No abstract

In order to better understand the particle diet of planktotrophic larvae of the eastern oyster Crassostrea virginica (Gmelin) we measured their ingest~on of naturally occurring food organisms. By using a dual radioisotope (3H and I4C) label~ng technique in conjunction w~t h 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 I1 oyster larvae cleared 0.2 to 30 pm I4C-labeled plankton (primarily phototrophs) at a rate of 0.0825 n~l larva-' h-', and 0.2 to 30 pm 3H-labeled plankton (heterotrophlc bacteria and phagotrophic protozoans) at a rate of 0.0017 m1 larva-' h-' This calculated clearance rate for 0.2 to 30 pm heterotrophs was low due to the predominance of small (0.2 to 0.8 pm), poorly retained bacteria in thls size class. Oyster larvae consumed a wide size range of food part~cles (0 2-0.8 pm to 20-30 pm) and selectively ingested 20 to 30 pm organisms. In other feeding experiments, oyster larvae cleared laboratory cultured heterotrophic flagellates (12 pm) at a rate of 0.0640 m1 larva-' h-' and cultured heterotrophic ciliates (12 X 20 pm) at a rate of 0.1093 m1 larva-! h" The inclusion of heterotrophic food organisms In the diet of C. virginlca 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 b e a reliable measure of food supply

We examined the feeding activity of planktotrophic larvae of the eastern oyster Crassostrea virginica on different size particles suspended in surface waters of 2 subestuaries of Chesapeake Bay, USA. Particle suspensions were characterized uslng particle counters and microscopic observations. At both sites, particle assemblages were dominated (in terms of particle number and volume) by particles (predominantly plankton cells) with equivalent spherical diameters <5 pm. Feeding experiments demonstrated that small (200 pm) oyster larvae typically ingested particles between 0.5 and 12 pm. However. in the presence of blooms of large (10 to 30 pm) dinoflagellates, small and large larvae extended their maximum ingested particle size to about 16 and 30 pm, respectively. These results indicate that oyster larvae generally exploit foods within the characteristic particle biomass peak present in these estuaries and opportunistically feed upon periodic blooms of large plankton cells. Larvae derived a large percentage (range 20 to 90%) of the total ingested particle volume from picoplankton-size (here defined as 0.5 to 3 pm) particles. These small particles constituted a larger percentage of the material ingested by small larvae than that ingested by large larvae. By comparison, both sizes of larvae derived about 2 to 4 0 % and about 5 to 20% of total ingested food volume from 3 to 5 pm and 5 to 10 pm particles, respectively. In experiments with abundant dinoflagellates > l 0 pm in size, both sizes of larvae derived 20 to 30% of their total ingested volume from these particles.