DEVELOPMENT OF DISEASE CAUSED BY THE PARASITE, PERKINSUS-MARINUS AND DEFENSE-RELATED HEMOLYMPH FACTORS IN 3 POPULATIONS OF OYSTERS FROM THECHESAPEAKE BAY, USA

The development of infection caused by the protozoan parasite, Perkins us marinus (Dermo) and some specific potential defense-related cellula r and humoral components in oysters collected from three geographic ar eas, Deepwater Shoal of James River (DW), Wachapreague (WP), and Mobja ck Bay (MJ) were examined over time. Oysters were maintained in estuar ine water (salinity = 20 ppt) or in water at a salinity similar to the ambient salinity of the collection sites. Oysters were sampled at the initiation of the experiment (day 0), day 35, and day 100 to determin e defense-related parameters and disease prevalence and intensity. All populations experienced a significant increase in P. marinus infectio n prevalence and intensity from the initiation of the experiment to th e termination of the study. Oyster mortality differed between oyster p opulations. None of the DW oysters perished while cumulative mortaliti es for WP at 32 ppt and 20 ppt and MJ oysters were respectively, 23, 2 5, and 35%. The experimental oyster populations demonstrated significa nt differences with respect to cellular and humoral defense-related va riables. As the study progressed, the mean number of total hemocytes d eclined in the WP and MJ populations and increased in the DW populatio n. The percentage of granulocytes in DW oysters was consistently highe r than other populations. DW oysters also had the highest concentratio ns of protein and lysozyme. This pattern persisted throughout the expe rimental period. Oyster condition index significantly decreased during the course of the study in all populations except the DW oysters at 1 0 ppt. Results suggest that the increase of hemocyte number and higher percentage of granulocytes, and lysozyme concentration in DW oysters may have contributed to the high (100%) survival rate of this populati on. Salinity may have affected disease development. Disease prevalence and intensity tended to be lower in the WP oysters maintained at low salinity than those maintained at high salinity. In the DW population, unexpectedly, oysters maintained at 20 ppt had lower infection preval ence and intensity than oysters maintained at 10 ppt. Salinity induced , to some extent, changes in certain hemolymph components: lysozyme co ncentration tended to be higher in oysters maintained at low salinity than those maintained at high salinity. Increase in percentage of gran ulocytes was also observed in WP oysters after transferring to a salin ity lower than ambient salinity.