Eastern pondmussel (Ligumia nasuta) COSEWIC assessment and status report: chapter 11

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Technical summary

Ligumia nasuta (Say 1817)

Eastern Pondmussel – Ligumie pointue

Extent and Area Information

Population Information

Threats (actual or imminent, to populations or habitats)

• Zebra mussels (aquatic invasive species) have destroyed most of the available habitat for L. nasuta throughout its range and continue to threaten the remnant population in the delta area of Lake St. Clair.
• Ligumia nasutain the St. Clair delta occur at depths of 1.5 m or less. Any changes in water levels in Lake St. Clair due, for example, to climate change could negatively impact the species.
• Threats to the population of L. nasuta in Lyn Creek have not been assessed.

Rescue Effect (immigration from an outside source)

Quantitative Analysis

Not available

Current Status

COSEWIC: Endangered (April 2007)

Status and Reasons for Designation

Status: Endangered

Alpha-numeric code: A2ce; B1ab(i,ii,iii,iv,v)+2ab(i,ii,iii,iv,v)

Reasons for Designation: This was one of the most common species of freshwater mussel in the lower Great Lakes prior to the invasion of the zebra mussel (Dreissena polymorpha) in the late 1980s. Zebra mussels attach to the shells of native freshwater mussels in the hundreds or even thousands, causing the native mussels to suffocate or die from lack of food. Over 90% of historical records for the species are in waters that are now infested with zebra mussels and therefore uninhabitable. The species has declined dramatically and now occurs as two small, widely separated populations, one in the delta area of Lake St. Clair and one in a tributary of the upper St. Lawrence River. There is evidence that declines may be continuing at one location. Although zebra mussels appear to be declining in some areas, their impacts on this species may be irreversible if insufficient breeding adults have survived. Climate change is likely to cause a drop in water levels in the delta and further reduce the amount of habitat available to the mussel. Recent surveys in Lake St. Clair, which were conducted as a collaborative effort between Environment Canada and the Walpole Island First Nation, resulted in the identification of a significant refuge for this species within First Nation territory. The refuge is being managed by the First Nation for the protection of this and other aquatic species at risk with which it co-occurs.

Applicability of Criteria

• Criterion A: (Declining Total Population): Meets Endangered A2ce: - population size reduction of > 50% inferred over the past 3 generations (~ 30 years, given that the average age of maturity for unionids is 6-12 years), where the cause (impacts of zebra mussels, which began in 1986) may not have ceased and may not be reversible - 2 - based on a decline in EO of 86% and a decline in the quality of habitat - c - and the effects of introduced taxa (zebra mussels) – e.
• Criterion B: (Small Distribution, and Decline or Fluctuation): Meets Endangered B2ab(i,ii,iii,iv,v):- AO < 500 km² (AO ~ 45 km²) - 2. - severely fragmented, i.e., known to exist at only 2 locations separated by more than 800 km - a - continuing decline projected in EO, AO, area, extent and quality of habitat and number of locations (e.g., a decline has been observed at one site in the St. Clair delta since 1999) - b(i,ii,iii,iv,v). Also meets Endangered B1ab(i,ii,iii,iv,v): - EO < 5,000 km² (EO ~ 3,400 km²) - 1- ab(i,ii,iii,iv,v) as documented above for Criterion B2.
• Criterion C: (Small Total Population Size and Decline): Does not apply because the number of mature individuals is unknown. The total number of individuals (mature and immature) in the Lake St. Clair population is estimated at 22,000-44,000; thus, the total number of mature individuals likely exceeds the threshold for Threatened (10,000). The size of the Lyn Creek population is unknown, but it is likely 2 orders of magnitude smaller than the Lake St. Clair population based on the relative sizes of the AOs.
• Criterion D: (Very Small Population or Restricted Distribution): Meets Threatened D2: - < 5 known locations (2 locations are known), and the species is prone to further declines due to impacts of zebra mussels because: (1) most individuals (~ 90%) are in one population (the St. Clair delta) where losses may be continuing and where 20-30% of individuals are found in one small bay; and (2) it is possible, although unlikely, that zebra mussels could be introduced into the Lyn Creek drainage.
• Criterion E: (Quantitative Analysis): Does not apply (no data available).