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In contrast to general settlement models of invertebrate larvae, recent Swedish studies of settlement of the shore crab Carcinus maenas indicate that many postlarvae close to metamorphosis emigrate from their preferred settlement habitats. In the present study, we assessed physical and biological factors affecting shore crab settlement and emigration from nursery habitats, and explored possible ultimate factors in this behavior. Postlarval abundance in the field was not affected by the tidal phase, but varied significantly according to the light conditions. Settlement densities on artificial substrates were significantly higher during the day compared to at night. In contrast, densities of planktonic postlarvae were highest at night. This result suggests that the postlarvae cling to shallow benthic habitats during the day and swim in the plankton at night. Mesocosm experiments demonstrated that most postlarvae remained in the benthos during the day, even in suboptimal habitats, but that a large proportion (on average 58%) actively emigrated at dusk, irrespective of settlement conditions. Emigration rates were significantly higher and metamorphosis rates lower in open sand compared to mussel habitats, whereas food levels and settlement densities had only small effects on the settlement process. Emigrated postlarvae were at a significantly earlier developmental stage and metamorphosed on average 1 d later than the megalopae that remained in the benthic habitat. Post-larvae >2 d from metamorphosis emigrated even from optimal settlement conditions. These results suggest that settlement in shore crabs is not an irreversible process once a late-stage postlarva has found a settlement habitat, but involves 2 separate behavioral components: (1) the selection of a benthic habitat during the day, which is mainly affected by the availability of refuges from predation; and (2) the choice to stay or emigrate the following night, where emigration is induced by a decrease in light and determined mainly by the development stage of the postlarvae. We propose a conceptual model of settlement in the shore crab where the dynamic behavior of the postlarvae is an adaptation to diel differences in predation risk between the settlement habitat and the plankton.

Settlement behavior in shore crabs Carcinus maenas: why do postlarvae emigrate from nursery habitats?