Master of Science (M.Sc.)
Virginia Institute of Marine Science
Standish K Allen, Jr.
Ryan B Carnegie
Roger L Mann
Jessica Moss Small
Unusual mortalities of cultured Crassostrea virginica in late spring have been reported from farms in the Chesapeake Bay from 2014 to 2017. None of the usual causes (e.g. disease, poor husbandry) were likely responsible, and mortalities occurred without clear signs of biological or physical stressors. Mortalities in the spring of 2014 were particularly high on over a dozen farms in Virginia, most of which were on the bayside of the Eastern Shore. Estimated losses were over 50%; however, mortalities only occurred within a four-week period between mid-May and early June. Farmers that had unusually high mortality in their crop were exclusively growing triploid oysters, which implicated triploidy as an important factor and even led to calling these events "triploid mortality." Many affected farmers were also growing a "northern cross," oysters made by crossing brood stock from New England to brood stock from Virginia, and relatively high fecundity was found in some triploid oysters sampled from these farms. It was hypothesized that the genetic contribution from the New England parent of the northern cross was causing aberrant gametogenesis in triploid oysters during the late spring, and that this caused triploid mortality. A controlled field test was conducted to evaluate this hypothesis and to further examine the role of genetics and gametogenesis in triploid mortality. to investigate the effect that ploidy and brood stock origin have on susceptibility to triploid mortality, four triploid and four diploid types of oysters, produced by crossing different combinations of brood stock of Virginia, Louisiana, and Maine origin, were deployed to three commercial oyster farms that experienced unusually high mortality in 2014, as well as to a control site lacking reports of unusual spring mortality. The survival and growth of oysters from each group were measured throughout the spring and summer of 2016. A mortality event (>20% mortality) was observed in late spring at one site. The mortality event was only associated with triploid groups, with cumulative mortality ranging from 12 to 24% among groups. Mortality in the "northern cross" was not especially high (23%). The effect that gametogenesis had on the mortalities was investigated by examining histological cross sections of triploid and diploid oysters from the site where triploid mortality occurred, as well as from the control site where there was no triploid mortality. Diploid oysters at the affected site became gravid and spawned earlier than diploid oysters at the control site. Gonad development in triploid oysters was abnormal and variable, so categories were developed for classifying gonad development in triploid oysters and making comparisons between sites. No obvious difference was observed in gonad development between triploid oysters at the two sites. Cross-sections from male triploids did not typically contain any spermatozoa, and cross sections from females usually contained only a few oocytes. Gonad development may not be strongly associated with triploid mortality, but the physiology associated with abnormal gametogenesis in triploid oysters could be a major contributing factor. More work is needed on the physiological consequences of gametogenesis in triploid oysters to explain triploid mortality.
© The Author
Matt, Joseph L., "The Relationship Between Reproduction and Mortality in Triploid Crassostrea Virginica" (2018). Dissertations, Theses, and Masters Projects. Paper 1550153660.