Variable Functional Responses of a Marine Predator in Dissimilar Homogeneous Microhabitats

Abstract

Adult soft-shelled clams {Mya arenaria) persist at low densities in Chesapeake Bay sandy habitats despite intense predation by blue crabs {Callinectes sapidus). Clam persistence may be a consequence of variation in blue crab foraging rates as a function of clam density and sediment composition. In laboratory aquaria, we measured the functional responses (prey consumption per predator as a function of prey density) of large blue crabs to six densities of adult soft-shelled clams buried at natural depths in two sediment types (mud and sand). Functional responses in sand and mud were differentiated statistically by analyses of (1) residuals and residual sums of squares of discrete and continuous-time models, and (2) the exponent (3 of a general functional response model. Crab predation rates were significantly higher in mud than sand, and functional responses differed significantly between these two substrates. Blue crabs displayed type III (sigmoid) density-dependent functional responses in sand, and type II (decelerating rise to an upper asymptote) inversely density- dependent responses in mud. Risk of mortality for clams decreased sharply in sand at low densities similar to those observed in the field near the end of the annual period of active predation. These observations (1) suggest that variable blue crab functional responses result in microhabitat-specific mortality rates of benthic prey, and (2) indicate that functional responses can differ significantly according to the physical properties of topographically simple habitats.