Predator-induced defenses in a salt-marsh gastropod

Abstract

Predator-induced defenses are among the most ecologically important forms of phenotypic plasticity. Although predation and induced defenses are well documented in rocky-intertidal systems, they have received less attention in soft-bottom communities. Shell-crushing predators are common in soft-bottom, vegetated habitats, which often exhibit substantial spatial heterogeneity in predation intensity. We examined variations in shell morphology of the salt-marsh periwinkle, Littoraria irrorata, among marsh microhabitats in the northern Gulf of Mexico that vary in their accessibility to predatory blue crabs, Callinectes sapidus. Littoraria from high-predation sites exhibited more extensively calcified apertural lips and narrower apertural openings relative to snails from low-predation sites. Thick apertural lips generally increased the handling time required by Callinectes to breach Littoraria shells in laboratory experiments, although the method of shell entry used by crabs was dependent on the crab:snail size ratio. Apertural-lip thickness was not related to past predation events in field-collected snails. Snails exposed to water treated with the effluent of Callinectes and crushed conspecifics produced significantly thicker apertural lips than controls, with a response time and morphological extent comparable to that of their rocky-shore counterparts. This study underscores the widespread occurrence of predator-induced plasticity in marine gastropods and emphasizes its role in soft-bottom, vegetated marine habitats, where shell-crushing predation can be as prevalent a selective force as in the rocky intertidal.