Impact of seasonality on the reproductive physiology of diploid and tetraploid eastern oyster, Crassostrea virginica

Abstract

Aquaculture production of triploid Eastern oyster, Crassostrea virginica, in the Gulf of Mexico (GoM) is reliant on naturally conditioned diploid females and tetraploid males. However, limited knowledge is available on their reproductive biology, including the gametogenic cycle. The aim was to quantify monthly changes in gametogenesis and gamete quality for triploid production in Eastern oyster in the GoM from June 2022 to July 2023. Monthly changes in temperature and salinity were recorded. Shell morphometrics and sex were determined. Sperm density, sperm activity, Spermatogenic Maturity Index (SMI), oocyte diameter, Oogenesis Maturity Index (OMI), and gonad coverage area were quantified. Mean monthly temperature ranged from 15.1 to 31.2 °C and salinity ranged from 12.5 to 27.6 PSU. Shell morphometrics increased over the annual cycle. When gametes were detectable, 48–80% of tetraploids and 45–70% of diploids were males. Hermaphrodites were identified in both ploidies in June; an additional tetraploid hermaphrodite was identified in March. Sperm density exhibited monthly fluctuations with high densities reported in June–July, September, and April–May. Sperm were motile when present, with high motility in June–July. High sperm velocity was also recorded in June–July and March–May. Tetraploids exhibited high SMI from June–September and April–May with increased variability in December–January. Mature sperm cells were predominant in tetraploids in June–July. Gonadal coverage was high from June–September and March–May. OMI was highest from June–September and March–May, while variability was high from August–March. Large oocyte diameters and gonad coverage areas were observed in June–July, September, and April–May. Together, these findings provide novel insights into reproductive dynamics for triploid production of Eastern oyster in GoM.