Adaptive plasticity in predator-induced defenses in a common freshwater snail: altered selection and mode of predation due to prey phenotype

Studies of putatively adaptive plasticity, such as inducible defenses, frequently explore the fitness consequences of expressing alternative phenotypes in alternative environments, but few studies examine how and why the pattern of selection changes in relation to trait induction. We induced snails in the presence/absence of nonlethal predatory crayfish, exposed both phenotypes (alone and combined) to selection by lethal crayfish, and quantified linear and nonlinear selection differentials. Crayfish induced an increase in mass, shell thickness, and absolute (but not relative) shell dimensions. Crayfish predation on uninduced snails was rapid, accomplished via shell-crushing and revealed strong selection for increased size (i.e., mass and shell dimensions). Conversely, crayfish predation on predator-induced snails was slower, often accomplished using an alternative mode of predation (shell-crushing 70% of the time, but shell-extraction 30% of the time), and revealed selection for wide apertures and thick shells. Crayfish selection on uninduced snails in the presence of predator-induced snails was stronger than predation on uninduced snails alone demonstrating that selection can be frequency dependent. Therefore, predator-induced changes in size and shell thickness appear to be adaptive and, along with reciprocal adjustments in the mode of predation, result in altered patterns of selection.