Generalist versus specialist strategies of plasticity: snail responses to predators with different foraging modes

Phenotypic plasticity is a common adaptation to environmental heterogeneity, and theory predicts that the evolution of constitutive versus plastic strategies should depend on the frequency of alternative environments, the magnitude of constraints and the costs of plasticity per se. However, it is unclear how species should evolve when they experience more than two environments that favour divergent phenotypes, particularly when they have absolute constraints on their morphology. We examined the plasticity of three freshwater snail species (Helisoma anceps, H.campanulata and H.trivolvis) in response to three environments: (i) no predator; (ii) shell-invading water bugs (Belostoma flumineum) and (iii) shell-crushing crayfish (Orconectes rusticus). We found distinct responses by each snail species to the predator treatments. Helisoma anceps starts with a relatively low, narrow and thick shell that becomes lower and thicker in response to crayfish but is unresponsive to water bugs. In contrast, H.campanulata starts with a relatively high, wide and thin shell that becomes lower and wider in response to water bugs but is unresponsive to crayfish. Helisoma trivolvis starts with a shell of intermediate height and width while the predators induce defences in different directions. These results suggest that H.trivolvis has a generalist plastic strategy while H.anceps and H.campanulata have specialised plastic strategies orientated against a single type of predator at the potential cost of being unable to respond to others. We then performed predation trials to determine predator preferences using a mixture of the three species. After 2weeks of exposure to crayfish cues, H.anceps had higher survival than both H.trivolvis and H.campanulata with uncaged crayfish. After 2weeks of exposure to water bug cues, both H.trivolvis and H.campanulata had higher survival than H.anceps with uncaged water bugs. When predation trials were conducted after 5weeks of exposure to predator cues, H.trivolvis and H.campanulata reached a size refuge from both predators and this shifted predation pressure to H.anceps. Collectively, these results suggest that closely related prey species with different absolute constraints in their morphology had different defences that are either specialised or generalised to alternative environments.