Late stage bivalve larvae in a well-mixed estuary are not inert particles

Depth-specific sampling at a single location was used to examine the vertical distribution of pediveliger larvae of bivalve mollusks in the York River, an estuary of Chesapeake Bay, Virginia. The water column at the sampling site was usually well mixed, lacking consistent temperature or salinity gradients for larvae. Four species showed strong vertical stratification when collected simultaneously at three depths. Pediveliger stage larvae of a clam (Cyrtopleura costata), an oyster (Crassostrea virginica), and a shipworm (Bankia gouldi) were most abundant near the benthos, and least abundant near the surface. A mussel (Geukensia demissa), showed the reverse trend, with most pediveliger larvae near the surface. Tidal stage had a slight effect on two species (C. costata and C virginica), but only to increase the relative abundance of larvae near the benthos during flood tide. Otherwise, neither tidal phase nor light levels (night versus day) had detectable effects on distribution patterns. Sampling very close to the sediment-water interface provided no evidence that pediveliger larvae spent a significant proportion of their time in this location. While the behavior of pre-competent bivalve larvae may tend to retain them within an estuary, that of competent-to-metamorphose pediveliger larvae does not appear to have that function. Pediveliger larvae may regulate their depth to best locate potential settlement substrate. Such an hypothesis is consistent with adult habitat zones of at least some of these species in Chesapeake Bay. Removing competent-to-settle larvae from analyses of larval distributions in estuaries will enhance or clarify depth distribution patterns observed for earlier-stage larvae.