Reproductive phenology of the eastern oyster, Crassostrea virginica (Gmelin, 1791), along a temperate estuarine salinity gradient

Low salinity can negatively affect reproduction in estuarine bivalves. The spatial and temporal extent of these effects are important to inform models of population dynamics, environmental risk assessments, restoration efforts, and predictions of climate change effects. We hypothesized that oysters at low salinity sites would have delayed gametogenesis compared to their higher salinity counterparts in downstream experimental cages. The timing of gametogenesis and spawning was observed June - August for 2-year-old oysters from three distinct ancestries (Native, Hatchery, Aquaculture), outplanted at age 1 month along the salinity gradient (3-30 psu) of a temperate estuary. A second season of data was collected from 3-year-old Aquaculture oysters (comparable to year 1 data) and Native adult oysters transplanted one year prior. Dermo was very low both years. A delay in gametogenesis and spawning was observed for all ancestries at low salinity relative to higher salinity sites during July and August of the first year but not the second year. In contrast, June showed the reverse pattern with northern low salinity sites having more advanced gonad index (2.65) than a high salinity site (1.46). This difference in average gonad index was 2.65 vs 1.46, respectively, for the Native line and 2.62 vs 2.08 for Aquaculture. Low salinity seemed to not only induce earlier gametogenesis in June, but also extended the reproductive season relative to higher salinity sites. Among oyster ancestries, the Aquaculture line stood out as having 30 - 48% lower gametogenic synchrony within sites, but only in 2018. Despite some dependence of reproductive phenology on salinity variation, the Native low salinity population demonstrates notable reproductive plasticity in the completion of a reproductive cycle across a wide range of salinities, an encouraging result for potential future restoration strategies.