Freshwater snails alter habitat use in response to predation

This study compared snail habitat use in Michigan lakes that contain a predator, the molluscivorous pumpkinseed sunfish, Lepomis gibbosus, with snail habitat use in adjacent lakes lacking pumpkinseeds. Snails in lakes with pumpkinseeds used the covered portion of standardized substrates more often than did snails in lakes lacking pumpkinseeds. The hypothesis that covered habitats provide a refuge from fish predation was experimentally tested by stocking Physella into artificial pools and manipulating the presence of cover and pumpkinseeds. After 3 days, survivorship of Physella was 15-fold higher in pools containing a covered habitat. To test for the existence of chemically mediated predator avoidance behaviours in Physella, they were exposed to water that had been in contact with (1) pumpkinseeds and (2) crushed conspecifics. Physella did not change their habitat use when exposed to water that had been in contact with pumpkinseeds, but increased their refuge use two-fold over controls when exposed to water that had been in contact with crushed conspecifics. The effects of habitat structure on the type of predator avoidance response used by Physella was tested by exposing them to mortality risk in wading pools with and without covered habitats. Physella did not move to the surface in response to mortality risk if a covered habitat was available, but in the absence of a covered habitat they responded to mortality risk by moving to the water's surface. Possible consequences of flexible behavioural choices by snails for population and community-level processes are discussed. (C) 1996 The Association for the Study of Animal Behaviour