Banff Springs snail (Physella johnsoni) COSEWIC assessment and status report: chapter 8

Biology

No primary publications solely on the biology of the Banff Springs Snail exist or have resulted since the research and recovery program began over 10 years ago; however, some biological data were included in the original COSEWIC Status Report (Lepitzki 1997a), the Alberta Status Report (Lepitzki 2002b), the publication on mtDNA sequencing (Remigio et al. 2001), and the hydrogeological articles on the springs (Grasby and Lepitzki 2002; Grasby et al. 2003). Data on all aspects of the species, including biology, collected up to 2002 were incorporated into the Parks Canada approved Resource Management Plan (RMP) for the recovery of the snail (Lepitzki et al. 2002b); more recent data were summarized for the Recovery Strategy and Action Plan (Lepitzki and Pacas 2007). Data up to 2001 are found within annual progress reports to Parks Canada (Lepitzki 1997b, 1998, 2000a, 2002a) and the Endangered Species Recovery Fund (ESRF) (Lepitzki 1999, 2000b, 2001, 2003a,b, 2004). More recent data are included in monthly and yearly data summaries to Parks Canada (Lepitzki unpubl. data) or are still being analyzed and summarized.


Life Cycle and Reproduction

Pulmonates are usually annual and semelparous (breed only once, then die) (Brown 1991; Dillon 2000). In general, as temperatures increase, snails grow faster, reproduce earlier, and can have multiple generations per year (McMahon 1983). Most likely, P. johnsoni is simultaneously hermaphroditic, similar to other members of the Physidae (Clarke 1973; Dillon 2000). In other physids, mating is not reciprocal in that one partner assumes the male role while the other the female with roles often being swapped (Wethington et al. 2000). Outcrossing is preferred (Dillon et al. 2002). In other species of Physella, reproduction is temperature dependent (DeWitt 1955, 1967; Sankurathri and Holmes 1976). It is uncertain if these generalities apply to P. johnsoni.

Transparent, crescent-shaped egg capsules have been observed in all thermal springs inhabited by P. johnsoni except Lower Middle and have been observed year-round in the Basin Spring pool although not in every month in a single year. Very small snails (length of shell about 1 mm), also have been observed in the Cave on most snail surveys suggesting that reproduction may not be seasonal. The cryptic nature of the egg capsules may contribute to their apparent density dependent abundance as weak although significant (P<0.05) regressions were found in both the Cave and Basin pools between snail and egg capsule numbers observed during snail surveys (see Population Sizes and Trends).

Egg capsules have always been found at or slightly above the water’s surface, attached to substrates (concrete pool wall, wooden post, floating microbial mat, deciduous leaves, sticks, other living snail shells) suggesting that atmospheric oxygen is required for development. In flow-through aquaria (39.7 litre, 10 U.S. gallon) containing Cave Spring water, egg capsules were, on average, 2.3 mm wide by 5.2 mm long (S.E.M. ± 0.03 and ± 0.05, range 1-4 and 3-8, sample size 280 and 282, respectively) and on average contained 12.3± 0.2 eggs per capsule (range 2-23, sample size 262). In contrast to thermal springs, egg capsules were found throughout the tanks. This may reflect the higher dissolved oxygen levels. Embryos within the eggs grew fully formed snails with shell lengths from 0.5 mm to 0.8 mm; they hatched within three to 10 days (average 5.9± 0.2, sample size 66) after deposition. The capsules initially observed in aquaria were noticeably smaller than those in thermal springs and were produced by snails as small as 3 mm shell length suggesting that adult snails, capable of reproduction, have shell lengths greater than or equal to 3 mm. Snails in tanks reached 3 mm within six weeks and began laying eggs after only nine weeks of age. The discrepancy between the number of eggs deposited in tanks and the absence of large number of newly hatched snails suggests that, at least in tanks and under crowded conditions, few of the eggs hatch or few of the newly hatched snails survive once they hatch. Cannibalism was observed where adult snails ate embryos within egg capsules but it was uncertain if the cannibalism was overt or accidental. The lifespan of individual snails is unknown but adult snails have lived for an additional 10 to 11 months once placed in aquaria. The rapidity with which snail populations can increase is shown in Population Sizes and Trends.


Predation, Parasites, and Competition

There are no direct observations of predation of this species by other animals, although waterfowl and other birds are suspected to be the main natural predators.

The potential effects of disease and parasites on mortality of P. johnsoni are unknown as no snails have been examined for parasites. Physids are known intermediate hosts for a variety of gastrointestinal flatworms whose definitive hosts (habitat of the adult parasite) are vertebrates such as waterfowl (Olsen 1974).

