Does space competition regulate density of juvenile dungeness crab cancer magister dana in sheltered habitats?

Abstract

Field experiments were performed to test the short-term response of juvenile Dungeness crab Cancer magister Dana to the density of conspecifics in intertidal shell habitats of the Grays Harbor estuary (46 ° N, 124°W). The experiments were conducted using oyster shell habitats constructed in the intertidal zone. First we tested whether crab losses from intertidal shell habitat were due to density-dependent emigration, density-dependent predation, or both. Our results show that although both processes account for crab losses, predation is not density-dependent, and most crabs had emigrated rather than being eaten at high crab density. Field experiments were also conducted to test the effect of crab density, food availability and conspecific physical contact on crab emigration. Treatments were different densities of free crabs to allow physical contact between them or individually caged crabs to prevent physical contact, with or without enhanced food (small clams). Emigration of crabs from experimental shell habitat increased with density of free crabs, irrespective of whether food was enhanced or risk of predation increased. Our results suggest that physical interactions regulate density because: (1) the per capita consumption rate of clams in the artificial shell plots was significantly lower at high crab density, and (2) density of enclosed crabs did not affect the immigration rate of conspecifics. Although the ultimate cause may be competition for food or avoidance of cannibalism, we suggest that when crab recruitment to epibenthic shell substratum has been successful, crab density over the short term is regulated primarily by space competition mediated by physical interactions.