Density-dependent foraging and mutual interference in blue crabs preying upon infaunal clams

Predator-prey dynamics between the blue crab Callinectes sapidus and an infaunal soft-shelled clam, Macoma balthica, were examined in laboratory experiments to assess the joint effects of varying predator and prey densities upon predator foraging rates and prey survival. A full-factorial experimental design involved 2 prey densities (4 and 16 clams m-2) and 3 predator densities (1, 2 and 4 crabs m-2) with 6 trials per treatment combination. Blue crabs exhibited density-dependent foraging under all conditions: proportionally more clams were consumed at the higher clam density. Furthermore, at the higher crab densities mutual interference was evident in the incidence of wounds and deaths to crabs resulting from cannibalism or intraspecific aggression. Thus, the combined impact of varying crab and clam densities resulted in (1) the maintenance of a density-dependent refuge from blue crab predation for large infaunal clams, irrespective of crab density, and (2) intraspecific aggression resulting in injury and mortality of blue crabs at high crab densities. The collective results indicate that both predator and prey densities must be examined experimentally for their joint impact upon predator-prey dynamics in marine systems