Hotwater physa (Physella wrighti) COSEWIC assessment and upate status report: chapter 6
Biology
The biology of freshwater pulmonates is generally well known because this group lives in accessible habitats, has relatively short generation times and survives well in captivity. No studies have been conducted on P. wrighti and no data are available on life history traits such as growth rate, longevity, age at sexual maturity, egg development rate, or seasonality of reproduction for this species in its natural habitat.
Life cycle and reproduction
Ectothermic animals, such as physids, cannot regulate their body temperature and so are predisposed to seek temperatures that optimize life history processes. Physids are oviparous (egg-laying) hermaphrodites. Eggs develop directly into substrate-dependent, crawling juveniles. In temperate climates, eggs are generally laid in the spring and juveniles mature into adults during the summer, lay eggs the following spring, and die. However, physids living at warmer temperature accelerate these life history traits (review in McMahon 1975). In Banff National Park, Alberta, juvenile P. johnsoni have been observed year round (Lepitzki et al. 2002). Elsewhere in Alberta, P. gyrina was found to reproduce year round in thermal effluents ranging from 15–31°C, although no egg development occurred at 32°C (Sankurathri and Holmes 1976). Captive P. wrighti Liard River Hotsprings Provincial Park (LRHPP) in a heated aquarium with water from laid egg masses containing 6–18 eggs above the water line at 25°C; hatching occurred after nine days (Lee and Ackerman 1999). As incoming water temperature at LRHPP probably remains relatively constant year round, the air temperature probably determines where the snails position themselves and their eggs. InAugust 2006 at ambient air temperature of 20°C, most snails and all egg cases were observed above the water/air interface. In September 1997, when the ambient temperature was much cooler, most visible snails were submerged (J. Lee, pers. obs.). P. wrighti positions itself and its eggs at optimal temperature to facilitate life history requirements.
Diet
Physids are generally considered to be detritivores and/or bacterial feeders (Brown 2001). Physella wrighti probably grazes on the aufwuchs, the algal and bacterial growth covering the submerged substrates on which it lives. P. wrighti kept in captivity in water from Alpha Pool survived and reproduced on a diet of mixed brewer’s yeast and fish food for 14 months (Lee and Ackerman 1999).
Predation
Nothing is known about direct predation of P. wrighti by other species. The shorebird Tringa flavipes (Lesser Yellowlegs) grazes in the marshes of LRHPP and might prey on P. wrighti, since the diet of this bird includes snails (Ehrlich et al., 1988). However, these two species have historically co-existed at this site, so predation by this bird would not appear to be a significant limiting factor for P. wrighti.
Physiology
Physella wrighti appears to be physiologically adapted or acclimated to its hot water habitat. Both adults and eggs are found at water margins and a sudden drop or change in water flow that exposes them to ambient air temperature might be lethal. These snails can only live in areas with no or very slow water flow.
Dispersal/migration
Freshwater snails disperse passively by being carried by wildlife or humans that inadvertently or purposefully pick up snails and deposit them in another water body. One example of snail dispersal is the observation that migratory birds might store snails in their feathers as a food source (Rees 1965). Wildlife or humans could pick up P. wrighti but if continuous warmth is required, it is unlikely that individuals would survive if introduced outside the current range.
The entire known population of P. wrighti is found within an EO of 0.02 km² so there is no migration and no possibility of rescue effect from other areas. Rescue effect from outside populations of P. gyrina is unknown.
Interspecific interactions
Physella wrighti does not display obvious reliance on other species for its survival at any time during its life cycle with the exception that it grazesalgal and bacterial growth on Chara and other surfaces.
Adaptability
Physids in general are able to withstand widely fluctuating temperatures (reviewed in McMahon 1975, Wethington and Guralnick 2004), and the degree to which P. wrighti is dependent on warm temperature is unknown. P. wrighti can tolerate changes such as loss of Chara as the preferred habitat, and specimens held in captivity tolerated changes to both habitat and food.