EXPERIMENTAL AND HISTOLOGICAL STUDIES OF 4 LIFE-HISTORY STAGES OF THEEASTERN OYSTER, CRASSOSTREA-VIRGINICA, EXPOSED TO A CULTURED STRAIN OF THE DINOFLAGELLATE PROROCENTRUM-MINIMUM

Effects of the dinoflagellate Prorocentrum minimum (strain EXUV) upon four life-history stages of the eastern oyster-embryos, feeding larvae , newly set spat, and juveniles-were investigated in laboratory exposu re studies. Embryonic development was not affected significantly by li ving, heat-killed, or sonicated cells, or by growth-medium extracts fr om P. minimum cultures. Feeding larvae, however, showed poor growth an d poor development of the digestive system when fed P. minimum, as com pared with larvae fed Isochrysis sp. (strain T-ISO). Growth of larvae fed mixed P. minimum + Isochrysis diets was intermediate. Larvae and n ewly set spat that had been fed a diet of 1/3 P. minimum + 2/3 Isochry sis exhibited distinctive changes in digestive-system anatomy. Spat sh owed an abnormal accumulation of lipid in the stomach epithelium. Abso rptive cells in the digestive glands of both larvae and spat contained accumulation bodies, often with a laminated, fibrous appearance in pr eparations for transmission electron microscopy. These accumulation bo dies were PAS (periodic acid-Schiff) positive and may correspond to au tolysosomal bodies within P. minimum cells. Juvenile oysters developed the ability to digest P. minimum, but only after a refractory period of about 2 weeks, during which most P. minimum was filtered but reject ed as pseudofeces. The linking of accumulation bodies within absorptiv e cells of oyster digestive diverticula with dinoflagellate autolysoso mal bodies suggests a mechanism by which some dinoflagellates interfer e with, feeding in phytoplankton grazers.