Heritability of acute low salinity survival in the Eastern oyster (Crassostrea virginica)

On the US east coast, Eastern oyster (Crassostrea virginica) aquaculture has grown substantially over the last century. As aquaculture expands into previously unexploited areas, there is a need for oyster lines bred for specific environmental conditions. In the Maryland portion of the Chesapeake Bay, and in other coastal areas, salinity tends to be below optimal for typical aquaculture productivity, and these areas frequently experience periods of acute low salinity following heavy rain events. While these areas may hinder oyster growth, they provide a refuge from common oyster diseases. Selective breeding focusing on survival in acute low salinity conditions could expand areas suitable for aquaculture, but the genetic framework underlying this trait is currently unknown. In this study, we estimated the heritability of survival at acute low salinity (< 3) by conducting two month-long low salinity exposure experiments with fifty half-sibling families. Hemolymph osmolality was analyzed during the first eight days of the second exposure experiment to track osmoconformation behavior and investigate potential physiological differences underlying variation in salinity survival among families. There were significant differences in mortality among families for both low salinity exposure experiments, with the majority of mortality occurring between 8 and 14 days of exposure for both experiments. Higher overall mortality was observed during the second experiment (53% in experiment 2 versus 23% in experiment 1), which was conducted during the summer when animals were reproductively active. Narrow-sense heritability estimates for survival were moderate to high using both a Bayesian (MCMCglmm, h2 = 0.34 for experiment 1 and 0.59 for experiment 2) and a likelihood-based (ASReml-R, h2 = 0.4 for both experiments) approach, and estimates from ASReml-R appeared to be lower compared to MCMCglmm for experiment 2 (ASReml-R h2 = 0.4, MCMCglmm h2 = 0.59). Finally, there were no differences in osmolality among families on either sampling day, but all families remained slightly hyperosmotic after salinity was held static at our desired level (2.5). This study provides the first quantitative genetic analysis of acute low salinity survival in C. virginica and results suggest that this trait is heritable and could be selectable in a breeding program.