EVALUATION OF VIBRIO SPP AND MICROPLANKTON BLOOMS AS CAUSATIVE AGENTSOF JUVENILE OYSTER DISEASE IN CRASSOSTREA-VIRGINICA (GMELIN)

Eastern oysters, Crassostrea virginica, cultured in the northeastern U nited States have experienced unexplained mass mortalities associated with a syndrome called juvenile oyster disease (JOD) for the past 7 y (1988-1995). Previous studies implicate bacteria, plankton blooms, or both as causes of this disease. The possibility that a bacterium in th e genus Vibrio, common aquatic pathogens, is the causative agent was e valuated by weekly monitoring of Vibrio spp. concentrations in water, oysters, sediment, and debris associated with suspended oyster nursery trays at an oyster nursery on Long Island, NY, from May to September 1993. Juvenile oysters experienced mortalities totaling 20-60% from Ju ly through August. Total Vibrio spp. counts rose exponentially in juve nile oysters' tissues immediately after water temperatures exceeded 20 degrees C, and preceding observed mortality by 1-2 weeks. The onset o f oyster mortalities did not correlate significantly with blooms of th e dinoflagellate, Gymnodinium sanguineum, or with salinity. Vibrio spp . concentrations in sediments rose significantly before oyster mortali ty was observed and decreased thereafter. However, trends in Vibrio sp p. concentrations in water and debris samples did not correlate with t he oyster mortalities. Healthy juvenile oysters were challenged with V ibrio spp. isolated from afflicted oysters before episodes of high mor tality periods. Oysters injected with two of nine isolates experienced significantly higher mortalities than controls or than those injected with other isolates. Vibrio spp. that were phenotypically identical t o the injected species were recovered from the experimentally infected oysters. Field and experimental observations strongly suggest a link between infection by a Vibrio strain and JOD.