FEEDING RATE RESPONSES OF OYSTER LARVAE (CRASSOSTREA-VIRGINICA) TO SESTON QUANTITY AND COMPOSITION

The functional response of oyster larvae [Crassostrea vilginica (Gmelin)] to in situ food variability was examined by measuring their feeding rates for samples of seston (Chesapeake Bay, USA) that differed in both particle amount and composition. Feeding rates and patterns were compared to those of larvae fed the cultured alga Isochrysis sp. (clone T-ISO), Larval feeding patterns appeared to be consistent with a Holling's Type 3 functional response for both natural particles and T-ISO. The incipient limiting concentrations of suspended particle volume where ingestion rates became maximal were similar for natural plankton (range 20 x 10(5) to 33 x 10(5) mu m(3) . ml(-1)) and T-ISO (19 x 10(5) mu m(3) ml(-1)) and were also similar to previously reported values for other species of bivalve larvae fed cultured algae. Incipient limiting concentrations for natural suspensions corresponded to 20-100% of ambient concentrations. Larval ingestion rates for seston collected over different times and from different locations appeared to be at saturated levels (range about 90 x 10(3) to 120 x 10(3) mu m(3) . larva(-1). h(-1)) but it is not clear whether larval growth is food-limited in Chesapeake Bay, Ingestion rates for these seston samples were similar despite considerable differences in the particle size structure and plankton composition of these samples. Larval ingestion rates for seston were greater than maximal ingestion rates of T-ISO (about 28 x 10(3) mu m(3) . larva(-1) h(-1)) and higher than previously reported maximal rates for bivalve larvae fed cultured foods, suggesting that oyster larvae perceived differences in the nutritional quality of natural and cultured foods. These experiments suggest that in nature, the ingestion rates of oyster larvae appear to be most strongly regulated by food quantity (volume) and nutritional quality and not particle size or numerical abundance.