The influence of body size on foraging facilitation and kleptoparasitic behavior in the green crab (Carcinus maenas)

Green crabs (Carcinus maenas) and dogwhelks (Nucella lapillus) frequently co-occur in intertidal communities and are both intensive predators of blue mussels (Mytilus edulis). Kleptoparasitic interactions, whereby crabs take prey from whelks, have been documented, although results from previous experiments are not consistent with respect to the presence and frequency of such interactions, perhaps because of the sizes of crabs used. We conducted a lab experiment to clarify the effects of interactions among crabs and whelks on consumption of blue mussels. Crab density, size, and whelk presence were manipulated, and number of mussels and mussel sizes consumed by crabs were assessed during a series of 10-day trials. In the absence of whelks, two small crabs foraging together consumed no more prey than a single crab, suggesting substantial inhibition of feeding through interference competition or other negative intraspecific interactions. Presence of whelks led to increased consumption, either through a reduction of this interference or other stimulatory mechanisms. Such effects were less apparent for large crabs and small crabs feeding without conspecifics. We found clear evidence of kleptoparasitism in all crab treatments, though this feeding mode only contributed substantially to the diet of small crabs in the presence of conspecifics. Small crabs feeding in pairs kleptoparasitized larger mussels than they consumed independently, allowing them to exploit prey larger than they usually would. Our size-specific results highlight the importance of considering predator size when studying such interactions.