P. johnsoni presumably grazes on plant material or aufwuchs (mixed algal, fungal, and bacterial slime communities growing on hard surfaces, McMahon 1983) like other physids (DeWitt 1955; Clampitt 1970; Brown 1991). The snails have been observed to ingest white-filamentous bacteria. Londry (2005a, b), using stable carbon and nitrogen isotopes, suggested that a white, Thiothrix-like organism is the dominant microbial food source for the snail although they also consume cyanobacteria. Thiothrix is a sulphide-oxidizing, filamentous bacterium. Soldier fly (Stratiomyidae) larvae, commonly found in the thermal springs on Sulphur Mountain (Lepitzki and Lepitzki 2003), consume a diet (Pennak 1978; Clifford 1991) similar to the snail. Londry (2005a, b) found the larvae ate a mixed diet of Thiothrix spp. and cyanobacteria in Banff’s thermal springs. Competition for food with soldier fly larvae may be occurring. Typically, the larvae appear to reach their maximum size in late-winter early-spring, as snail populations begin their annual decline (see Population Sizes and Trends).

The population effects of these potential natural mortality factors are unknown but may contribute to springtime declines in snail numbers (see Population Sizes and Trends). They also could contribute to subpopulation extirpation especially if they occurred during extreme population lows. One of the many actions in the Recovery Strategy and Action Plan (Lepitzki and Pacas 2007) is to produce a response plan that could reduce predation and competition pressures when snail populations are at their lowest.


Physiology

Based on the microdistribution of the snail in the thermal springs, some inferences can be made on the physiological tolerance, or at least preference, of the species. Physella johnsoni prefers the upper reaches of the thermal springs that, in comparison to downstream areas (occupied by fewer snails), are significantly (P<0.05) warmer and have higher levels of sulphide but lower pH and dissolved oxygen. However, an anomaly is found at the Cave Spring where snails are also thriving in areas wetted by slow flowing and dripping thermal spring water which is typically between 20°C and 26°C, cooler than that found in the Cave Spring pool (range 27°C to 33°C). Perhaps this anomaly and typically coolest (Figure 6) of the springs currently occupied by P. johnsoni helps explain the slight genetic difference of snails in the Cave Spring (see Genetic Description).

Preliminary experiments in flow-through aquaria containing Cave Spring water examined the snail’s thermal preference. After each aquarium was delineated into 27 volumetric cells by dividing each tank face into nine quadrants, an immersion heater was placed into the corner of one of three aquaria. The immersion heater increased water temperature with significant (P<0.05) differences observed among cells only in the tank with the heater (temperature in the cells ranged from 32.5°C to 34.4°C in the tank with the heater and 31.1°C to 31.5°C in the tanks without the heater). This temperature differential did not result in changes in snail microdistribution. No significant differences were observed among cells in any of the other water parameters measured (pH, dissolved oxygen, conductivity) either in the tank containing the heater or the other tanks.

The ability of the species to survive desiccation has not been experimentally tested; however, behaviour in the field has been observed. In general, it appears that snails are able to cope with gradual decreases or increases in water levels by following the change in waterline but drastic drops such as 50 cm in less than 15 minutes have stranded and killed many snails. This magnitude of drop has occurred when valves controlling water levels in the Basin and Cave pools require manipulation to prevent flooding of built resources. Egg capsules, because they adhere to the concrete-edged pools, desiccate and/or freeze with dropping water levels or are most likely asphyxiated with rising water levels.


Dispersal/Migration

Active dispersal (via the foot) is limited within springs and extremely unlikely among springs. Microdistributional observations of re-introduced snail subpopulations prove that upstream migration through crawling is possible. Most snails moved upstream immediately upon translocation to the Upper Middle Spring; however, it took 17 weeks before snails were observed upstream within the Upper Middle Spring cave (~5.6 m from their point of translocation) and 45 weeks before they were observed completely around the periphery of the cave (~7.8 m from point of translocation). At Kidney Spring, snails were not observed at the cliff face (4.2 m upstream) until 40 weeks after they were translocated into the cistern. Snails could travel upstream at Kidney both through the underground pipe and through the surface stream. Downstream dispersal, also commonly observed, may be active or, more likely, passive via water currents. Snails have been observed to release their grip on substrates and tumble downstream. Because snail numbers typically drop with increasing distance from thermal spring origin, the importance of the downstream outliers to the subpopulation within each spring is uncertain. Lepitzki (2006) concluded that while these downstream areas have appeared to be population “sinks” over the last 10 years of study, they could become “sources” if thermal spring origins moved or were subject to very localized catastrophes whose effects dissipated by the time they reached downstream areas.

