Freshwater snail responses to fish predation integrate phenotypic plasticity and local adaptation

Predators shape the phenotype of prey by means of local adaptation, within-generation plasticity, and transgenerational plasticity. Theory predicts that the influence of these three mechanisms on phenotype is determined by the number of generations exposed to the predator and the autocorrelation between parent and offspring predator environments. To test theoretical predictions about the relative effects of local adaptation, within-generation plasticity, and transgenerational plasticity on prey phenotype, we exposed two generations of freshwater snails (Physa acuta) from two populations—one from a large pond containing predatory fish and one from a set of small ponds that lacked predatory fish—to the cues from a predaceous bluegill sunfish (Lepomis macrochirus). Physa acuta exhibit both local adaptation and phenotypic plasticity in response to predation risk; thus, we predicted that snails from the population with fish and snails experimentally exposed to fish cues would produce more globose (or spherical), crush-resistant shells. In addition, given the prolonged exposure to fish predation, we predicted that snails from the population with fish would exhibit reduced phenotypic plasticity. We found evidence that snails from the population with fish were generally more globose and more crush resistant than snails from the fish-less population and exposure to fish cues led snails from both populations to develop more globose shells with increased crush resistance. However, contrary to our predictions, the offspring of snails from both populations exposed to fish cues had elongated shells and reduced crush resistance. This may reflect the negative effects of parental stress or variation in resource investment.