OMNIVOROUS FEEDING BY PLANKTOTROPHIC LARVAE OF THE EASTERN OYSTER CRASSOSTREA-VIRGINICA

In order to better understand the particle diet of planktotrophic larvae of the eastern oyster Crassostrea virginica (Cmelin) we measured their ingestion of naturally occurring food organims. By using a dual radioisotope (H-3 and C-14) labeling technique in conjunction with plankton size fractionation procedures we demonstrate that oyster larvae feed upon bacteria, phagotrophic protozoans and phototrophs present in the diverse summer plankton assemblages of Chesapeake Bay, USA. Prodissoconch II oyster larvae cleared 0.2 to 30-mu-m C-14-labeled plankton (primarily phototrophs) at a rate of 0.0825 ml larva 1 h-1, and 0.2 to 30-mu-m H-3-labeled plankton (heterotrophic bacteria and phagotrophic protozoans) at a rate of 0.0017 ml larva-1 h-1. This calculated clearance rate for 0.2 to 30-mu-m heterotrophs was low due to the predominance of small (0.2 to 0.8-mu-m), poorly retained bacteria in this size class. Oyster larvae consumed a wide size range of food particles (0.2-0.8-mu-m to 20-30-mu-m) and selectively ingested 20 to 30-mu-m organisms. In other feeding experiments, oyster larvae cleared laboratory cultured heterotrophic flagellates (12-mu-m) at a rate of 0.0640 ml larva-1 h-1 and cultured heterotrophic ciliates (12 x 20-mu-m) at a rate of 0.1093 ml larva-1 h-1. The inclusion of heterotrophic food organisms in the diet of C. virginica may enhance its growth and development by providing energy and nutrients that supplement those of ingested phytoplankton. We suggest that because oyster larvae ingest non-phytoplankton cells, estimates of standing stocks of phytoplankton may not always be a reliable measure of food supply.