Relative importance of habitat selection and predation for the distribution of blue crab megalopae and young juveniles

Abstract

We assessed the relative role of 3 potentially important processes affecting the distribu- tion of young juvenile blue crabs Callinectes sapidus Rathbun: (1) habitat selection at settlement, (2) selection of habitats by dispersing juveniles, and (3) habitat-specific predation rates, using cage experiments. The results suggest that active habitat selection by postlarvae and young juvenile crabs determines the habitat-specific distribution of juvenile blue crabs. Densities of blue crab settlers (megalopae and first instar crabs) in caged habitat patches were high and similar in artificial sea- grass, live shoal grass Halodule wrightii, and live oyster habitats (on average 69 to 82 settlers m -2 ), but significantly lower in mud (26 settlers m -2 ), indicating active selection for any structurally com- plex habitat at settlement. Second and third instar juvenile blue crabs (J2 and J3 crabs) also colonized the structurally complex habitats in higher numbers (34 to 115 crabs m -2 ), compared to mud (14 crabs m -2 ), demonstrating that young juvenile blue crabs are highly mobile and redistribute soon after metamorphosis. Densities of J2 and J3 blue crabs in uncaged treatments were significantly lower in all habitats than in caged treatments (on average 44% loss), suggesting high predation mortality. However, the loss of settlers and juvenile crabs was similar in all habitats, and had no significant effect on the juvenile crab distribution. Densities of potential predators were on average 5 times higher in the structurally complex habitats than in mud. An aggregation of predators in the refuge habitats, coupled with a refuge at low prey densities in unstructured habitats, appears to decrease the proximate effect of predation on the distribution of juvenile crabs. Concurrent high settlement of a small xanthid crab (cf. Eurypanopeus sp.) occurred almost exclusively in the oyster habitat, sug- gesting that species with a limited ability to redistribute after metamorphosis may display a more specific selection of the adult habitat at settlement.