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 p redator abundance or diversity over a range of temporal and spatial scales. These patterns are assumed to reflect natural selection via differential p redation on genetically determined, fixed phenotypes. Phenotypic variation, however, also can reflect within-generation developmental responses to env ironmental 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 scal es. Here, we demonstrate adaptive phenotypic plasticity in prey from geogra phically separated populations that were reared in the presence of an intro duced predator. Marine snails exposed to predatory crab effluent in the fie ld increased shell thickness rapidly compared with controls. Induced change s were comparable to (i) historical transitions in thickness previously att ributed to selection by the invading predator and (ii) present-day clinal v ariation predicted from water temperature differences. Thus, predator-induc ed phenotypic plasticity may explain broad-scale geographic and temporal ph enotypic variation. If inducible defenses are heritable, then selection on the reaction norm may influence coevolution between predator and prey. Trad e-offs may explain why inducible rather than constitutive defenses have evo lved in several gastropod species.