuing habitat trends

Increases in anthropogenic sources of stress, such as land-use changes, pollution, and urbanization, and invasive species, are the main historical, long-term, and continuing threats acting on Lilliput and its habitat. Below is a description of the trends for each watershed where Lilliput is known to occur.

The most serious threats to aquatic life in the Sydenham River are sediment loads, excess nutrients, toxic chemicals, temperature changes, and invasive species (SCRCA 2022). Over 80% of the land in the Sydenham River watershed is used for agriculture and 60% of this land has tile drainage (Dextrase et al. 2003; SCRCA 2021). Only 12% of the original forest cover remains, and large areas of the river have little to no riparian vegetation. Agricultural lands, particularly those with little riparian vegetation and large amounts of tile drainage (a common method that uses networks of perforated pipes to drain subsurface water from agricultural fields), contribute large inputs of sediment to the watercourse. Over the last 30 years, the Sydenham River has had high nutrient levels, including total phosphorus levels consistently exceeding provincial water quality objectives (0.03 mg/L) (SCRCA 2018; Government of Ontario 2023a). The watershed has been characterized as a “major contributor” of phosphorus to the St. Clair–Detroit River system. Mean levels in the East Sydenham River from 2001 to 2015 ranged from approximately 0.06 mg/L to 0.12 mg/L. The levels in the North Sydenham basin were higher, ranging from 0.12 mg/L to 0.23 mg/L (SCRCA 2018). These high phosphorous loads are from non-point sources such as agricultural activities and, to a lesser extent, septic systems (Oldfield et al. 2020). Not surprisingly, nitrogen has replaced phosphorus as the limiting nutrient in the system. According to information for the period 2013 and 2020 from the Provincial Water Quality Monitoring Network (PWQMN), chloride levels in the East and North branches of the Sydenham where Lilliput occurs rarely exceed 50 mg/L and 80 mg/L, respectively (Government of Ontario 2023b). However, the levels are increasing (Sorichetti et al. 2022). They are below the threshold set out in the Canadian Water Quality Guidelines for the Protection of Aquatic Species at Risk (640 mg/L in the short term and 120 mg/L in the long term; CCME 2011). However, since road salt continues to be used for de-icing, chloride concentrations may increase (Dextrase et al. 2003; SCRCA 2008; Lawson and Jackson 2021). The human population (over 147,000) in this area is not dense, and the lower portion of the river is subject to commercial shipping activities which tend to fluctuate in response to economic conditions (SCRCA 2021). Non-native species have also invaded the Sydenham River. Dreissenid mussels are found in the lower portion of the river (downstream of the sites where Lilliput were collected); however, recent brail surveys targeting deeper water in the north and east branches of the Sydenham River in 2022 provided no records of dreissenids on any live unionid mussels (Lebaron et al. 2023). There is concern, however, that if these invasive mussels were to become established in the Strathroy Reservoir, there would be negative impacts on native unionid mussels in the east branch of the river (Carroll pers. comm. 2021; Patterson 2021). Round Goby (Neogobius melanostomus) is also present, with a distribution extending upstream to Strathroy, which includes the known range of Lilliput. Although Common Carp (Cyprinus carpio) is present throughout the Sydenham River watershed (SCRCA 2022), no detailed analyses are available (Aguiar et al. 2021; Patterson et al. 2021). Information from the DFO Program Activity Tracking for Habitat (PATH) database showed that between 2013 and 2022, works, undertakings, or activities (typically related to infrastructure projects such a bridge repairs/rehabilitation) have occurred within the known Lilliput range in the North Sydenham River and Black Creek (FFHPP pers. comm. 2022).

The lower Thames River watershed is subject to intense pressure from agriculture, with over 80% of the land devoted to agricultural activities (LTVCA and UTRCA 2008). Water quality in the Thames River basin has historically suffered greatly from agricultural activities. Tile drainage, wastewater drains, manure storage and spreading, and insufficient soil conservation have all contributed to poor water quality within this basin (Taylor et al. 2004). There are also a number of wastewater treatments plants along the Thames River and one on Tilbury Creek (Nürnberg and LaZerte 2015). According to the Lower Thames Valley watershed report (LTVCA 2018), surface water quality in Tilbury, Baptiste, and Jeannettes creeks is considered poor and has not changed since 2013. Not surprisingly, the Thames River was identified in the Lake Erie Binational Phosphorus Reduction Strategy as one of the 14 rivers that is believed to contribute significantly to phosphorous loads in Lake Erie (Great Lakes Water Quality Agreement Nutrient Annex Subcommittee 2019). Since 2013, mean phosphorus levels in the main stem of the Thames River and in Jeannettes Creek have remained four to six times higher than the provincial water quality guidelines (Government of Ontario 2023b). According to McKay et al. (2020) and the Government of Ontario (2023b), chloride levels in Baptiste and Jeannettes creeks have not exceeded the provincial water quality guidelines over the last decade. However, exceedances of 120 mg/L have occurred in Big Creek (close to Tilbury Creek) and the main branch of the Thames River (McKay et al. 2020; Government of Ontario 2023b). During surveys in Tilbury, Baptiste, and Jeannettes creeks in 2022, dreissenid shells were abundant and found at most sites where Lilliput specimens were found, which is consistent with the observation of a large number of shells in Baptiste Creek during the 2010 surveys. However, during the DFO sampling campaign in 2022, only two sites had live dreissenids: one in Tilbury Creek and one in Baptiste Creek (DFO unpubl. data). A large number of live dreissenid mussels (~100) were observed in nearby Big Creek during the 2022 surveys (DFO unpubl. data). Round Goby and Common Carp appear to be common throughout the lower portions of the Thames River watershed, with recent records obtained for both in Big Creek (close to Tilbury Creek) and Jeannettes Creek, and for Common Carp in Baptiste Creek (Aguiar et al. 2021; GLLFAS Biodiversity Science Database 2022). Other fish species that have been found in the Thames River watershed are Tubenose Goby (Proterorhinus semilunaris) and Flathead Catfish (Pylodictis olivaris; Illes et al. 2019; V. McKay pers. comm. 2023). Regarding works, undertakings, or activities in the lower Thames River and its tributaries, shoreline stabilization, building rehabilitation (Thames River), and bridge repair (Baptiste and Jeannettes creeks) have occurred, and some of these activities took place in the Lilliput distribution (FFHPP pers. comm. 2022). In addition, a number of pumping stations are located in the watershed towards the lower end (for example, Jeannettes Creek) where Lilliput have been observed (V. McKay pers. comm. 2023).

