Trait compensation and cospecialization in a freshwater snail: size, shapeand antipredator behaviour

We examined relationships between individual differences in antipredator be haviour and prey morphological characters (size, shape) that influence prey vulnerability. Behavioural responses of Physa gyrina to chemical cues asso ciated with predation by crayfish Orconectes rusticus, were assayed in the laboratory for 6 days over a 13-day period. Snails displayed consistent, in dividually repeatable responses to the predation cues, including hiding (re fuge use) and substratum avoidance (crawling to the water surface or out of the water). We assessed shell morphology using morphometric techniques tha t isolate geometrically independent components of size and shape variation corresponding to aperture width and apertural obstruction. Previous studies indicate that large size, narrow apertures and obstructed apertures reduce morphological vulnerability to the shell-entry predation tactics used by c rayfish. In the present study, small, and thus more vulnerable, prey tended to show stronger antipredator behaviour than large prey (i.e. behavioural compensation for morphological vulnerability). In contrast, behavioural and shape-based defences were positively correlated; snails with narrow apertu res showed strong antipredator responses. We refer to this 'double defence' against predators as cospecialization. With either compensation or cospeci alization, suites of correlated behavioural or morphological traits must be studied in tandem to understand the adaptive value of prey responses to pr edators. (C) 1999 The Association for the Study of Animal Behaviour.