Temperature mediates non-competitive foraging in indigenous rock (Cancer irroratus Say) and recently introduced green (Carcinus maenas L.) crabs from Newfoundland and Labrador

Abstract

The green crab, Carcinus maenas, was recently (2007) introduced to predominantly cold water ecosystems of Newfoundland and Labrador (NL). The known broad diet of the green crab in warmer ecosystems of the northwest Atlantic raised concerns about its ability to interfere with species indigenous to NL, especially the rock crab, Cancer irroratus, which exhibits common preference with the green crab for the blue mussel, Mytilus edulis. We used two experiments to determine the effects of low water temperature representative of southern NL (4°C and 12°C), body size (small, medium, and large), chela loss, and chemical cues from live and dead conspecifics and heterospecifics on capture and selection of blue mussels in six size classes (10 to 40mm in shell length) and five associated behaviors (handling, feeding, sitting, moving, and burying in sediments or using an artificial cavity) in rock and green crabs from NL held individually in microcosm tanks. Experiment 1, which addressed the effects of changes in water temperature, body size, and chela loss on foraging in both rock and green crabs, indicated that mussel capture was nearly three times higher in large than small individuals in both species. Large rock and green crabs captured between 43 and 46% of the largest (>30mm) mussels, while small rock and green crabs captured 25 and 76% of the smallest (<20mm) mussels, respectively. Chela loss caused a twofold decrease in mussel capture in rock crabs, though did not affect green crabs. Experiment 2, which addressed the effects of changes in water temperature and chemical cues on foraging in rock crabs, showed that cues from rock crabs (live and dead) and green crabs (live) had no effect on mussel capture, while causing only subtle changes in mussel size selection. However, small rock crabs buried in sediments or were located within the artificial cavity twice as often in the presence of cues from dead rock crabs than in the absence of cues. Overall, the crabs captured approximately three times more mussels at 12°C than 4°C regardless of crab species, crab size, chela loss, and the presence or absence of chemical cues, which therefore demonstrate the pervasive effect of low temperature on foraging in both rock and green crabs. We conclude that the recent introduction of the green crab to Newfoundland and Labrador may have a greater impact on foraging in small than large rock crabs and those missing one chela, while altering mussel populations through marked preference by small green crabs for small mussels.