FUNCTIONAL-RESPONSES OF BLUE CRABS CALLINECTES-SAPIDUS RATHBUN FEEDING ON JUVENILE OYSTERS CRASSOSTREA-VIRGINICA (GMELIN) - EFFECTS OF PREDATOR SEX AND SIZE, AND PREY SIZE

Laboratory experiments assessed functional responses (prey consumption-predator-1 as a function of prey density) of male and female blue crabs Callinectes sapidus Rathbun to six densities of juvenile American oysters Crassostrea virginica (Gmelin) as a function of predator sex and size, and prey size. Crusher chela heights of large male and female crabs (135-165 mm carapace width; CW) were compared to identify sex-related differences in cheliped morphology that might account for the different functional responses. Initial identification of the form of the functional response (positively density-dependent, inversely densitydependent) was based on an ANOVA of the pattern of proportional mortality rates with prey density. The fit of several functional response models was compared statistically. Large female blue crabs exhibited a positively density-dependent functional response to large juvenile oysters (35 mm shell-height; SH), whereas large male crabs demonstrated an inversely density-dependent response. These different functional responses were due to specific features of both the predator and prey: (l) the higher crushing strength associated with a larger chela height to carapace width ratio of large male crabs compared to females, and (2) density-dependent prey features involving a reduction in the number of oysters attached to the outside edges of the oyster-shell cultch at low densities, which are easily crushed. Large crabs feeding on small oysters (25 mm SH) exhibited inversely density-dependent functional responses. Large juvenile oysters attained an absolute prey refuge in size from small blue crabs (60-85 mm CW) of both sexes. These results suggest that (1) sex-related differences in claw morphology can lead to different functional responses, and (2) juvenile oysters gradually attain a partial prey refuge at low densities (< 16.7 oysters · m-2) and large shell-heights (> 35 mm SH) prior to attainment of an absolute prey refuge from all crabs at ≈ 45-50 mm SH. © 1990.