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 extents of these effects are important to inform models of population dynamics, environmental risk assessments, restoration efforts, and predictions of climate change effects. A hypothesis of delayed gametogenesis for oysters at low salinity sites was tested relative to their higher salinity counterparts in downstream experimental cages. In 2018, 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 in 2019 from a 3-year-old aquaculture line and mixed-age native adult oysters dredged and transplanted 1 year prior. Dermo was tested in 2019 and prevalence was 1.3% (n = 240). Gametogenesis and spawning were retarded for all ancestries at low salinity relative to higher salinity sites during July and August. The reverse pattern was found in June, with low salinity sites having more advanced gonad index than at a high salinity site. 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. Because the native oysters used in this study have been restricted to low salinity conditions for many generations, demonstration of their reproductive plasticity across salinities is notable and broadens the range of potential future restoration strategies.