CHANGES IN BACTERIAL DENSITIES AND HEMOCYTE PARAMETERS IN EASTERN OYSTERS, CRASSOSTREA-VIRGINICA, AFFECTED BY JUVENILE OYSTER DISEASE

Juvenile oyster disease (JOD) in the eastern oyster, Crassostrea virgi nica, is characterized by a conchiolin deposit on the inner surface of the valves. Similarities to pathological syndromes affecting other oy sters (pearl oyster mortality) and clams (brown ring disease), has sug gested an infectious origin and a possible bacterial etiology. Bacteri ological analysis (Total Heterotrophic Bacteria, THE and Total Vibrio sp., TVS) of oyster soft tissues, shell fluid and inner shell surfaces were monitored during the course of JOD onset and development. A sign ificant increase in THE (but not TVS) occurred in shell fluid and at t he surface of the inner shell of JOD affected oysters. At the same tim e, changes in cytometric parameters (Total Hemocyte Counts, THC; Diffe rential Hemocyte Counts, DHC) in hemolymph and shell fluid were docume nted in symptomatic oysters. THC in the shell fluid showed a decrease in late-stage JOD oysters. The disease was also correlated with altere d cell ratios in both hemolymph and shell fluid, resulting in an incre ase in the percentage of the granulocytes in both locations. Our resul ts have showed that main pathological changes associated with JOD occu rred in extrapallial region rather than in the soft tissues. The disea se was transmitted in the laboratory by extracts from diseased oysters : anomalous conchiolin developed within 4 weeks and the prevalence was higher when two inoculations were performed rather than one. A number of bacterial strains were found at high levels in diseased oysters an d isolated. However, none produced the JOD deposit when injected into asymptomatic oysters. The bacterial etiology hypothesis for JOD should be explored further, expanded beyond the family Vibrionaceae, and sho uld include consideration of a multiple etiology.