The Lake St. Clair tributaries (Pike Creek, Puce River, Belle River, Duck Creek, Moison Creek, Ruscom River, and Little Creek) are all located in the Essex Region and cover an area of approximately 550 km² (ERCA 2015). Land use in these areas consists predominantly of agriculture, with wetlands, riparian forest, and forest cover making up ~17% of the area of the subwatersheds (ERCA 2018). Water quality within these seven tributaries has been graded as poor to very poor (ERCA 2018). Over the last decade, total phosphorus concentrations in the Essex Region have continued to greatly exceed the provincial water quality guidelines (ERCA unpubl. data 2022a). Chloride levels vary widely throughout these systems, but exceedances of both the short- and long-term Canadian Water Quality Guidelines have occurred (ERCA unpubl. data 2022a). During the 2022 surveys, a large number of live dreissenids were observed in Pike Creek (>100) at the site where live Lilliput were found. Dreissenid shells were abundant at the site where Lilliput occurred in Little Creek; however, only four live dreissenids were observed. No live dreissenids were found in Puce River, or Duck and Moison creeks at sites where Lilliput specimens were found; however, there was an abundance of shells in each waterbody. No dreissenids were observed at the sites where Lilliput specimens were found in the Belle or Ruscom rivers (DFO unpubl. data), although the species is known to occur at the mouths of these rivers. Round Goby and Common Carp are known to inhabit most, and likely all, of these Lake St. Clair tributaries (Aguiar et al. 2021; ERCA unpubl. data 2022b; GLLFAS Biodiversity Science Database 2022). These two invasive fish species are found within most Lake St. Clair tributaries. Further research may be required to determine their distribution and status within these creeks and rivers. Since the last COSEWIC status report (2013a), works, undertakings, or activities that have occurred in these Lake St. Clair tributaries include breakwaters (Pike Creek), bridge work (Puce River), culverts (Belle and Ruscom rivers), dredging/excavation (Pike Creek, Puce River), shoreline protection (Duck and Pike creeks, Belle and Ruscom rivers), bank stabilization (Ruscom River), and water outfalls (Belle and Ruscom rivers) (FFHPP pers. comm. 2022). Some dredging has also occurred in the past (timeframe unknown) for recreational purposes (for example, removal of junk/debris) at certain river mouths as sand can build up there and cause issues for boaters (J. Bryant pers. comm 2023).

The Canard River is found within the Detroit River subwatershed and represents the largest subwatershed (347.7 km²) within the ERCA boundary (ERCA 2018). Wetlands, riparian forest, and forest cover make up approximately 0.6%, 4%, and 8% of the Canard River subwatershed, respectively, with most land used for agriculture (ERCA 2018). Non-point source pollution is a key issue in the region and is primarily affected by runoff from agricultural and urban areas (ERCA 2018). Based on data collected between 2012 and 2016, the area containing the Canard River received a poor surface water quality rating (ERCA 2018). Total phosphorus levels were found to far exceed the provincial guideline (ERCA unpubl. data 2022a). Chloride levels between 2013 and 2020 also exceeded the long-term provincial guidelines during 66% of sampling events (ERCA unpubl. data 2022a). Data on mean aluminum concentrations were collected at PWQMN sites throughout waterbodies in the Essex Region Conservation Authority (ERCA) region. The highest aluminum level (3,490 µg/L), along with elevated levels of iron, lead, and copper, was recorded in the Canard River (ERCA 2015). Very high levels of Escherichia coli bacteria are found throughout the region and likely come from sources such as faulty septic systems (ERCA 2006, 2018). Evidence of dreissenid mussels has been obtained from the Canard River, including one site where live Lilliput were found (Morris et al. 2020). Round Goby and Common Carp have been captured in the Canard River within the known Lilliput distribution (Aguiar et al. 2021; ERCA unpubl. data 2022b). Works, undertakings, or activities that have occurred within the Lilliput distribution in the Canard River include bridge replacement and repairs, and culvert installations and replacements (FFHPP pers. comm. 2022).

Pelee Island is one of the nine islands that make up the Township of Pelee in Lake Erie, the southernmost point in Canada (The Corporation of the Township of Pelee 2020a). A large portion of the island is devoted to agriculture (soybeans, wheat, grapes, organic crops; Zelinka Priamo Ltd. 2011; The Corporation of the Township of Pelee 2020b). Although some water quality information is available for natural/created ponds and Lake Henry on Pelee Island (Ward and Hossie 2020; DFO unpubl. data), no information could be found on water quality in the drains where the species occurs. Common Carp and Round Goby have been found in Lake Erie around Pelee Island (Johnson et al. 2005; GLLFAS Biodiversity Science Database 2022); however, it is unknown if these fish species occur in the drains on the island. Drainage maintenance and pumping takes place and will continue to occur on the island (Natural Resource Solutions Inc. 2020; E. Chamberlain pers. comm. 2023).

Over the last 50 years, mussel communities in the Grand River have undergone a significant decline and subsequent recovery (Kidd 1973; Mackie 1996; Metcalfe-Smith et al. 2000). When Kidd (1973) sampled the river in 1970 to 1972, he reported a 55% decrease in mussel species diversity, much of which was attributed to impaired water quality related to agricultural activity and habitat fragmentation resulting from the construction of three large and 11 small impoundments. In 1995, Mackie (1996) indicated that anthropogenic threats, particularly below urban centres, were likely driving the species' declines. In 1997 to 1998, Metcalfe-Smith et al. (2000) noted an increase in species richness when compared with Kidd’s (1973) results, and suggested that the improvement in mussel communities in the Grand River was most likely due to improved water quality, the addition of fish ladders promoting fish movement (allowing dispersal through host activity), and the reconnection of formerly fragmented habitat. A similar increasing trend in mussel density and richness was observed in the lower Grand River during quadrat surveys conducted by the Unionid Monitoring and Biodiversity Observation (UMBO) Network between 2010 and 2018 (DFO unpubl. data). While the health of the Grand River has improved over time as a result of an increased investment in wastewater treatment and urban and rural stormwater management (GRCA 2020), there are still numerous threats to the unionid community, including Lilliput. Pollution from nutrient (primarily phosphorus) loading is a major issue in the Grand River because of the dense human populations in urban centres as well as agricultural practices (GRCA 2015, 2020). Currently, 86% of the watershed is considered rural with agriculture accounting for 61% of land use (GRCA 2020). While rural area still accounts for the vast majority of the watershed, urban area has increased from 5% to 14% since 1999 and supports a population of ~1,000,000 people (GRCA 2020, 2022a). The primary sources of phosphorus input into the Grand River are wastewater treatment plant and stormwater discharge from urban areas (GRCA 2015) and runoff from agricultural land (GRCA 2020). The Grand River is home to 30 wastewater treatment plants, all of which are located upstream of Lilliput habitat (GRCA 2022b). Median total phosphorus levels in areas where Lilliput occur have reached four times the provincial objective during high spring flows, and even 12 times the objective (Water Quality Working Group 2011). Water quality at Dunnville (the most downstream water quality sampling site), where most of the species’ current Lilliput distribution occurs, is characterized as poor given all of the cumulative upstream inputs (GRCA 2021). The dams located along the Grand River also contribute to the high levels of phosphorus in the watershed (Water Quality Working Group 2011). Throughout the watershed today, there are 27 dams, including seven large multi-purpose dams and reservoirs (GRCA 2022c,d). While nutrient loading is the primary threat in the Grand River, a number of secondary threats exist, including dredging undertaken to improve passage for recreational and commercial boat activity in the lower Grand River, residential and commercial development including in-water work (for example, docks, marinas, shoreline hardening), and invasive species (Allan pers. comm. 2022). Dreissenids are found in the Grand River downstream of the Dunnville dam, which is within the Lilliput distribution (GRCA 2022e). Common Carp is also present in the Grand River but appears to be in low abundance (Aguiar et al. 2021). Round Goby is found both above and below the dam at Dunnville; however, abundances are higher above the dam (GLLFAS Biodiversity Science Database 2022; GRCA 2022e). Three sterile (that is, cannot reproduce) Grass Carp (Ctenopharyngodon idella, an invasive species of Asian carp) have been caught in the lower portion of the Grand River at Lake Erie (Marson et al. 2014, 2016; Marson pers. comm. 2022). According to FFHPP (2022), very few works, undertakings, or activities have occurred within the distribution of Lilliput in the Grand River, with the largest project being bridge work that required mussel relocation in Caledonia; however, this relocation occurred upstream of current Lilliput distribution.

