Predator-induced defenses differ between sympatric Mytilus edulis and M. trossulus

This is the first study to compare defenses to predation in closely related species of Mytilus and draws attention to the role of predation in determining their distribution, abundance and co-existence. We experimentally determined whether 2 blue mussel species, Mytilus edulis and M. trossulus, that co-exist within a hybrid zone in Newfoundland, Canada, differ in their predator-induced responses. Both species recognized cues from predators, although the magnitude of the responses to these cues varied with the predator (Asterias rubens or Cancer irroratus) and the traits measured (growth, attachment strength, shell thickness and adductor muscle mass) after 122 d of exposure. Both mussel species showed decreased growth following exposure to sea stars. Both predator species induced an increase in attachment strength in both mussel species (M. edulis > M. trossulus). M. edulis displayed plasticity in partitioning resources among defensive traits in the presence of sea stars (growth in adductor muscle) or crabs (increased shell thickness). These responses were not observed in M. trossulus, which is therefore likely more susceptible to these predators than is M. edulis. The preference of sea stars for M. trossulus supports this conclusion. Differences in predator-inducible defenses increase the likelihood of M. edulis succeeding M. trossulus in mussel aggregations, which in turn explains the often abrupt increase in the proportion of M. edulis among larger mussels. Such predator-induced defenses, and the resultant predator-prey interactions, affect the temporal and spatial distributions of mussel species and their potential to coexist and hybridize.