ACTIVE HABITAT SELECTION BY MEGALOPAE AND JUVENILE SHORE CRABS CARCINUS-MAENAS - A LABORATORY STUDY IN AN ANNULAR FLUME

We studied megalopae (postlarvae) and young juveniles of the shore cra b (Carcinus maenas L.) in laboratory experiments to examine four poten tially important processes for juvenile distribution and recruitment: (1) hydrodynamic processes and passive deposition of megalopae, (2) ac tive habitat selection of megalopae, (3) habitat specific predation ra tes, and (4) active habitat selection by juveniles. In an annular flum e, simulating natural current velocities in nursery areas on the Swedi sh west coast, we assessed the distribution of dead megalopae, Live me galopae, live megalopae with predators (juvenile conspecifics and brow n shrimp, Crangon crangon), and first instar crabs, in four simultaneo usly presented habitats: blue mussels (Mytilus edulis), eelgrass (Zost era marina), filamentous green algae (Cladophora sp. and Chaetomorpha linum) and bare sand. In a second experiment we studied the distributi on of Live megalopae between four different ephemeral macroalgae with different structural complexity (Ulva lactuca, Enteromorpha sp., Clado phora sp. and Ectocarpus siliculosus). Dead megalopae were evenly dist ributed between the four habitats, whereas all other treatments showed significantly lower proportions of megalopae and juvenile crabs in th e sand habitat (0-2%) compared to the structurally complex habitats (2 4-40%). The distribution between mussels, eelgrass and filamentous alg ae of live megalopae in absence of predators did not differ significan tly from the hydrodynamical null hypothesis, i.e. distribution of dead megalopae. However, predation increased the proportion of megalopae s ignificantly in the filamentous algae, providing the best refuge from predation of these habitats. First instar crabs showed a significantly different distribution compared to megalopae, with higher proportion in the algal habitat, whereas juvenile predatory crabs were found in s ignificantly higher proportion among mussels. Megalopae selected all f our different macroalgae species over open sand, but a significantly l ower proportion were found in the algae with the highest structural co mplexity (Ectocarpus siliculosus; 14%) compared to the other algal spe cies (26-30%). These results indicate that passive deposition have Lit tle influence on the small scale (< 10 s of meters) distribution of sh ore crab megalopae during normal current velocities, but that active h abitat selection by megalopae is the major process responsible for the non-random distribution of megalopae and juvenile shore crabs. The re sults further suggest that the initial distribution of megalopae betwe en nursery habitats is quickly modified by habitat specific predation rates and size-specific movements and habitat choices by juveniles. Th e correlation between the habitat choice of megalopae and juvenile cra bs, and the refuge value of the examined habitats suggests that habita t specific predation rates is a major selective force behind the behav ior of active habitat selection in this species.