The Welland River has the largest watershed within the jurisdiction of the Niagara Peninsula Conservation Authority (NPCA) and is part of the Niagara River Area of Concern (NPCA 2010a,b, 2021). Over 70% of this area is rural, with agriculture consisting mainly of poultry, eggs, grain, and oilseed production (NPCA 2010b, 2012, 2021). Only 22% of the watershed is forest cover; however, this represents an increase from 15% forest cover in 2010 (NPCA 2010b, 2012). There are 30 water quality monitoring stations in the Welland River watershed, with 14 of these in the main channel of the Welland River and two in Oswego Creek (NPCA 2021). A Water Quality Index (WQI) rating is calculated at each station and incorporates “…the number of parameters where water quality objectives have been exceeded, the frequency of exceedances within each parameter, and the amplitude of each exceedance” (NPCA 2010a). According to data collected between 2016 and 2020, seven of the 14 stations on the Welland River had a WQI rating of poor, five stations had a marginal rating, and two stations had a fair rating (NPCA 2021). Both stations in Oswego Creek were had a poor rating. In these systems, the low WQI ratings were mostly due to exceedances of copper, lead (Welland River only), zinc, chloride, nitrate, total phosphorus, E. coli, and total suspended solids. They were also attributable to stressors including agricultural, urban, and roadway runoff, sewage treatment plant effluent, and dreissenid mussels (NPCA 2021). Elevated concentrations of phosphorus (consistently well above provincial water quality guidelines) are a primary contributor to the poor water quality in the Welland River watershed (NPCA 2021). Suggested causes of high phosphorus concentrations are related to animal waste, sewage discharges, soil erosion, and agricultural land use (NPCA 2010a, 2021). In addition, Zebra Mussels have become established in the lower portion of the Welland River, including the sites where live Lilliput have been observed (NPCA 2010a; Wright et al. 2017; NPCA 2021; DFO unpubl. data). Both Common Carp and Round Goby have been caught in the Welland River since the original Lilliput status report was published. At this time, there is distributional overlap between Lilliput and Common Carp and to a lesser extent with Round Goby (GLLFAS Biodiversity Science Database 2022). Works, undertakings, or activities that have occurred in the Welland River watershed since 2013 include culvert replacements, weir repair, drain maintenance, and pile wall installation (FFHPP 2022).

The Twenty Mile Creek subwatershed drains just over 300 km² of land and represents the second largest watershed in the NPCA (NPCA 2021). This subwatershed drains into Jordan Harbour and then into Lake Ontario. Water quality in the Twenty Mile Creek subwatershed is similar to the Welland River watershed in that it is considered stable, but five of the six monitoring stations on the main channel have “poor” water quality according to the WQI rating, and the remainder are classified as “marginal.” There are exceedances of total phosphorus (some at concentrations that were over 30 times the provincial guidelines), nitrate, total suspended solids, chloride, E. coli, copper, lead, and zinc (NPCA 2021). The stressors that are the suspected causes of this generally poor water quality are agricultural, urban, and roadway runoff (NPCA 2021). Both Common Carp and Round Goby are known to occur in Jordan Harbour (Aguiar et al. 2021; GLLFAS Biodiversity Science Database 2022). Two fertile Grass Carp have been captured by DFO’s Asian Carp Monitoring Program in Jordan Harbour, Lake Ontario (Marson et al. 2018; Aguiar et al. 2021).

In 1985, Hamilton Harbour was identified as an Area of Concern by the International Joint Commission (IJC) as it was one of the most degraded areas in the Great Lakes region (Hall et al. 2006; O'Conner 2010). Cootes Paradise Marsh and Grindstone Creek where Lilliput is found are part of the western end of Hamilton Harbour. Loading sources in the area consist of wastewater treatment plants, steel mills, urban runoff, combined sewer overflows, creeks (Red Hill and Grindstone), and Cootes Paradise (O’Conner 2010). The Hamilton Harbour watershed is approximately 36% agriculture, 49% commercial/ industrial/urban, and 15% open space/vacant land (Bowlby et al. 2009). Phosphorus concentrations have decreased since the implementation of the Remedial Action Plan in Hamilton Harbour; however, they remain well above the provincial water quality objectives (Hiriart-Baer et al. 2016; SLR Consulting (Canada) Ltd. 2020). This includes the area of outer Grindstone Marsh, which is one of the areas where Lilliput occurs (Richer and Theijsemeijer 2017; Bowman 2022). The harbour also experiences high levels of ammonia and suspended solids (Ramin et al. 2012; Vanden Byllaardt and O’Conner 2018). Examples of toxic chemicals that are present in the harbour at levels exceeding the provincial and/or federal guidelines (water, sediment, or tissue thresholds) for the protection of aquatic species are polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), arsenic, cadmium, iron, lead, zinc, and mercury (O’Conner 2010; Vanden Byllaardt and O’Conner 2018). Cootes Paradise has seen an increase in water clarity and a decrease in total phosphorus concentrations since the installation of the Fishway; however, phosphorus levels continue to exceed the provincial guideline with a mean concentration of 0.126 mg/L recorded in 2021 (Reddick and Theÿsmeÿer 2012; RBG 2019; Bowman 2022). The average concentration of total suspended sediment was ~29 mg/L in 2020 (Bowman 2022). The Cootes Paradise Marsh continues to be eutrophic and degraded (Thomasen and Chow-Fraser 2012; Theijsmeijer et al. 2016). According to RBG staff, the number one threat to Cootes Paradise Marsh is sewage spills. A number of these have occurred in Hamilton Harbour since the last status assessment (Mitchell 2021; City of Hamilton 2022a,b), including one that led to 24 billion litres of raw sewage leaking into Chedoke Creek and then into Cootes Paradise over a four-year period starting in 2014 (RBG 2021a). Another major threat to Lilliput in Hamilton Harbour is invasive species. Round Goby and Common Carp are both present in Lake Ontario, including Hamilton Harbour. The Fishway, which is situated where Cootes Paradise Marsh flows into Hamilton Harbour, was designed to prevent Common Carp from entering the marsh but allow the continued flow of water and movement of native fishes. This has led to a decrease in the number of Common Carp found in Cootes Paradise; however, some remain in the marsh (RBG 2019). It has been noted that there are more mussels in areas that are protected from Common Carp (Theijsmeijer pers. comm. 2022). Other invasive species that are of concern include additional fish species (for example, Goldfish [Carassius auratus] and Rudd [Scardinius erythropthalmus]) as well as various plant species such as Giant Reed Grass (Phragmites arundinacea) (Theijsmeijer et al. 2016).

Current and projected future threats

Lilliput is vulnerable to the cumulative effects of various threats, especially natural system modifications, pollution, climate change and severe weather, human intrusions and disturbance, and invasive and other problematic species and genes. The nature, scope, and severity of these threats are described in Appendix 1, following the IUCN-CMP (International Union for Conservation of Nature–Conservation Measures Partnership) unified threats classification system (see Salafsky et al. 2008 for definitions and Master et al. 2012 for guidelines). The threat assessment process consists of assessing impacts for each of 11 main categories of threats and their subcategories, based on the scope (proportion of population exposed to the threat over the next 10-year period), severity (predicted population decline within the scope during the next 10 years or 3 generations, whichever is longer, up to ~100 years), and timing of each threat. The overall threat impact is calculated by taking into account the separate impacts of all threat categories and can be adjusted by the species experts participating in the threats evaluation.

