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 crab (Carcinus maenas L.) in laboratory experiments to examine four potentially important processes for juvenile distribution and recruitment: (1) hydrodynamic processes and passive deposition of megalopae, (2) active habitat selection of megalopae, (3) habitat specific predation rates, and (4) active habitat selection by juveniles. In an annular flume, simulating natural current velocities in nursery areas on the Swedish west coast, we assessed the distribution of dead megalopae, Live megalopae, live megalopae with predators (juvenile conspecifics and brown shrimp, Crangon crangon), and first instar crabs, in four simultaneously presented habitats: blue mussels (Mytilus edulis), eelgrass (Zostera marina), filamentous green algae (Cladophora sp. and Chaetomorpha linum) and bare sand. In a second experiment we studied the distribution of Live megalopae between four different ephemeral macroalgae with different structural complexity (Ulva lactuca, Enteromorpha sp., Cladophora sp. and Ectocarpus siliculosus). Dead megalopae were evenly distributed between the four habitats, whereas all other treatments showed significantly lower proportions of megalopae and juvenile crabs in the sand habitat (0-2%) compared to the structurally complex habitats (24-40%). The distribution between mussels, eelgrass and filamentous algae of live megalopae in absence of predators did not differ significantly from the hydrodynamical null hypothesis, i.e. distribution of dead megalopae. However, predation increased the proportion of megalopae significantly 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 significantly higher proportion among mussels. Megalopae selected all four different macroalgae species over open sand, but a significantly lower proportion were found in the algae with the highest structural complexity (Ectocarpus siliculosus; 14%) compared to the other algal species (26-30%). These results indicate that passive deposition have Little influence on the small scale (< 10 s of meters) distribution of shore crab megalopae during normal current velocities, but that active habitat selection by megalopae is the major process responsible for the non-random distribution of megalopae and juvenile shore crabs. The results further suggest that the initial distribution of megalopae between nursery habitats is quickly modified by habitat specific predation rates and size-specific movements and habitat choices by juveniles. The correlation between the habitat choice of megalopae and juvenile crabs, and the refuge value of the examined habitats suggests that habitat specific predation rates is a major selective force behind the behavior of active habitat selection in this species.

Subject headings

NATURVETENSKAP  -- Biologi -- Ekologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Ecology (hsv//eng)

Keyword

Carcinus maenas; hydrodynamics; predation; passive deposition

post-settlement movements

lobsters homarus-americanus; mobile epibenthic fauna; zostera-marina

dungeness crab; hydrodynamical processes; intertidal barnacle

callinectes-sapidus; swimming behavior; cancer-magister; panulirus-argus

Publication and Content Type

ref (subject category)

art (subject category)