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 and C. gigas, held in flumes receiving water from the York River, VA, where two protozoan parasites, Haplosporidium nelsoni and Perkinsus marinus, are endemic. Gametogenesis in C. virginica was characterized by a general lack of development, maturation, and spawning. Only two C. virginica developed mature gametes, and none showed evidence of spawning. From July onward, most individuals exhibited gamete resorption. In contrast, gamete development, maturation, and spawning were well defined and synchronous in C. gigas. Mature individuals predominated in June, and spawned individuals predominated in July. Mean gonadal area indices (GAI) were significantly different (P less than or equal to 0.001) between months and species. Mean oocyte areas were significantly different (P less than or equal to 0.001) between months. Significant month x species interactions (P less than or equal to 0.001) for both GAI and oocyte areas supported the differences in gametogenic cycles observed between species by the use of subjective staging. Among mature females, C. gigas had both a significantly greater (P less than or equal to 0.001) GAI and mean oocyte area than C. virginica. Combined prevalence of the parasites H. nelsoni and P. marinus increased from 0 to 86.7% in C. virginica between May and August. Infection intensity increased from epithelial infections to systemic infections from June through September. These parasites were never detected in C. gigas. The difference in gametogenic cycles observed between oyster species could be related to differences in susceptibility to the parasites H. nelsoni and P. marinus, genetic differences in gametogenic cycles, or a combination of both factors. This study establishes the ability of C. gigas to produce gametes and spawn in the environment prevailing in lower Chesapeake Bay.