Predation by Rock Crabs (Cancer irroratus) on Diseased Sea Urchins (Strongylocentrotus droebachiensis) in Nova Scotia

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Abstract
Predation of morbid sea urchins (Strongylocentrotus droebachiensis) by rock crabs (Cancer irroratus) and other predators was observed using SCUBA during an outbreak of disease in southwestern Nova Scotia in August 1983. Disease increases susceptibility of sea urchins to predation by precluding natural defensive behaviors including spine projection, strong attachment to the substratum, and aggregation. In laboratory feeding experiments, rock crabs preferred diseased or narcotized sea urchins over healthy ones and fed upon them at a much higher rate than crabs given only healthy sea urchins as prey. Rock crabs showed no significant preference between diseased and narcotized sea urchins. Rock crabs clearly preferred mussels over healthy or diseased sea urchins. Although rock crabs do not appear to be important predators of healthy sea urchins, from field and laboratory findings I conclude that they contribute to mass mortality of sea urchins by preying upon morbid individuals during outbreaks of disease.