Competition for Shelter Between Lobsters (Homarus americanus) and Jonah Crabs (Cancer borealis): Effects of Relative Size

American lobsters (Homarus americanus) and Jonah crabs (Cancer borealis) inhabit rocky subtidal areas where they take shelter in crevices or burrows. In previous studies, lobsters dominated crabs of equivalent size in competition for shelter; however, we predicted that large crabs could dominate small lobsters. SCUBA surveys revealed a broad overlap in size of lobsters and Jonah crabs cooccurring in Narragansett Bay, Rhode Island; thus, opportunity exists for interaction between animals disparate in size. In laboratory experiments, relative size did not affect the outcome of competition for shelter: lobsters were competitively superior to crabs of all sizes. However, some crabs excluded lobsters from shelter. Fish predation on lobsters increased significantly when shelter was limited, but predation on crabs did not. Crabs were able to escape predation by rapid burrowing, while lobsters were dependent on locating preexisting refuges. Differential vulnerability to predation may explain the lack of size effect in competition between lobsters and crabs. The cost of not obtaining shelter is lower for crabs than for lobsters; thus, crabs utilize alternatives rather than compete with lobsters.