PREDATION ON POSTLARVAE AND JUVENILES OF THE SHORE CRAB CARCINUS-MAENAS - IMPORTANCE OF SHELTER, SIZE AND CANNIBALISM

Settlement and early juvenile stages are considered a bottleneck in th e Life history of many epibenthic organisms because of high predation mortality. Nursery habitats may play an important role in mitigating s ettlement and post-settlement mortality by providing refuge from preda tion. We examined these relationships in postlarvae and early juvenile stages of the shore crab Carcinus maenas L. in laboratory and field t ethering experiments. We studied habitat and size related habitat mort ality using postlarvae and young juvenile crabs as prey, and various p redators, including juvenile conspecifics, in several habitats common in shallow (0 to 1 m) soft bottom nursery areas on the Swedish west co ast. Settling mortality was high in open sand (80 to 90%), whereas a s ignificant habitat refuge was obtained in mussel beds, eelgrass and fi lamentous green algae, the latter yielding the lowest mortality (13 to 14%). Small differences in structural complexity of ephemeral macroal gae dramatically affected predation mortality of first instar crabs, w ith a significant refuge obtained only in algae of medium complexity. Predation rate on tethered crabs in the field was high (52 to 67%) onl y on the smallest crabs (<5 mm carapace width, CW), which obtained a s ignificant refuge in the eelgrass habitat compared to open sand. Morta lity for larger crabs (5 to 25 mm CW) was low (<10%) and similar in sa nd and eelgrass habitats. Our results indicate that predation is an im portant process that can create a bottleneck for juvenile shore crab p opulations during settlement and early juvenile stages, mediated by th e availability of nursery habitats. Postlarvae obtained refuge from pr edation in several different habitats, suggesting that the recruitment of juvenile shore crabs will be less affected by temporal and spatial variation of any single habitat type. The strong size refuge for crab s larger than 4 mm CW indicates that key predators are small. We sugge st that cannibalistic juveniles, which caused predation rates similar to or higher than all other investigated predators, are dominant preda tors on settling postlarvae and young juvenile crabs in nursery areas. We further propose that habitat- and size-specific predation by small epibenthic predators are an important selective force in habitat sele ction by postlarvae and ontogenetic shifts in habitat use by juveniles .