Effects of Perkinsus marinus on reproduction and condition of the eastern oyster, Crassostrea virginica, depend on timing

The protistan parasite Perkinsus marinus has been responsible for high mortality of eastern oysters, Crassostrea virginica, along the Atlantic and Gulf Coast of the United States. It also causes sublethal effects, although an impact on reproduction has heretofore been difficult to demonstrate. We examined the effect of P. marinus infection on growth, reproduction (as measured by relative gonadal size, and the proportional representation of gametogenic tissue and gametes in the gonad), and condition index of eastern oysters. Oysters of known age and genetic history were exposed to P. marinus infection and sampled at 2-4 week intervals over an 11-month period during the 1990-1991 epizootic in Delaware Bay. During this time, 50 to 100% of the oysters had detectable infections and nearly 55% died. Shell growth rates were inhibited as infections intensified during initial exposure in autumn 1990, but resumed in the spring and remained high, even after infection became heavy. The effect on reproduction varied with the stage of the gametogenic cycle. The relative gonadal size and the proportion of gametogenic tissue in the gonad, as well as the condition index, were most strongly depressed during the spring gametogenic period; however, there was no significant effect of parasite burden when the oysters were reproductively mature and ready to spawn. Data suggested that infected oysters, recovered, produced gametes, and spawned in early summer. Other studies have found inconsistent evidence of sublethal deleterious effects of P. marinus on reproduction. By sampling throughout the year and employing oysters of known genetic background, of the same age, reared in a common environment, and for which the timing of initial infection was known, we were able to show that P. marinus does have significant negative effects on oyster reproduction, measurable primarily during gametogenesis, rather than at spawning.