Functional diversity among predators of a freshwater snail imposes an adaptive trade-off for shell morphology

We explored how functional diversity among predators of the freshwater snail Physa creates an adaptive trade-off for the snail’s shell morphology. Physid shells range continuously between elongate and rotund in overall shape. The protection conferred by alternative shell shapes depends on the mode of attack employed by predators. Predators attack Physa primarily through shell entry (by crayfish, Orconectes obscurus) and shell crushing (by fish, Lepomis gibbosus, Cyprinus carpio, Carrasius auratus). The narrow apertures associated with elongate shells were expected to restrict apertural access by crayfish, as it does in several marine gastropod–decapod interactions. The orbicular body whorls of rotund shells were predicted to disperse better the crushing forces applied by fish. Resistance to one predator then implies susceptibility to the other because shells can be relatively rotund or elongate but not both. Three levels of evidence from this study supported the idea of a trade-off: (1) measurement with laboratory instruments indicated that rotund shells were relatively crush-resistant, whereas elongate shells were more entry-resistant; (2) foraging trials showed that rotund shells required more handling time and were rejected more frequently by fish, whereas snails with elongate shells were more likely to survive with crayfish; and (3) field samples revealed rotund shell morphology in habitats where fish were common. The physical tests, foraging trials and field survey of the present study demonstrate the functional basis for predator-induced plasticity, which has been documented elsewhere for physid shell morphology and serves as a fourth line of support for the existence of an adaptive trade-off. Implications of the trade-off for the behaviour of predators and prey are also discussed.