Microgeographical Variations in Gametogenesis and Sex Ratios in the Eastern Oyster at Two Marsh Sites in Georgia

Abstract

This study examined gametogenesis of the eastern oyster Crassostrea virginica along an intertidal gradient. Twenty eastern oysters were taken from two tidal heights (high-intertidal, HI, and low-intertidal, LI) on a mostly biweekly basis from two sites (House Creek and Skidaway River) in Wassaw Sound, Georgia, from June 1993 to September 1994. Gametogenic condition was evaluated by histological staging of the gonads and by subsequent image analysis. No retardation in gametogenic maturation and spawning activity was seen in the HI eastern oysters when compared with the LI eastern oysters. Also, the HI eastern oysters tended to maintain higher gametogenic variables throughout the year than the LI eastern oysters. At the Skidaway River site, HI female eastern oysters had significantly higher reproductive variables than the LI eastern oysters. Multiple spawning events occurred at each tidal height. We concluded that no retardation in gametogenesis occurred in HI eastern oysters, and being located high in the intertidal zone appeared to enhance the reproductive ability of the oysters. Sex ratios differed at the different tidal heights. Female:male abundance ratios were greater in HI areas (3.45:1 at House Creek and 3.12:1 at Skidaway River) than in LI areas (1.95:1 at House Creek and 1.85:1 at Skidaway River). The higher proportion of males and smaller-sized eastern oysters found in the LI zone at both sites probably indicated a younger population of eastern oysters. Younger or smaller animals could have resulted from stressors on the eastern oysters induced by nonlethal predatory activity, siltation, and disease.