Induced defenses in response to an invading crab predator: an explanation of historical and geographic phenotypic change.

The expression of defensive morphologies in prey often is correlated with predator abundance or diversity over a range of temporal and spatial scales. These patterns are assumed to reflect natural selection via differential predation on genetically determined, fixed phenotypes. Phenotypic variation, however, also can reflect within-generation developmental responses to environmental cues (phenotypic plasticity). For example, water-borne effluents from predators can induce the production of defensive morphologies in many prey taxa. This phenomenon, however, has been examined only on narrow scales. Here, we demonstrate adaptive phenotypic plasticity in prey from geographically separated populations that were reared in the presence of an introduced predator. Marine snails exposed to predatory crab effluent in the field increased shell thickness rapidly compared with controls. Induced changes were comparable to (i) historical transitions in thickness previously attributed to selection by the invading predator and (ii) present-day clinal variation predicted from water temperature differences. Thus, predator-induced phenotypic plasticity may explain broad-scale geographic and temporal phenotypic variation. If inducible defenses are heritable, then selection on the reaction norm may influence coevolution between predator and prey. Trade-offs may explain why inducible rather than constitutive defenses have evolved in several gastropod species.