Habitat selection and predator avoidance: ontogenetic shifts in habitat use by the Jonah crab Cancer borealis (Stimpson)

Abstract

Ontogenetic changes in behaviour and habitat use of Jonah crabs Cancer borealis Stimpson were examined as part of a study on habitat competition between Jonah crabs and American lobsters Homarus americanus Milne-Edwards. Crabs, lobsters and predatory fish ( Tautoga onitis Linnaeus) were placed in large circular tanks, each containing three “habitats”: cobble, featureless sand, and sand with shelter boxes. Two treatments of shelter availability were used: one shelter for every two decapods (limited) or two shelters per decapod (excess). Three size-classes of crabs (encompassing 65–158 mm carapace width) were tested with lobsters 49–59 mm carapace length. When excess shelter was available, crabs and lobsters occupied shelters significantly more frequently than the other habitats. When shelter availability was limited, crabs were displaced to other areas. The alternative habitat selected by crabs and the way in which they used it depended on crab size. Most small and medium crabs chose cobble, while large crabs were found equally in cobble and sand. Small crabs burrowed more deeply than large crabs, and burrowed more deeply in sand than in cobble. Interspecific aggressive interactions increased in intensity with crab size. T. onitis readily kill and consume Jonah crabs in tanks with no substratum or shelter. Based on the literature and my qualitative observations, I predicted that smaller crabs would be more vulnerable to predation than large crabs. However in my habitat choice experiments, crab mortality due to predation was low and was not related to crab size or habitat type. I hypothesize that size-dependent patterns of crab habitat use and behavior compensated for the greater vulnerability of smaller crabs.