Predation Risk, Prey Abundance, and the Vertical Distribution of Three Brachyuran Crabs on Gulf of Maine Shores

Abstract

Three large brachyuran species are common in the intertidal and shallow subtidal of New England rocky shores: two native crabs Cancer borealis (Jonah crab) and Cancer irroratus (rock crab), and the introduced crab Carcinus maenas (European green crab). For these three co-occurring species in the Isles of Shoals (Gulf of Maine, USA), we compared distribution and abundance to survivorship and prey availability along a depth gradient and examined stomach contents and prey preference. The three species show differences in vertical distribution: Carcinus is more abundant in the intertidal, while both species of Cancer are more abundant in the subtidal. Survivorship of both species of Cancer increases with increasing depth, while survivorship of Carcinus decreases with increasing depth, perhaps corresponding to differential vulnerability to predation by gulls in the intertidal and by decapods and fish in the subtidal. There were notable differences in laboratory prey preference experiments: C. irroratus consumed both small mobile and non-mobile prey (amphipods, small snails, and small mussels), while Carcinus consumed primarily small mobile prey (amphipods and isopods). In contrast, C. borealis consumed larger, heavier bodied prey (larger snails and mussels) but did not eat amphipods or isopods. However, differences in prey preference among crab species were greater than the differences in realized diets. Based on stomach content analysis, the blue mussel Mytilus edulis was the majority component of stomach contents for all three species. Some differences were evident in the remaining diet components: Carcinus was the most omnivorous (. 30% green algae), C. borealis consumed more snails and arthropods, and C. irroratus consumed the most mussels. Overall, species distribution does not track the distribution of the preferred prey of each species; rather, the distribution corresponds with patterns of survivorship, indicating predominant top-down control of crab distribution.