The role of predation by the red rock crab, Cancer productus, on the invasive European green crab, Carcinus maenas, in Yaquina Bay, Oregon

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In Yaquina Bay, Oregon, I observed very little overlap in the distribution of theinvasive green crab, Carcinus maenas, and the larger red rock crab, Cancer productus.Red rock crabs dominate the more saline, cooler lower estuary and green crabs, the less saline, warmer upper estuary. Because caged green crabs survive well in the lowerestuary, I decided to test the hypothesis that red rock crabs prey on green crabs and thus contribute to their exclusion from the more physically benign lower estuary. A laboratory species interaction experiment was designed to determine whether red rock crabs prey on smaller green crabs at a higher rate than on smaller crabs of theirown species. Crabs of both species were collected and sorted by weight into three size classes: small, medium and large. Small and medium crabs of both species were pairedwith green crabs or red rock crabs of various sizes. Crab pairs were housed in individual arenas and allowed to interact for seven days. When conspecifics were paired, mortality was less than 15 %, even in the presence of larger crabs. Smaller red rock crabs survived well in the presence of larger green crabs, but the reverse was not true. When small green crabs (60-67 mm carapace width) were matched with medium and large red rock crabs, their mortality increased to 52% and 76% respectively. A less dramatic pattern was observed for medium green crabs (73-80 mm) in the presence of medium and large C. productus. Thus on the West Coast of North America, the more aggressive red rock crab, Cancer productus, has the potential to reduce the abundance of Carcinus maenas in the more saline and cooler lower estuaries.