The overall threat impact for Lilliput is considered to be medium due to the spatiotemporal overlap of threats. These values are to be interpreted with caution, as they may be based on subjective information, such as expert opinion, although efforts have been made to corroborate the scores with available studies and quantitative data.

Natural system modifications (IUCN 7; overall threat impact medium-low)

7.2 Dams and water management/use

The presence of impoundments and dams on freshwater rivers and streams has been shown to negatively affect mussel communities (Blalock and Sickel 1996; Vaughn and Taylor 1999; Watters 2000; Parmalee and Polhemus 2004). Impoundments and dams typically result in siltation, stagnation, loss of shallow water habitat, pollutant accumulation, poor water quality, flow alteration, and temperature changes (Bogan 1993; Vaughn and Taylor 1999; Watters 2000). Dams also disrupt habitat connectivity and may create barriers that prevent host fish movement and therefore mussel distributions, and cause sediment retention upstream and scouring downstream (Watters 1996; Gardner et al. 2011). In addition, poor management of water control structures can potentially dewater areas, leading to unsuitable habitat for mussels as the bottom of the watercourse may become exposed. There are a number of pumping stations in certain areas where Lilliput is found (see Historical, long-term, and continuing habitat trends). Although there are a number of small dams and culverts throughout the Lilliput distribution, the only significant dam within the species’ range is the dam at Dunnville on the Grand River, and there are no plans to remove the dam at this time (Allan pers. comm. 2022). As a result, the potential impact of the dams themselves on Lilliput is minimal.

Dredging for drain maintenance, agricultural purposes, or storm water management can have environmental, physical, and biological consequences (Ebert 1995). Specific effects of dredging on Lilliput are uncertain; however, it has caused the direct destruction of mussel habitat, leading to siltation and sand accumulation in local and downstream mussel beds, created unstable substrate, resuspended contaminants, caused changes in benthic and fish communities, and lowered water velocity (Aldridge et al. 1987; Watters 2000; Tuttle-Raycraft et al. 2017). In addition, dredging can lead to removal of mussels and the redistribution of these individuals into suboptimal habitat (Miller and Payne 1998; Aldridge 2000). Grace and Buchanan (1981) found no mussels present in an area that had been dredged 15 years earlier. Dredging events have occurred recently within Lilliput habitat (see Historical, long-term, and continuing habitat trends for details).

7.3 Other ecosystem modifications

The presence of the invasive Round Goby indirectly impacts unionid mussel populations. This species has been implicated in the declines of native benthic fishes through competition and egg predation (Young et al. 2010). A number of fish species appear to be impacted by the Round Goby’s presence, including at least one suspected Lilliput host, Johnny Darter (French and Jude 2001; Lauer et al. 2004; Steinhart et al. 2004; Thomas and Haas 2004; Baker 2005; Reid and Mandrak 2008). Although there are no specific studies that show that the Round Goby negatively affects Green Sunfish, Bluegill, or White Crappie, it does alter the ecosystem where these species occur. Tubenose Goby, although not as abundant as Round Goby, may have similar indirect impacts as there is some overlap in preferred habitat (shallow coastal wetland habitat; Dawson et al. 2020). The invasive species is known to compete with darter species (Kocovsky et al. 2011; State of Michigan 2023), and prey on young “bottom-dwelling” fishes (Ministry of Natural Resources and Forestry 2022). These indirect impacts on host fish may lead to disruptions in the Lilliput reproductive cycle and should also be important considerations for conservation and management of the species (Clark et al. 2022).

Common Carp (non-native) and Grass Carp (invasive) are fishes that may indirectly impact unionid mussels. Their presence significantly alters habitat due to the nature of their feeding. This may include increasing turbidity, nutrient or water quality modifications, and significant impacts or changes to aquatic macrophytes (Miller and Crowl 2006; Pípalová 2006; Weber and Brown 2009; Wittman et al. 2014; Moore et al. 2019). These changes in habitat may not only impact unionid mussels, but also their host fishes (for example, reduction in abundance, reduction in habitat required for spawning or shelter; Petr 2000; Pípalová 2006).

The non-native Flathead Catfish has also been caught and is reproducing in the Thames River (Illes et al. 2019). This species may indirectly impact Lilliput through its host fishes, as it has been shown to cause a decline in a number of recreational fish species (Bonvechio et al. 2009). The impact over the next 10 years will likely be minimal across the entire population.

The presence of dreissenid mussels has also impacted unionid mussels indirectly, largely through their filtering capabilities which cause changes to the ecosystem. Examples include food reduction and increases in waste products and water clarity (Hebert et al. 1991; Strayer 1999; Haag 2012, Schwalb et al. 2021), all of which can also impact host fishes.

European Water Chestnut (Trapas natans) is an invasive plant that can alter habitat by creating dense, floating mats, which in turn can decrease native plant diversity and decrease dissolved oxygen levels in the water column (Ontario Federation of Anglers and Hunters 2022). During removal of this invasive species, Lilliput can be displaced by getting caught in the roots while the plant is being pulled up.

Dredging for recreational purposes is also occurring in certain areas where Lilliput is found and is discussed under this threat category. Because this type of dredging typically removes debris and increases boat access at the mouth of certain waterbodies, the impacts would be similar to other dredging activities described in IUCN threat 7.2 Dams and water management/use.

Pollution (IUCN 9; overall threat impact medium-low)

Pollution has been deemed one of the primary threats affecting mussel populations (COSEWIC 2013b, 2021b; DFO 2019). Although the specific impact of pollution on Lilliput is uncertain, pollution has been shown to affect mussel populations throughout North America. A variety of threats are associated with “household sewage and urban waste water,” “industrial and military effluents,” and “agricultural effluents,” including sediment loads (siltation), nutrient loads, contaminants (including contaminants of emerging concern, whose name reflects “…the limited knowledge of their scope and potential impacts”; Woolnough et al. 2020), and toxic substances (for example, road salts and spills), which fall into all three categories of waterborne pollution. Giri and Qui (2016) have found that global water quality is degrading as a result of agricultural activities associated with urbanization. Freshwater mussels, particularly glochidia and juveniles, are sensitive to a number of aquatic pollutants (Bringolf et al. 2007a,b; Woolnough et al. 2020); therefore, the current levels of pollution observed in these watersheds may have negative effects on the remaining subpopulations of Lilliput. Below is a brief description of how these threats affect mussels.

