Cannibalism in Dungeness crab Cancer magister:effects of predator-prey size ratio, density, and habitat type

Abstract

Cannibalism between age classes has been reported for Dungeness crab Cancer magister Dana, and hypothesized to cause the cyclic behavior of the fishery. Recently, cannibalism among young-of-the-year (YOY) Dungeness crabs has been documented, and proposed as an alternative hypothesis to explain recruitment failure of later cohorts of megalopae in intertidal shell habitats already occupied by older YOY crabs. Furthermore, cannibalism among juveniles has also recently been reported for other crab species. This study experimentally addressed whether: (1) cannibalism rate by age classes 0+, 1+ and 2+ of Dungeness crabs Cancer magister of both sexes on YOY conspecifics is affected by habitat type; (2) cannibalism rate among YOY is affected by conspecific density and prey-predator asymmetry; and (3) YOY mutual interference and prey-predator asymmetry affect consumption rate. Predation rate by older crabs (age classes 1 and 2) on YOY was highest in mud habitat, intermediate in low shell density, and lowest in the high shell density treatment. These results show that shell habitat enhances production of YOY crâbs by increasing survival. Despite the fact that shell habitat decreased cannibalism rate of larger conspecifics on YOY, it did not affect cannibalism rate among juveniles. YOY crabs did not consume carapaces of conspecifics, while larger crabs consumed entire prey; this difference in amount of hard parts consumed suggests that although cannibalism may be common among YOY, it may not be detected by stomach analysis. Consumption of megalopae was affected by prey density, and also by the asymmetry between the prey and the predator. As predator density increased, the total consumption of megalopae increased, but proportional mortality per predator decreased. When other instar stages were used as prey, prey-predator asymmetry and prey density were not as important factors as predator density on cannibalism rate. Megalopae were the most vulnerable prey, and under certain circumstances YOY conspecifics may be able to decimate cohorts of settling megalopae. However, megalopae may find refuge from cannibalism when predator-prey size ratio is low, predator density is high, and probably when relative abundance of alternative prey is high (laboratory experiments were conducted using only conspecifics as prey). Cannibalism, or other 'processes that may have evolved in response to cannibalism, may be important factors regulating crab recruitment in natural populations and limiting crab production in artificial habitats.