Crab Predation, Waterborne Cues, and Inducible Defenses in the Blue Mussel,mytilus Edulis

Inducible defenses are found in both terrestrial and aquatic organisms and appear to be a common defensive strategy of both sessile and mobile species. Here we report the presence of previously undescribed inducible defenses in marine mussels, a dominant member of temperate rocky intertidal communities. In the Gulf of Maine, estuarine shorelines with low water flow are characterized by high crab predation on the common blue mussel, Mytilus edulis, compared to nearby shorelines with high water flow. Mussels at these high predation sites had thicker shells, relatively more shell mass, and were more tightly attached to the substrate than mussels at low predation sites. Field and laboratory experiments were used to test whether the between-site differences in these traits were the result of phenotypic plasticity. Mussels outplanted to high predation sites produced more byssal threads and attached more firmly to the substrate than did mussels outplanted to low predation sites. In the laboratory, thicker shells were induced by waterborne cues from both the common predator, Carcinus maenus, and damaged conspecifics, and these thicker shells were mechanically stronger than thinner, noninduced ones. Our results contribute to growing evidence that inducible defenses are a pervasive feature of intertidal assemblages. In addition to the bivalve example presented here, gastropods, barnacles, bryozoans, and seaweeds are also known to have inducible defenses. Phenotypic plasticity for defensive traits has likely evolved repeatedly in intertidal organisms and appears to play a major role in the dynamics of rocky intertidal communities.