GAMETOGENESIS OF EASTERN OYSTERS, CRASSOSTREA-VIRGINICA (GMELIN, 1791), AND PACIFIC OYSTERS, CRASSOSTREA-GIGAS (THUNBERG, 1793) IN DISEASE-ENDEMIC LOWER CHESAPEAKE BAY

Gametogenic cycles were compared for oysters, Crassostrea virginica an d C. gigas, held in flumes receiving water from the York River, VA, wh ere two protozoan parasites, Haplosporidium nelsoni and Perkinsus mari nus, are endemic. Gametogenesis in C. virginica was characterized by a general lack of development, maturation, and spawning. Only two C. vi rginica developed mature gametes, and none showed evidence of spawning . From July onward, most individuals exhibited gamete resorption. In c ontrast, gamete development, maturation, and spawning were well define d and synchronous in C. gigas. Mature individuals predominated in June , and spawned individuals predominated in July. Mean gonadal area indi ces (GAI) were significantly different (P less than or equal to 0.001) between months and species. Mean oocyte areas were significantly diff erent (P less than or equal to 0.001) between months. Significant mont h x species interactions (P less than or equal to 0.001) for both GAI and oocyte areas supported the differences in gametogenic cycles obser ved between species by the use of subjective staging. Among mature fem ales, C. gigas had both a significantly greater (P less than or equal to 0.001) GAI and mean oocyte area than C. virginica. Combined prevale nce of the parasites H. nelsoni and P. marinus increased from 0 to 86. 7% in C. virginica between May and August. Infection intensity increas ed from epithelial infections to systemic infections from June through September. These parasites were never detected in C. gigas. The diffe rence in gametogenic cycles observed between oyster species could be r elated to differences in susceptibility to the parasites H. nelsoni an d P. marinus, genetic differences in gametogenic cycles, or a combinat ion of both factors. This study establishes the ability of C. gigas to produce gametes and spawn in the environment prevailing in lower Ches apeake Bay.