Negative interactions between two species of invasive crab, Carcinus maenas and Hemigrapsus sanguineus, along the US east coast have been abundantly demonstrated.  The result of these interactions has been the elimination of Carcinus from rocky intertidal habitats in southern portions of the invaded range (Long Island Sound).  Previous work concluded that this species replacement occurred because of predation by Hemigrapsus on settling postlarvae and juvenile Carcinus.  However, theoretical evidence suggests that Hemigrapsus predation is not solely responsible for this species replacement.  Further, a similar species replacement does not seem to be occurring further north within the Gulf of Maine.  Instead, the two species coexist in the Gulf of Maine.  The question therefore remains: what is the mechanism for this species replacement, and why does it differ in different geographical regions.  I address these questions using data on crab diets and physiology.

Results/Conclusions

I demonstrate that interactions between these crabs within the Gulf of Maine are strongly lopsided.  Specifically, Carcinus responds to the presence of Hemigrapsus by eating fewer mussels and more algae, while interactions between the two species have little impact on Hemigrapsus.  I further demonstrate that this diet shift reduces energy stores and ultimately reduces reproductive output for Carcinus.  Finally, I use this information, together with prevailing coastal current patterns along the US east coast, to present an alternative hypothesis that explains why Hemigrapsus has replaced Carcinus as the dominant crab in Long Island Sound, but not in the Gulf of Maine.