Predation and Functional Responses of Carcinus maenas and Cancer magister in the Presence of the Introduced Cephalaspidean Philine orientalis

An increasing number of examples suggest that interactions among introduced species are ecologically important and relevant to the management of invaded systems. We investigated the potential for the introduced cephalaspidean sea slug Philine orientalis to interfere with the feeding of the introduced European green crab (Carcinus maenas) and the native Dungeness crab (Cancer magister). We observed co-occurrence of crab species and P. orientalis at field sites in Bodega Harbor and Tomales, San Pablo, and San Francisco Bays. In laboratory and field experiments, we determined whether crab feeding was suppressed by P. orientalis and the duration of this suppression for individual crabs. We also used foraging response models to explore changes in the feeding rate of crabs with varying densities of P. orientalis and small bivalve prey. We found that P. orientalis deterred predation by green and Dungeness crabs on small clams in laboratory feeding trials, but not in field experiments with green crabs and P. orientalis. Foraging models predicted that P. orientalis would only affect crab feeding in the field under specific conditions of crab, P. orientalis, and prey densities. These foraging models bridged an important gap between lab and field experiments and allowed us to predict how changes in species abundances at two trophic levels might alter the importance of crab suppression by P. orientalis.