The green crab Carcinus maenas invaded the Gulf of Maine over a hundred years ago and has become a well-established member of coastal ecosystems. Carcinus is most abundant in the intertidal zone despite higher availability of invertebrate prey in the nearby shallow subtidal. A previous study revealed a higher mortality rate for Carcinus in the subtidal than in the intertidal zone. Two other decapods, the American lobster Homarus americanus and Jonah crab Cancer borealis, have their highest abundance in the shallow subtidal. These three species have similar diets and predation among and within species occurs. We performed a laboratory experiment to investigate the effects of H. americanus, C. borealis, and larger conspecifics on mortality and behavior of Carcinus. Homarus americanus was the most effective predator on Carcinus, consuming 27% within 24 h. In contrast, only 5% of Carcinus were eaten by C. borealis and none by conspecifics. The presence of American lobsters had dramatic effects on Carcinus behaviors: hiding and climbing increased while walking and standing on the open substrate decreased. In contrast, Carcinus showed no significant changes of behavior in the presence of C. borealis or larger conspecifics. The consumptive and non-consumptive (behavioral) effects of lobsters on Carcinus appear to be one major cause of the scarcity of Carcinus in subtidal communities. These effects are also likely to reduce Carcinus predation on the small invertebrates that are common prey for all the large decapods in this ecosystem.
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Vol. 29 • No. 3