Mechanisms governing ontogenetic habitat shifts: role of trade-offs, predation, and cannibalism for the blue crab

Nursery habitats play a major role in the population dynamics of marine and estuarine species, with the blue crab Callinectes sapidus serving as a model invertebrate. The current paradigm of blue crab habitat use postulates that juvenile survival decreases with size in submerged aquatic vegetation (SAV) due to a reduction in suitably scaled refuge, triggering an ontogenetic shift from SAV to unvegetated habitats. However, alternative mechanisms for this habitat shift have not been examined. We evaluated the paradigm of blue crab habitat use by conducting field tethering experiments in York River (Virginia, USA) nursery habitats using a broad range of juvenile size and SAV cover. Cameras were deployed to identify key predators of juvenile blue crabs and to assess the relative importance of predation and cannibalism as sources of juvenile mortality. Probability of survival increased significantly and additively with crab size and SAV cover. The absence of an interaction between crab size and SAV cover is inconsistent with the current paradigm. Rather, the ontogenetic habitat shift by juvenile blue crabs is likely driven by a density-dependent trade-off between predation risk and foraging efficiency. In images of predation events, adult blue crabs, northern puffers Sphoeroides maculatus, striped burrfish Chilomycterus schoepfi, and oyster toadfish Opsanus tau were identified as predators of juveniles in sea-grass beds and sand flats. The high frequency of successful predation events by adult blue crabs suggests that cannibalism is an important source of juvenile mortality and may be as, or more influential, to blue crab population dynamics than finfish predation.