Abstract
The invasive Asian shore crab (Hemigrapsus sanguineus) has rapidly expanded its range and become the dominant intertidal crab species in southern New England, but little is known about the factors that regulate its population size. To estimate the role of cannibalism as a potential factor limiting population growth, this study investigated potential cannibalism rates in H. sanguineus in the laboratory, and estimated the impact of cannibalism by adults on H. sanguineus recruitment in the field. In the laboratory, cannibalism rates were measured for various size class combinations of conspecific predator and prey crabs in the presence and absence of an alternate food item. Predators were categorized based on carapace width (CW): adult (12-15 mm), large juvenile (7-10 mm), and small juvenile (3-6 mm). The prey size classes consisted of megalopae (final larval stage that recruits and settles in the intertidal zone; ~1.5 mm), stage 1 crabs (the initial juvenile stage; ~1.6 mm), and small juveniles (3-6 mm). Additionally, cannibalism rates were compared for prey that had previously experienced a predator cue versus those that had not, for high versus low prey densities, and for laboratory-reared versus wild-caught prey. Adults cannibalized all prey smaller than 6 mm CW, and large and small juveniles cannibalized all smaller size classes, with and without food present. Cannibalism rates were influenced by the interaction between predator size, prey size, and food availability, and by the size difference predator and prey. Cannibalism rates increased with increasing predator size, decreasing prey size, and increasing predator-prey size difference. Cannibalism rates were highest when adults preyed on megalopae. The availability of an alternate food item was not as influential as crab sizes in affecting cannibalism rates. Cannibalism rates were not significantly affected by prey experience, density, or origin (lab-reared vs. wild-caught). Based on these results, a field caging study investigated the impact of cannibalism by adults on the recruitment of megalopae and juveniles to the intertidal zone. Cage treatments consisted of: No Predator cages (contained no adult), Predator cages (contained an adult that could freely move throughout the cage), and Barrier cages (contained an adult confined to one side of the cage). The number of H. sanguineus megalopae and juveniles (greater than or equal to 10 mm CW) that recruited to each cage and to each side of each Barrier cage was measured after 10 days of deployment. Recruitment was not different between the Predator and No Predator cages, but was significantly higher in the Barrier cages in which almost all of the recruits were found on the side without the predator. These results suggest that H. sanguineus adults and juveniles cannibalize smaller conspecifics regardless of food availability, cannibalism rates are strongly influenced by interacting crab sizes, and the cannibalism rates are particularly high between adults and megalopae in this species. If cannibalism occurs similarly in nature, then cannibalism of recruits to the intertidal zone could limit their population growth in the invaded range..