Abundance and vertical distribution of drifting, post-larval Macoma spp. (Bivalvia : Tellinidae) in the York River, Virginia, USA

We sampled the early drifting post-larvae of a complex of 2 species of tell inid bivalves, Macoma spp., at a station in the lower York River, Chesapeak e Bay, USA. Plankton samples were collected by pump every 3 h from 3 depths (surface, mid-depth, and bottom) on 4 dates corresponding to full and new moons. Macoma spp. post-larvae (size range 400 to 500 mu m) were abundant i n the plank ton throughout the sampling period. The environmental factors i nfluencing the abundance and vertical distribution of drifting post-larvae were evaluated using linear and logistic regression. Post-larvae were alway s more abundant during night as compared to day and were more abundant duri ng nocturnal, flooding tides than during ebbing tides. In general, they wer e closer to the surface at night and during flood tides, though these patte rns were highly variable. These data indicate that drifting post-larval biv alves use 'selective tidal stream transport' to promote upstream dispersal as observed in the postlarvae of other estuarine taxa (e.g, crabs and fish) . The post-larval stage generally re-invades juvenile habitats following th e export of larvae to the mouth of the parent estuary or nearshore continen tal shelf. We suggest that small drifting post-larval bivalves exert behavi oral control over suspension in the water column. This life-history stage s erves to maintain high densities of juveniles and adults in the upstream po rtions of the York River estuary despite downstream transport of early larv al stages.