Density-dependent predation by blue crabs Callinectes sapidus on natural prey populations of infaunal bivalves

We used field and laboratory mesocosm experiments to examine the effects of the functional response of the blue crab Callinectes sapidus foraging on Balthic clams Macoma balthica in the upper Chesapeake Bay. Field experiments measured the density-dependent effect of blue crabs on clam patches in both mud and sand substrates at multiple sites spanning the natural range of clam densities, allowing us to examine the effects of larger-scale variation in prey abundance on prey mortality patterns. We compared the results of our field experiments to mesocosm experiments with 1 and 2 blue crabs to determine differences in the effects of single and multiple predators on the density-dependence of clam mortality. We observed predator behavior to identify mechanisms responsible for differences in prey mortality patterns. In the field, M. balthica mortality was not significantly density dependent, but the natural density of prey surrounding experimental patches had a negative density-dependent effect on clam mortality in mud. In the mesocosms, 1 or 2 blue crabs caused density-dependent mortality. Density dependence was weaker in mesocosms with 2 crabs. Agonistic behaviors were not significantly affected by clam density, but the presence of a conspecific increased a crab's foraging time at the lowest clam density. Changes in behavior when multiple crabs forage together may partly account for the reduction in density dependence of clam mortality. Predator responses such as the effects of conspecifics on foraging and patch choice that were lacking in the laboratory appear to be key in determining prey mortality patterns in the field. Larger-scale patterns of prey density variation were more important in determining prey mortality rates than the small-scale variation represented by the experimentally manipulated patches.