Testing predictions of sex allocation theory for a sequential hermaphrodite: Age, size, and sex ratios in restored oyster populations

The eastern oyster, Crassostrea virginica, is a sequential hermaphrodite in which most individuals shift from being male to female over time.  Sex change in oysters is thought to be, at least in part, a phenotypically plastic response to their local environment.  Studying these patterns in relation to age for field populations is normally complicated by recruitment of successive generations and associated difficulties in assigning year classes to individuals.  However, restored oyster reefs in the upper portion of Chesapeake Bay provide a unique opportunity to study sex ratio patterns in populations of known age which have extremely low natural recruitment rates (< 1 spat m^-2).  By sampling the oyster populations on these reefs, we were able to assess the correlation between oyster sex ratios and oyster age, oyster size, oyster density, and local environmental factors.  We sampled a total of 25 reefs, ranging in age from 1 to 5 years old, and collected data on oyster length, sex and density.  Our results demonstrate high variability of oyster size within year class, a positive relationship between the proportion of females in the population and both oyster age and size.  Within most size classes, there is a positive relationship between oyster age and the proportion of females.  Predictions based on sex allocation theory, specifically the size advantage hypothesis, are being tested by examining sex ratio patterns in relation to oyster density, distance to nearest neighbor, disease history and other environmental parameters.