Salinity impacts gamete quality in eastern oyster, Crassostrea virginica

The demands on aquaculture for high-value products, like the eastern oyster (Crassostrea virginica), continue to rise. To aid commercial production, hatcheries mate diploid females with tetraploid males to produce triploid offspring, which, in many instances, exhibit faster growth than diploid oysters and maintain meat quality during the reproductive season. Many factors limit triploid production by hatcheries, including low gamete quality. While it is known that abiotic factors can influence gamete performance in eastern oyster, the effects of salinity on triploid production are not well understood. To investigate these effects on gametogenesis and gamete quality, adult tetraploids and diploids were held in recirculating aquaculture systems (RAS) at 10, 20, and 30 PSU for 30 days. Each treatment contained six 140 L tanks with 15 oysters per ploidy. All morphometric measurements were recorded and the genotypic sex of the oysters was determined. Sperm density was quantified from tetraploid males. Sperm motility and velocity from tetraploid males was also analyzed by Computer Assisted Sperm Analysis, while their viability was determined using flow cytometry. ATP and lipid peroxidation were quantified for tetraploid sperm via bioluminescence and fluorescence, respectively. Eggs from diploid females were assessed for fecundity and size. For tetraploid males and diploid females, lipids from gonadal tissues were analyzed by gas chromatography. Results showed that survival, shell morphometrics, and sperm density were unaffected by salinity. However, variability in sperm density increased when tetraploids were held at 10 PSU. In general, sperm motion was suppressed for oysters held at 10 PSU, relative to the other salinity treatments. Sperm membrane viability was lowest and variability highest in oysters held at 30 PSU. Salinity did not impact the quantity of ATP in sperm. Lipid peroxidation was higher in oysters held at 30 PSU compared to 10 PSU, while those at 20 PSU demonstrated no significant difference compared to other treatments. Variation in lipid peroxidation was highest at 10 PSU. Oocyte irregularities were more prevalent when diploids were held at 10 and 30 PSU. Developmentally crucial lipids from females, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), were highest when oysters were held at 10 PSU, compared to 30 PSU, while the amount of these lipids from either salinity treatment was statistically different from 20 PSU. Fatty acid concentrations within the sperm from tetraploid males did not differ between salinities. In conclusion, gametes collected from both ploidies held at 20 PSU, performed as well or better than those from the other salinities for every hatchery and gamete-quality metric measured.