Passive dispersal by birds, a recognized mechanism for gastropod transportation (Roscoe 1955; Rees 1965; Malone 1965a, b, 1966; Dundee et al. 1967; Boag 1986), has been proposed as a mechanism for colonization of additional thermal springs after the species evolved in situ at the Basin and Cave Springs (Remigio et al. 2001). Passive dispersal through pipes linking the Middle Springs with the C&BNHS (see Habitat Trends) could have occurred. At one time, pipes also linked the Upper C&B and Lower C&B Springs to the Basin Spring (Van Everdingen 1972; Van Everdingen and Banner 1982). The current linkage between the origin pools and outflow streams at the C&BNHS are pipes. Water physicochemistry, dye-tracer, and snail linkages have been observed via the pipes over the past 10 years. For example, a week after reopening a blocked drain at the Lower C&B Spring following provisions outlined in an emergency Environmental Assessment (Leeson 2001), an unprecedented but brief increase in snail numbers was observed in the eastern Cave outflow stream. This was most likely due to a pulse of snails being flushed passively through the pipes.

Anthropogenic dispersal of snails among thermal springs was proposed as a possible mechanism to reduce probability of extinction (Tischendorf 2003). Given low levels of natural dispersal among snail subpopulations and their natural lack of genetic heterogeneity (see Genetic Description), the appropriateness of moving snails from spring to spring is questionable.


Interspecific Interactions

The Banff Springs Snail is dependent on thermal spring water and the corresponding microbial community for food and habitat.


Adaptability

Habitat preferences and tolerances of snails and eggs to water level fluctuations and desiccation have been discussed in Physiology. Data on the rapidity of annual population increases and success of re-introductions will be shown in Population Sizes and Trends. Some responses to changes in habitat were given in Dispersal/Migration. Limits to tolerance to natural disturbances and normal, natural seasonal fluctuations in water physicochemistry and microbial community were discussed in Habitat Requirements, as were potential consequences of thermal spring drying in Habitat Trends.

Mixed results were obtained from captive-breeding experiments in aquaria. Snails eventually died in tanks re-circulating thermal spring water even though up to half the water volume was changed weekly. This prompted a change to flow-through tanks where Cave Spring water was pumped into three tanks and allowed to drain by gravity. Tank populations, initiated with four snails per tank, increased to peaks ranging from 120 to 285 snails without supplemental feeding; however, difficulties in excluding snails continually entering the tanks through the water pump, even through the various filters, complicated results. These tank populations even began cycling, with population declines apparently related to cessations in egg laying. A change in water physicochemistry in the Cave Spring as a result of cleaning a blocked drain following an emergency Environmental Assessment (see Dispersal/Migration) and a corresponding change in the tank thermal water regime stopped captive-breeding success. Essentially no reproduction occurred in the captive-breeding tanks for 21 months even though supplemental feeding and other actions were tried. Success also did not occur in three newly established tanks suggesting that the cause of no reproduction was not related to age of snail cultures. It is speculated that something as simple as a change in microbial community diversity, where a microbe that secretes a substance detrimental to the snails becomes established or increases in abundance, could have caused the lack of reproduction in the captive-breeding tanks. After a lull of almost two months, the captive-breeding program was re-initiated with six snails per tank and tank populations reached unprecedented levels, ranging from peaks of 1216 to 1256 snails per tank. An increase in the amount of supplemental food appeared to result in an increase in tank carrying capacity but since supplemental feeding was not continually increased, tank populations levelled.

An additional experiment was tried in the tanks. Six snails from the captive-population were added to each of two tanks containing re-circulating municipal tap water, a third tank acted as control with no snails added. The purpose was to see if snails could survive in tap water as an emergency measure in case a thermal spring dried. Snails thrived and reproduced with tank populations peaking at 479 and 773 snails, well below levels observed in the flow-through tanks, even though fish food flakes were added equally to all tanks containing snails. When tap-water snails were returned to flow-through thermal water tanks, they continued to reproduce suggesting that the Banff Springs Snail could be maintained in tap water as an emergency measure. However, significant (P<0.05) differences were found between tap-water and flow-through thermal spring water populations: egg capsules in tap-water tanks were smaller, contained fewer eggs, but also hatched sooner than those in thermal spring water. It is also suspected that shell morphology changed but these conclusions await additional shell measurements.

The captive-bred snails were transferred to the Basin outflow stream when the tanks were decommissioned (Lepitzki 2005). Initially, the Basin outflow stream subpopulation reached unprecedented levels. Whether habitat enhancement, in the form of an in-stream weir and the addition of woody debris, will continue to maintain a higher outflow stream carrying capacity is being monitored. Much of the woody debris added both in the Basin outflow stream and as mitigation in the Lower C&B Spring pool for reverting drainage back into the pipes is now underwater and does not provide emergent habitat for the snail.

Another example of the adaptability or response of the snail also was observed in the Basin outflow stream. Once flow was restored to the Basin outflow stream by manipulating a valve in March 1999, the Basin outflow stream subpopulation rebounded from four snails to more typical levels.

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