Specificity of induced defenses in Mytilus edulis and asymmetrical predator deterrence

Induced defenses of prey have become widely recognized in several marine taxa, yet their specificity to particular predators and impacts on subsequent predation are seldom investigated. In this study, Mytilus edulis showed highly specific induced defenses in response to predators with different attack strategies. The mussels developed significantly heavier shells only in the presence of waterborne cues from Carcinus maenas, a crushing crab predator, and significantly heavier adductor muscles only in the presence of waterborne cues from Asterias vulgaris (= Asterias rubens), a predatory sea star that pries bivalves open. However, mussels effectively thickened their shells in response to cues from predators by either increasing allocation to shell (in response to C. maenas), or reducing linear shell growth (in response to A. vulgaris and, to a lesser extent, the predatory whelk Nucella lapillus). These different mechanisms of shell thickening in response to all 3 predators defended the mussels from subsequent crab predation, increasing handling times of mussels by predatory C. maenas by more than 48%. In contrast, only mussels with increased adductor muscle weight (an induced response to A. vulgaris) were subsequently protected from the sea star. These results suggest that although induced defenses in M. edulis are specific to predators with different attack strategies, diffuse selection by C. maenas may allow predator specific responses to be adaptive even when predator composition changes.