Accommodation of the sex-ratio in eastern oysters Crassostrea virginica to variation in growth and mortality across the estuarine salinity gradient

Abstract

Protandric oysters generate a relatively uniform reproductive potential over a wide range of environmental conditions that impose variations in growth rate and life span. Sex-at-length keys applied to survey data show that the female fraction routinely fell between 0.4 and 0.5, regardless of location in the salinity gradient. About 70% of population biomass is female over the same salinity range. Due to the necessary local modulation of the rate of male-to-female conversion to limit the influence of varying growth and life span over the salinity gradient, the number of males always exceeds by a small amount the number of females; thus improving the likelihood of a female having neighbouring males, a necessity for an immobile broadcast spawner. However, oysters at the extremes of the estuarine gradient all yielded populations with divergent sex-ratios. One consequence of reduced generation time brought about by increased mortality from disease should be selection favouring the switch from male to female at smaller size, if disease mortality is strongly female-biased. The site with the longest record of high mortality manifests such an increase. Sites up coastal rivers, putative refuges from disease, harbour animals with the slowest male-to-female conversion rates. Arguably these animals are most similar to the ancestral oyster pre-disease. Marketed animals range from 62% to 69% female. The principal influence of the fishery, and of oyster disease, would seem to be a reduction in lifetime egg production. Dermo disease may have reduced lifetime fecundity of females by nearly a factor of four.

A modelling study of the role of marine protected areas in metapopulation genetic connectivity in Delaware Bay oysters

“…In each generation, a protandric male is given the chance to convert to a functional female. A conversion probability was obtained from empirical data from Delaware Bay (Powell et al ., ) using age–length relationships developed by Kraeuter et al . ().…”

“…Powell et al . () found that the relationship between the fraction of the population that is female, Ff , and age could be modelled as a Gompertz curve: where α and β are population‐specific parameters (Table ). The first derivative of Equation gives the rate at which any animal can change from male to female ( Df ) as: where γ is a population‐specific parameter (Table ).…”

“…As all oysters that are protandric begin life as male, all recruits are male. However, some recruits convert to female before first spawning, as appears to be the case in populations from Delaware Bay (Powell et al ., ).…”

“…Potential parents are drawn randomly, without replacement, from a list of all animals greater than 1 year of age (Kennedy, ; Powell et al ., ) until enough males and females accrue to provide nParents , or until the list of animals is exhausted. Each pair of parents, taken randomly, without replacement, from the parents' list, produces a number of offspring up to a maximum number, which represents a typical larval settlement at the beginning of the simulation.…”

Temporal Structure and Trends of Parasites and Pathologies in U.S. Oysters and Mussels: 16 Years of Mussel Watch

“…One obvious option would be an increasing maleness in the population with declining adult length, if that decline was a function of declining longevity or reduced growth rate. Powell et al (2013) noted that the sex change was a complicated function of both age and length, modulated by within-population processes. The association of increasing maleness with decreasing length is dramatically apparent in oysters from the Gulf of Mexico over the 1995 to 2010 time series and also is present in oysters taken from the Southern Mid-Atlantic.…”