TRADE‐OFFS BETWEEN GROWTH AND SURVIVAL: RESPONSES OF FRESHWATER SNAILS TO PREDACIOUS CRAYFISH

Foraging organisms must often balance the conflicting demands of feeding and avoiding predators. I investigated whether the freshwater snail Amnicola limosa trades off obtaining food for reducing the risk of mortality from predatory crayfish ( Orconectes ). Field surveys and experiments were conducted to determine, first, whether snails altered habitat use when presented with predation risk, and second, whether this behavior had consequences for growth. Surveys of 20 northern Wisconsin lakes revealed that snail occupancy of food‐poor macrophytes was positively related (and occupancy of food‐rich bottom sediments was negatively related) to crayfish abundance. Similarly, in a lake where crayfish ( O. rusticus ) are abundant, but differentially active among seasons, seasonal sampling indicated that snails occupied sand during the spring period of low crayfish activity but were absent from this habitat during summer. These distributional patterns derive from a combination of active habitat shifts and differential mortality among habitats, as indicated by laboratory experiments and field estimates of snail dispersal rates. Furthermore, aggregation of snails on macrophytes reduces snail growth. The average length of adult snails collected from bottom sediments was greater than the length of snails on macrophytes. Similarly, in the laboratory, snails that migrated up vertical surfaces in response to chemical alarm signals from feeding crayfish exhibited a growth depression. This trade‐off of maximum growth rates for increased survival from crayfish by A. limosa likely poses constraints on snail fitness, which, in turn, can affect interactions with other predator species.