9.1 Domestic and urban wastewater

Contaminants and toxic substances can come from a variety of sources including industrial processes (threat 9.2), wastewater treatment plants, steel mills, and sewage and stormwater/road/urban area runoff. Toxic chemicals from both point and non-point sources are believed to be one of the major threats to mussel populations (Strayer and Fetterman 1999). Due to the nature of their life cycle, freshwater mussels are particularly sensitive to increased levels of contaminants and toxic substances (Cope et al. 2008). Because adult mussels are unable to move large distances, they are exposed to and accumulate these substances from their environment. In addition, contaminants can influence mussels at various life stages, from the molecular level to the population level (Newton and Cope 2007). Freshwater mussels are sensitive to a number of contaminants, including but not limited to PCBs, DDT, malathion, rotenone, copper, heavy metals, and contaminants of emerging concern (Fuller 1974; Havlik and Marking 1987; USFWS 1994; Gillis et al. 2008; Rzodkiewicz et al. 2023). Contaminants can impact respiration, growth, filtration, enzyme activity, recruitment ability (Rzodkiewicz et al. 2023), and behaviour (Naimo 1995). Glochidia and juveniles are particularly sensitive to heavy metals (Keller and Zam 1991; Bringolf et al. 2007a,b; Gillis et al. 2008), acidity (Huebner and Pynnonen 1992), salinity (Liquori and Insler 1985), chloride (Gillis 2011; Prosser et al. 2017; Gillis et al. 2022), ammonia (Newton and Bartsch 2007; Wang et al. 2007), and potassium (Wang et al. 2018). Salerno et al. (2020) also showed that the co-occurrence of some of these threats could pose risks to glochidia and juvenile mussels and therefore impact reproduction. Lilliput has been found in urban areas and in areas that are predominantly agricultural (threat 9.3), therefore, this threat is ongoing, with the concentrations of some of these contaminants already increasing (for example, chloride; Todd and Kaltenecker 2012; Sorichetti et al. 2022).

Exposure to municipal effluent has been shown to negatively affect unionid health (Havlik and Marking 1987; Gagné et al. 2004, 2011; Gagnon et al. 2006; Gillis et al. 2014). As the human population in Ontario continues to grow and expand, it can be expected that there will be an increase in the amount of wastewater entering aquatic environments. Pharmaceuticals enter aquatic ecosystems largely via effluent from sewage treatment plants and disrupt gonad physiology and reproduction in mussels. Gagné et al. (2011) showed that there has been a dramatic increase in the number of females of Eastern Elliptio (Elliptio complanata) downstream of a municipal effluent outfall in Quebec. In addition, males were found to exhibit a female-specific protein (Gagné et al. 2011). Flutedshell (Lasmigona costata) living downstream of a municipal wastewater outfall in the Grand River have shown significantly reduced condition factor and mussel age as well as impacts on immune status when compared to individuals living upstream of the outfall (Gillis 2012). Gillis et al. (2014) showed that caged mussels situated downstream of a municipal wastewater treatment plant exhibited physiological stress when chronically (4 weeks) exposed to effluents. In addition, Gillis et al. (2017) found a 7 km stretch of the Grand River downstream of a major wastewater treatment plant to be devoid of mussels and suggested that the poor water quality has either directly or indirectly created an extirpated zone in this system.

9.2 Industrial and military effluents

Oil spills can result in limited oxygen exchange, interference with respiration, blanketed substrates, consumption effects (Crunkilton and Duchrow 1990), and changes in fish communities, all of which impact the survival of mussels. A spill in a tributary of the Kalamazoo River in Michigan released over 800,000 gallons (>3 million litres) of crude oil affecting many of the organisms in the area (Murray and Korpalski 2010). Oil transmission trunk lines run through the Grand, Welland, Thames, Ruscom, and Belle rivers and some of their tributaries (Canada Energy Regulator 2022). Lilliput subpopulations in the Grand, Ruscom, and Belle rivers would be severely impacted if a spill were to occur as they are found just downstream of the trunk line. Subpopulations in the Thames and Welland rivers are located well downstream of the trunk lines (which occur in the headwaters, whereas Lilliput occur in the lower portions of these rivers); therefore, a spill would not have a great impact on these extant subpopulations.

9.3 Agriculture and forestry effluents

Loading of suspended solids causing turbidity and siltation is presumed to be one of the primary limiting factors for most aquatic species at risk (SAR) in southern Ontario (DFO 2011; Bouvier et al. 2014). Lilliput, however, appears to prefer muddy or silty substrates (Metcalfe-Smith et al. 2005; Watters et al. 2009). Therefore, increased turbidity may not impact Lilliput subpopulations as much as other mussel species. It is important to note, however, that the transport and increase in abundance of fine particles can degrade streams as well as negatively impact mussel feeding, growth, respiration, and reproduction (Wood and Armitage 1997; Strayer and Fetterman 1999; Tuttle-Raycraft et al. 2017, 2019; Goldsmith et al. 2021; Hyvärinen et al. 2022). Sedimentation effects may be more severe in that the accumulation of silt on the streambed may reduce flow rates and dissolved oxygen concentrations below the surface by clogging interstitial spaces in the stream substrate (Österling et al. 2010). In addition, increased turbidity and/or siltation would decrease the likelihood that a host fish will be able to visually locate a Lilliput or its conglutinate (see host attracting strategy in Life cycle and reproduction). Further research is required to determine if sediment loading is having any negative effects on Lilliput. Turbidity alone does not appear to be a major threat; however, it may affect the presence of the host species. Siltation may be a larger threat if levels are high.

Strayer and Fetterman (1999) identified increased nutrient loads from non-point sources, especially from agricultural activities, as a primary threat to freshwater mussels. Other sources include municipal wastewater discharges, domestic septic systems, and runoff associated with lawn maintenance, parking lots or roads in areas of urban and residential development (threat 9.1). Increased nutrient loads can have indirect effects on freshwater mussels as they lead to poor water quality. For example, decreased levels of oxygen are often associated with increased concentrations of phosphorus and nitrogen, and this can stimulate growth and decomposition of algae and plants (Ardón et al. 2021; RBG 2021b). This, in turn, reduces respiration and can cause death (Chen et al. 2001; Tetzloff 2001), as well as changes in fish communities (Jackson et al. 2001), which can impact reproduction. Nutrient loading can also change the algal biomass and species composition, which may lead to poor food quality or create unsuitable environments for juvenile mussels which are typically found burrowed in the substrate (Strayer 2014).

Climate change and severe weather (IUCN 11; overall threat impact medium-low)

The degree to which Lilliput is impacted by climate change and severe weather is uncertain. However, Brinker et al. (2018) state that “Climate change will affect the distribution and abundance of species in the Ontario Great Lakes Basin.” The threat was scored as having a medium-low impact. Brinker et al. (2018) also showed that of the 10 taxa included in the analysis, molluscs were one of the most vulnerable, which is consistent with other analyses conducted in New York and Michigan, USA (Schlesinger et al. 2011; Hoving et al. 2013). All of the Level 2 climate change and severe weather threats are likely to interact as well. Although the direct impact of climate change on Lilliput is uncertain, according to the Foden et al. (2013) framework for assessing a species’ vulnerability to climate change, Lilliput may be considered highly vulnerable because (i) it will be exposed to climate change impacts, (ii) its obligate host may be sensitive to the potential impacts, and (iii) it is sedentary as an adult and therefore unable to relocate away from the impacts.

11.2 Droughts

Any climate change-induced droughts will have a direct impact on adult mussels due to their sedentary nature. A drop in water level may be particularly important to freshwater mussels inhabiting shallow wetlands (for example, Cootes Paradise Marsh), as it may lead to a loss of aquatic habitat. Decreased water levels can cause community-wide declines, changes to flow regimes and current velocity, changes in host fish availability and/or habitat, local extinction, or forced movement into deeper waters where habitat may not be as suitable or impacts from invasive species may be more pronounced (Lemmen and Warren 2004; Spooner et al. 2011; DuBose et al. 2019; Tarter et al. 2022; Vaughn et al. 2015; Lopez et al. 2022).

