Following larval development in coastal waters, postlarvae (megalopae) of the blue crab Callinectes sapidus Rathbun enter inlets and, against the net seaward flow of estuarine waters, move upstream to juvenile habitats. Abundance and vertical distribution of blue crab megalopae in the York River, Virginia, USA, a subestuary of Chesapeake Bay, was quantified to examine the hypothesis that megalopae are not transported simply as passive particles, but display behaviors that augment their immigration. Megalopal abundance and depth distribution and environmental variables were measured at shallow (3 to 4 m) sites in 1988 and 1989 and at a deep (10 m) site in 1990. Megalopae were more abundant in the water column during flood than ebb, indicating a net upstream flux of megalopae. Densities were always low during ebb. Highest densities occurred during night flood tides, when megalopae were aggregated near the surface. During day flood tides, megalopae were generally less abundant and distributed deeper in the water column. Occasionally, however, they were abundant during day flood, concentrated near the bottom in deep water. At shallow sites, megalopae were never abundant during day, apparently not ascending into well-lit waters. Abundance and depth distribution of megalopae were not affected by current speed, wind speed, water temperature or salinity. A conceptual model of vertical migration of estuarine blue crab megalopae is presented. Megalopae which behave according to this model should lower their susceptibility to predation in the water column by selectively utilizing flood currents to rapidly reach juvenlle nursery grounds while avoiding well-lit waters.

We quantified (1) spatio-temporal variation in molt stage (developmental state) of blue crab Callinectes sapidus megalopae (postlarvae), and (2) the relationship between planktonic availability, molt stage and settlement of megalopae during peak settlement in Chesapeake Bay, USA. Settlement was significantly correlated with the planktonic availability of megalopae. Developmental state of megalopae also appeared influential in settlement because (1) blue crab megalopae displayed quantifiable changes in molt stage; (2) molt stage of megalopae varied on a temporal scale of days to 1 mo or more, and a spatial scale of kilometers, apparently reflecting the physiological progression through the molt cycle by megalopae pulsing through settlement habitats; and (3) molt stage of megalopae advanced in collections from the plankton, on artificial settlement habitats, and in the benthos, indicating the approach to settlement, metamorphosis and a benthic existence. In addition, settlement was not necessarily obligatory untd sometime in late premolt, as many megalopae collected in the plankton were in late premolt. Hence, megalopae have the time in which to exercise some degree of habitat selection. Our findings show that settlement of a benthic invertebrate is regulated partly by the planktonic availability of postlarvae, and that these postlarvae progress through a predictable series of changes in developmental state as they invade settlement habitats. Thus, a quantitative measure of postlarval developmental state appears useful in relating planktonic availability of postlarvae to natural settlement patterns. Similar alterations in developmental state occur in the larval and postlarval stages of marine fish and a wide range of invertebrate taxa, indicating the potentially broad applicability of quantitative measures of developmental state in studies of settlement.