Plasticity constrained: over-generalized induction cues cause maladaptive phenotypes

The adaptive value of phenotypic plasticity depends upon the degree of match achieved between phenotype and environment. This match is governed in part by how well organisms discern environmental states (e.g. presence or absence of particular predators). To examine the specificity with which organisms use cues to induce defensive phenotypes, we raised freshwater snails, Physella virgata, with either molluscivorous or non-molluscivorous sunfish species (Lepomis cyanellus, L. gibbosus, L. macrochirus, L. megalotis, L. microlophus, Micropterus salmoides). Regardless of the predator class with which they were raised, snails exhibited two induced responses: (1) reduced growth and (2) development of rotund shells. Reduced growth typically results from predator avoidance behaviours and so is associated with a net survival benefit in environments containing molluscivores. Rotund shells increase crush resistance and so increase both handling time and rejection rates by molluscivorous sunfish. Despite the adaptive nature of these responses to molluscivorous species, responding unnecessarily to non-molluscivores is maladaptive. Growth reduction limits fecundity and prevents snails from attaining size refugia for most predators. Rotund shells increase vulnerability to shell-entry predators, which are typically more common than shell-crushing predators. Thus the induced responses entail costs but award no advantages in the presence of non-molluscivorous sunfish. Lack of specificity in use of cues (e.g. all sunfish treated alike) may be a major constraint on the evolution of phenotypic plasticity. Furthermore, these induced responses may produce trait-mediated indirect effects that cascade throughout the food web.