11.3 Temperature extremes

Water temperatures in the Great Lakes have already increased and are projected to continue increasing into the future (see many examples of these studies in Poesch et al. 2016; Brinker et al. 2018; Zhang et al. 2020; Xue et al. 2022). Increased water temperature can impact mussel growth (Hastie et al. 2003), longevity (Hastie et al. 2003), reproduction (Cope et al. 2008; Jeffrey et al. 2018), host fish availability (Hastie et al. 2003; Spooner et al. 2011; Poesch et al. 2016; Modesto et al. 2018), and species richness (Lopez et al. 2022). However, it could also lead to an increase in habitat availability for potential host fishes as the northern range boundaries expand for centrarchids (for example, Bluegill) (Alofs et al. 2014); hence, the severity was scored as unknown.

11.4 Storms and flooding

The impact of intense weather events on Lilliput specifically is unknown; however, the effects may be direct or indirect. Although some species are able to cope with flooding events by burrowing (Schwalb and Pusch 2007), these events may cause mussel displacement if the individual is unable to burrow fast enough. Indirectly, storms and floods can lead to altered water quality, which is known to impact mussel species (see Current and projected future threats for details). For example, Carpenter et al. (2018) showed that extreme precipitation increases phosphorus loadings and can “…intensify the eutrophication of lakes.”

Human intrusions and disturbance (IUCN 6; overall threat impact low)

6.1 Recreational activities

Recreational activities include any activity that occurs along the shoreline or involves shallow water wading (for example, such as boating, canoeing, fishing, magnet fishing) which may negatively impact Lilliput either directly or indirectly. These activities could cause increases in suspended solids or accidental dislodgement of Lilliput. Boating occurs frequently within the Lake St. Clair tributaries and the lower Grand River as these waterways provide easy access to lakes St. Clair and Erie. Canoeing and magnet fishing also occur in Cootes Paradise and the Detroit River. Further research is required to determine if Lilliput may be at risk from these particular human intrusions or disturbances.

6.3 Work and other activities

Although the impacts of work and other activities are suspected to be minimal (dislodgement, handling effects; Haag and Commens-Carson 2008), the collection of Lilliput for scientific research is required to gain a better understanding of the species, its population, threats, and conservation status. Mitigation (for example, mussel relocations) does occur for some in-water work; however, the intent is to either remove individuals from a known, planned threat (for example, dredging, construction, etc.) or decrease potential impacts that may occur from any works, undertakings, or activities.

Invasive and other problematic species, genes and diseases (IUCN 8; overall threat impact low)

8.1 Invasive non-native/alien species

The presence of dreissenid mussels (Zebra and Quagga mussels) has been one of the most significant threats to unionid mussels in the Great Lakes. Dreissenid mussels attach to unionid mussels using byssal threads and this attachment interferes with and can often prevent unionid feeding, respiration, reproduction, excretion, and locomotion (Haag et al. 1993; Baker and Hornbach 1997). Although dreissenids almost completely eradicated native unionid mussels from Lake St. Clair, Lake Erie, and the Detroit River (Schloesser and Nalepa 1994; Nalepa et al. 1996; Schloesser et al. 2006), recent work suggests that at least some co-existence is possible (Crail et al. 2011; Strayer et al. 2011; Zanatta et al. 2015; Bossenbroek et al. 2018; Keretz et al. 2021) and that the effects may be decreasing (Burlakova et al. 2014). In addition, Karatayev et al. (2015) suggest that there is a shift towards higher numbers of Quagga than Zebra mussels in the lower Great Lakes and that Quagga Mussels may have fewer direct impacts on native mussel species. Given that most Lilliput are found in areas of slower moving creeks, rivers, and marshy areas, dreissenid mussels do remain a threat. During recent surveys conducted by DFO, live Zebra Mussels were observed at some sites where Lilliput occurs. A few Lilliput had Zebra Mussels attached to their shells and a few others showed signs of previous attachment (DFO unpubl. data 2022); however, most showed no evidence of Zebra Mussel infestation (see Historical, long-term, and continuing habitat trends for details).

Direct impacts of Round Goby to native mussels include predation on juveniles and small adults (Ray and Corkum 1997; Poos et al. 2010; Clark et al. 2022). Given the small size of Lilliput, it would likely be at risk of being eaten by Round Goby. In addition, Round Goby has been shown to act as a sink for glochidia, which then limits unionid recruitment (Tremblay et al. 2016). The invasive Common Carp may also be impacting Lilliput. Direct impacts include the accidental consumption of juvenile mussels and dislodgement of adult mussels during feeding (Moore et al. 2019).

8.5 Viral/prion-induced diseases

In 2017, a Common Snapping Turtle (Chelydra serpintina) found in Cootes Paradise marsh was confirmed to have ranavirus. This was the first documented case of the virus in the species and the first reported case of the infection in a reptile in Canada (McKenzie et al. 2019). Ranavirus has also been reported in a number of fish species including Bluegill and Crappie, which are both suspected Lilliput hosts. However, mortality events seem to be rare in these species when compared to other fish species (Duffus et al. 2015; Kipp et al. 2022). Although no studies have been completed on the potential impacts of ranavirus on mussel reproduction, the virus is present in Cootes Paradise.

Number of threat locations

The most serious and plausible threats identified (Medium-Low impact) for Lilliput include Natural System Modifications (IUCN threat 7: dams and water management/use, other ecosystem modifications), Pollution (IUCN 9, from sediment and nutrient loading, contaminants, and toxic substances), and Climate Change and Severe Weather (IUCN 11, for example, droughts). These threats are expected to act at a scale below that of the drainage (n = “3 to 4” as used in the original assessment); therefore, this unrealistic minimum number of locations (that is, 3) was not included in the range of possible locations. The maximum number of locations is at the waterbody scale (n = 32). Thus, the most likely number of locations, at the subwatershed/waterbody scale, is 20 (Table 4).

Protection, status, and recovery activities

Legal protection and status

Lilliput was listed as Threatened in Schedule 3 of the Ontario Endangered Species Act in 2013 (Government of Ontario 2023c). The collection of freshwater mussels may require a provincial collection permit issued by the Ministry of the Environment, Conservation and Parks.

Other provincial acts in Ontario that will indirectly benefit Lilliput include the following: Lakes and Rivers Improvement Act (Government of Ontario 2023d), Nutrient Management Act (Government of Ontario 2023e), Clean Water Act (Government of Ontario 2023f), Water Resources Act (Government of Ontario 2023g), and the Environmental Protection Act (Government of Ontario 2023h).

Freshwater mussels and their habitat also are protected under the federal Fisheries Act. and the species was listed as Endangered in Schedule 1 of the federal Species at Risk Act in 2019 (Government of Canada 2021b). This listing includes the identification of Critical Habitat in the Belle, Ruscom, East Sydenham, and Grand rivers, Hamilton Harbour (Cootes Paradise Marsh and Grindstone Creek estuary), Jordan Harbour, and Welland River and Oswego Creek. There are no global designations for this species.

