Effects of temperature, body size, and chela loss on competition for a limited food resource between indigenous rock crab (Cancer irroratus Say) and recently introduced green crab (Carcinus maenas L.)

The green crab, Carcinus maenas, is one of the most successful marine invasive species worldwide. The recent (2007) discovery and rapid explosion of populations of green crab in predominantly cold water ecosystems of Newfoundland and Labrador (NL) provided an opportunity to investigate interactions with a potential, indigenous competitor, the rock crab, Cancer irroratus, under presumably adverse thermal conditions for green crab. We used a laboratory microcosm experiment with rock crab and green crab from NL to determine how changes in temperature (4°C and 12°C), body size (small, medium, and large), and the loss of one chela affect the ability of one rock crab to compete with one large, intact green crab for one blue mussel, Mytilus edulis. Green crab was first to grasp the mussel in >90% of trials, regardless of temperature. In cold water, large, intact rock crab held the mussel as frequently as green crab and 25- and 12-fold more frequently than small, intact rock crab and large rock crab missing one chela, respectively. Changes in temperature and relative body size did not affect the number of contests between species, although strong physical interactions occurred ~20% more often in contests with large than small rock crab. Large, intact rock crab initiated contests with green crab more frequently than smaller conspecifics in warm water only, while winning 5-fold more contests than small rock crab, regardless of temperature. The ability of large rock crab to win contests was not affected by chela loss. Small and medium rock crab spent at least threefold more time buried in sediments or inside a shelter than large crab, regardless of chela loss. This study suggests that the recent introduction of green crab to NL reduces foraging in rock crab, especially in competitive interactions between green crab and smaller rock crab, and that this effect is exacerbated during warmer times of year. ► We studied competition between rock crab and green crab for one blue mussel. ► Effects of water temperature, rock crab size, and chela loss were tested. ► Green crab primarily grasped the mussel before rock crab in all treatments. ► Body size, but not chela loss, affected the ability of rock crab to win contests. ► Low temperature limits the ability of green crab to compete with rock crab.