Immigration of blue crab (Callinectes sapidus) megalopae in the York River, Virginia: Patterns and processes

Following larval development in coastal waters, the postlarval stage, the megalopa, of the blue crab enters estuaries and colonizes juvenile/adult habitats. I examined the dynamics of the immigration of blue crab megalopae in the York River, Virginia. Plankton samples were collected nightly during flood tide in the York River Jul-Nov 1987, 1988 and 1989 to quantify variation in abundance of blue crab megalopae during the recruitment season. The abundance of megalopae was highly episodic, and significantly correlated with wind stress to the west (1987, 1988 and 1989), wind stress to the north (1989), and observed tidal range at Gloucester Point (1987 and 1988). These factors accounted for 22-56% of the variation in megalopal abundance. Megalopal abundance was not correlated with current speed, water temperature, salinity, or Chesapeake Bay subtidal volume. While in the estuary, megalopae vertically migrated in response to light and tide. Blue crab megalopae were more abundant during flood than ebb, indicating that megalopae selectively occupy flood waters. Ascent of megalopae into the water column appeared to be light limited. at night, megalopae were concentrated at the surface; during day, they were concentrated near the bottom in deep water, but apparently did not ascend from the bottom in shallow water. Thus, megalopae utilize flood currents for transport up the estuary while reducing the risk of predation to visual predators. Spatial patterns of abundance of megalopal and juvenile blue crabs were not consistent across habitats (plankton, grassbeds, or artificial settlement substrates) or time (days, months). Densities of megalopae and first-stage juveniles in grass beds correlated with megalopal abundance in the plankton; settlement on artificial substrates did not. Total juvenile abundance exhibited lower spatial and temporal variability in grass beds than that of megalopae or first-stage juveniles, suggesting that high variability in settlement is tempered by post-settlement processes. In laboratory studies, sand shrimp, Crangon septemspinosa, consumed &\u3e&99% of available megalopae, while grass shrimp, Palaemonetes pugio, consumed 37% of megalopae in the first trial and 5% in the second trial. Predation by grass shrimp was not related to the rate of metamorphosis of megalopae, but may have been related to water temperature