Non-legal status and ranks

Lilliput is considered globally Secure (G5; last reviewed January 2024) and nationally Secure (N5) in the U.S. In Canada, however, it is considered Critically Imperilled at both the national level (N1) and provincially in Ontario (S1) (National General Status Working Group 2020; NatureServe 2024). In Michigan (S1), Pennsylvania (S1/S2), and New York (S2), the status ranges from Critically Imperilled to Imperilled although the species is considered Secure (S5) in Ohio. These are the U.S. states bordering lakes St. Clair, Erie, and Ontario where Lilliput is found. See NatureServe (2024) for a full list of other U.S. state conservation ranks ranging from Critically Imperilled (S1) in Iowa and South Dakota to Not Ranked (SNR) in Florida, Minnesota, Nebraska, New Jersey, and Oklahoma. Lilliput has been assessed as Least Concern by the IUCN (Cummings and Cordeiro 2012).

Land tenure and ownership

A majority of the land adjacent to the rivers where Lilliput is found is privately owned and unprotected; however, the river bottom is generally owned by the provincial Crown. Some Lilliput are found within conservation areas, for example, Tremblay Beach Conservation Area, along Little Creek (owned by ERCA), Chippewa Creek Conservation Area along the Welland River (owned by NPCA), and Byng Island Conservation Area along the Grand River (owned by Grand River Conservation Authority). Those in Cootes Paradise Marsh are found within the Royal Botanical Gardens’ property.

Recovery activities

Since the original status and assessment report on the Lilliput in 2013 (COSEWIC 2013a):

A federal recovery potential assessment was undertaken (DFO 2014).

A federal recovery strategy and action plan was published including the identification of Critical Habitat in Belle/Ruscom rivers, East Sydenham River, Grand River, Hamilton Harbour (Cootes Paradise, Grindstone Creek Estuary), Jordan Harbour, and Welland River/Oswego Creek (DFO 2022).

A provincial recovery strategy was published (Ministry of the Environment, Conservation and Parks 2023).

Surveys were conducted in Cootes Paradise Marsh in 2015 by staff from the Royal Botanical Gardens in collaboration with DFO (Wright et al. 2020). Results widened the known distribution of Lilliput in the area.

Surveys were conducted by staff at the Royal Botanical Gardens in Cootes Paradise Marsh and Grindstone Creek.

Wetland restoration projects have been implemented by the Royal Botanical Gardens (RBG 2022).

Surveys or mitigation activities were conducted in the Canard River (Morris et al. 2020) and on Pelee Island (Natural Resource Solutions Inc. 2020).

Lilliput age and growth estimates have been completed (including creation of VBGF, estimated age at sexual maturity and generation time (see Life cycle and reproduction) (DFO unpubl. data 2022).

Targeted Lilliput surveys were completed in 2022 in Essex County along Lake St. Clair and Haldimand/Norfolk counties along Lake Erie (Gibson et al. 2023).

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Collections examined

No collections were examined for the preparation of this report. All inquiries were restricted to obtaining records or viewing photographs to confirm species identity. Much of the information came from the Lower Great Lakes Unionid Database (2024). The following description has been modified from the COSEWIC status report (2013a).

Fisheries and Oceans Canada’s Great Lakes Laboratory for Fisheries and Aquatic Sciences in Burlington, Ontario is home to a computerized, GIS-linked database known as the Lower Great Lakes Unionid Database. It was created in 1996 and contains all of the available information on historical and current records of freshwater mussel species found throughout the lower Great Lakes drainage. Original data sources included the primary literature, natural history museums, federal, provincial, and municipal government agencies (and some American agencies), conservation authorities, Remedial Action Plans for the Great Lakes Areas of Concern, university theses, and environmental consulting firms. Mussel collections held by six natural history museums in the Great Lakes region (Canadian Museum of Nature, Ohio State University Museum of Zoology, Royal Ontario Museum, University of Michigan Museum of Zoology, Rochester Museum and Science Center, and Buffalo Museum of Science) were the primary sources of information, accounting for over two-thirds of the initial data acquired. Janice Metcalfe-Smith (Biologist, Environment Canada) personally examined the collections held by the Royal Ontario Museum, University of Michigan Museum of Zoology, and Buffalo Museum of Science, as well as smaller collections held by the Ontario Ministry of Natural Resources. The database continues to be updated with new data and now contains over 23,000 records of unionids from across Canada, The majority of records in the database are now from recent (post-1990) collections or incidental observations made by Fisheries and Oceans Canada, Environment and Climate Change Canada, provincial agencies, universities, consulting companies, naturalists, members of the public, conservation authorities, iNaturalist (iNaturalist 2022, Burgbirder 2024a;b), and any data received from the DFO “Data Request for DFO Aquatic Species Distribution Maps”. Data were also requested from the Canadian Museum of Nature and Natural Heritage Information Centre; however, these data were already included in the Lower Great Lakes Unionid Database. The status report writers have, at some point, personally verified (either in person or digitally) live Lilliput specimens from all of the subpopulations described in this report.

Information from the Essex Region Conservation Authority (ERCA) Surface Water Quality Data and Station Locations and Fish Database was used to inform the threats section (if applicable) when discussing water quality or the presence and/or distribution of Round Goby and Common Carp in waterbodies found within the ERCA. The following statement is required under the data sharing agreement between ERCA and DFO regarding water quality data: “Water quality data copyright ERCA 2022. Locations of monitoring sites are approximate”. The following statement regarding data from the Fish Database is also included: “The following records of fishes have been provided by the Essex Region Conservation Authority including records up to and including December 2018. These records represent only those within our database. Within the vicinity of the subject area there may be species at risk present (species listed by COSEWIC and or OMNR). Please contact Fisheries and Oceans Canada. Collection locations represented as shown/reported are approximate.”

Authorities contacted

Allan, C. Natural Heritage Supervisor. Grand River Conservation Authority. Cambridge, Ontario.

Anderson, R. Research Scientist. Canadian Museum of Nature. Gatineau, Quebec.

Badra, P. Aquatic Zoologist. Michigan Natural Features Inventory. Michigan State University. Lansing, Michigan.

Benshoff, A. Project Manager/Senior Malacologist. Edge Engineering and Science, Kent, Ohio.

Blaise, P. Project Canaiad administrator, iNaturalist. La Prairie, Quebec.

Bryant, J. Director of Watershed Management Services. Essex Region Conservation Authority. Essex, Ontario.

Carroll, E. Director of Biology. St. Clair Region Conservation Authority. Strathroy, Ontario.

Crosthwaite, J. Coordinator, Conservation Biology, Southwestern Ontario. Nature Conservancy of Canada. London, Ontario.

Filion, A. Scientific and GIS Project Officer COSEWIC Science Support, Canadian Wildlife Service, Environment and Climate Change Canada.

Frohlich, K. Ecologist. Niagara Peninsula Conservation Authority. Welland, Ontario.

Grant, P. Research Scientist. Department of Fisheries and Oceans. Sidney, British Columbia.

Hossie, T. Assistant Professor. Trent University. Peterborough, Ontario.

Jones, C. Provincial arthropod zoologist. Ontario Ministry of Northern Development, Mines, Natural Resources and Forestry. Peterborough, Ontario.

McDonald, R. Senior Environmental Advisor. National Defence. Ottawa, Ontario.

MacVeigh, G. Senior Aquatic Biologist. Natural Resource Solutions Inc., Waterloo, Ontario.

Marson, D. Senior Aquatic Biologist, Field Operations Lead Aquatic Invasive Species Program. Fisheries and Oceans Canada, Burlington, Ontario.

Martel, A. Malacologist. Canadian Museum of Nature. Gatineau, Quebec.

McKay, V. Species at Risk Biologist. Lower Thames Valley Conservation Authority. Chatham, Ontario.

Pickett, K. Species at Risk Biologist. Canadian Wildlife Service, Ontario Region: species in Ontario. Toronto, Ontario.

Richer, S. Species at Risk Biologist. Royal Botanical Gardens. Burlington, Ontario.

Shepard, P. Species Conservation and Management Ecosystem Scientist III. Parks Canada. Vancouver, British Columbia.

Shoobs, N.F. Curator of Mollusks, Museum of Biological Diversity, Dept. of Evolution, Ecology, and Organismal Biology. The Ohio State University. Columbus, Ohio.

Smith, P.D. Naturalist. Volunteer with Royal Botanical Gardens. Hamilton, Ontario.

Stammler, K. Water Quality Scientist/Source Water Protection Project Manager Essex Region Conservation Authority. Essex, Ontario.

Ste-Marie, P. Assistant Collection Manager. Canadian Museum of Nature. Gatineau, Quebec.

Taylor, T. Natural Heritage Information Centre, Ontario Ministry of Northern Development, Mines, Natural Resources and Forestry (NDMNRF). Peterborough, Ontario.

Theysmeyer, T. Head of Natural Areas. Royal Botanical Gardens. Burlington, Ontario.

Turner, S. Project Canaiad Curator, iNaturalist. Guelph, Ontario.

Woolnough, D. Research Associate Professor. Biology Department and Institute for Great Lakes Research. Central Michigan University. Mount Pleasant, Michigan.

Wu, J. ATK Coordinator, COSEWIC Secretariat, Canadian Wildlife Service, Environment and Climate Change Canada. Gatineau, Quebec.

Zanatta, D.T. Professor, Biology Department and Institute for Great Lakes Research. Central Michigan University. Mount Pleasant, Michigan.

Acknowledgements

Funding for the preparation of this report was provided by Environment and Climate Change Canada. These authorities provided valuable data and/or advice: Essex Region Conservation Authority (T. Dufour); St. Clair Region Conservation Authority (E.C. Paterson, N. Drumm); and Fisheries and Oceans Canada (A. Conway). Thanks are extended by the report writer to all of those who contribute information to the Lower Great Lakes Unionid Database and answer the “Data Request for DFO Aquatic Species Distribution Maps” call as well as to naturalists who are part of the Canadian iNaturalist project. The report writer wishes to offer thanks to J. Colm from Fisheries and Oceans Canada for creating the distribution map and to A. Saini from the COSEWIC Secretariat for calculating the EOO and IAO maps. Thanks are offered by the report writer to L. Bouvier, who was a co-writer on the original Lilliput status and assessment report.

Biographical summary of report writer(s)

Kelly McNichols-O’Rourke has 20 years of experience working with mussel species at risk in Ontario. She has a B.Sc. (Hons.) in Marine and Freshwater Biology from the University of Guelph Ontario (2001), and an M.Sc. in Integrative Biology from the University of Guelph (2007) that focused on identifying host species and population dynamics of mussel species at risk in Ontario. She has co-written three COSEWIC status reports, including the original status report for Lilliput. She has co-authored two recovery strategies, and edited/updated four COSEWIC reports on 11 listed freshwater mussel species. She has served on the Mollusc Specialist Subcommittee of COSEWIC since 2016, and is a member of the Ontario Freshwater Mussel Recovery Team. Ms. McNichols-O’Rourke is an Aquatic Science Biologist with the Great Lakes Laboratory for Fisheries and Aquatic Sciences with Fisheries and Oceans Canada in Burlington, Ontario, Canada and her research interests focus on the life cycle, lifespan, and distribution of native unionids and their host fishes in aquatic ecosystems.

Dr. Todd J. Morris is a Research Scientist with the Great Lakes Laboratory for Fisheries and Aquatic Sciences with Fisheries and Oceans Canada in Burlington, Ontario, Canada. He has a B.Sc. (Hons.) in Zoology from the University of Western Ontario (1993), a Diploma in Honours Standing in Ecology and Evolution from the University of Western Ontario (1994), an M.Sc. in Aquatic Ecology from the University of Windsor (1996), and a Ph.D. in Zoology from the University of Toronto (2002). Dr. Morris’s research interests focus on the biotic and abiotic factors structuring aquatic ecosystems, and he has worked with a wide range of aquatic taxa, from zooplankton to predatory fishes. He has been studying Ontario’s freshwater mussel fauna since 1993, has authored three recovery strategies for eight listed freshwater mussel species, written or co-written eight COSEWIC status reports and one COSEWIC status appraisal summary. He co-chairs the Ontario Freshwater Mussel Recovery Team, is Co-chair of the Molluscs Specialist Subcommittee of COSEWIC, and past member of the American Fisheries Society Endangered Mussels Subcommittee.

Meg Goguen studied Ontario’s freshwater mussels from 2014 to 2023. She has a B.Sc. (Hons.) in Wildlife Biology and Conservation from the University of Guelph, Ontario (2016). Ms. Goguen was an Aquatic Science Technician with the Great Lakes Laboratory for Fisheries and Aquatic Sciences with Fisheries and Oceans Canada in Burlington, Ontario, Canada from 2016 to 2023. She has co-written one COSEWIC status report.

Appendix 1. Threats calculator assessment for Lilliput (Toxolasma parvum)

Threats assessment worksheet

Species or Ecosystem Scientific name

Toxolasma parvum, Lilliput

Date

6/13/2023

Assessor(s)

Report writers: Kelly McNichols-O'Rourke, Todd Morris (also Mollusc Co-chair), Meg Goguen; Facilitator and responsible Co-chair: Dwayne Lepitzki; Mollusc SSC members: Suzanne Dufour, Olwyn Friesen, Cam Goater, Andrew Hebda, Erin Herder, Tim Rawlings, Daelyn Woolnough; DFO: Jennifer Diment; ON: Brent Patterson; Secretariat: Dean Whitehead; External experts: Katie Stammler (Essex Region Conservation Authority), Craig Paterson (St. Clair Region Conservation Authority), Tys Theijsmeijer (Royal Botanical Gardens), Vicki McKay (Lower Thames River Valley Conservation Authority), Eric Chamberlain (Manager, Roads and fleet - drainage superintendent), Amanda Conway (FFHPP Biologist), Tony Zammit (GRCA Watershed Ecologist), Gina MacVeigh (Natural Resources Solution Inc.).

References:

draft calculator (6 June 2023) provided by report writers based on draft status report by writers Kelly McNichols-O'Rourke, Todd Morris, Meg Goguen, J. Rosie Goguen

Assigned Overall threat impact:

C = Medium

Impact adjustment reasons:

Spatiotemporal overlapping threats so adjusted to a straight medium

Overall threat comments

generation time 3 years so timeframe for severity and timing = 10 years; most likely uses a variety of host fishes (although not confirmed for Canada); occurs in SE ON; no abundance estimates due to no quantitative sampling; stable or increasing but limited data; 15 subpopulations; highest numbers on Pelee Island, Grand River, Hamilton Harbour ("largest, reproducing Lilliput subpopulation" although no abundance estimates); no evidence of continuing decline; "does not appear to be a projected, or a suspected decline within the next three generations, based on currently available data" "The habitat where Lilliput are currently found is highly degraded and little is expected to change in respect to habitat quality." EN B2ab(iii), May 2013.