Synergistic effects of salinity and temperature on the eastern oyster Crassostrea virginica throughout the lifespan

The eastern oyster Crassostrea virginica has a wide salinity tolerance, but all life stages are vulnerable to environmental extremes, and elevated temperatures can truncate the expected salinity tolerance. The rising water temperatures and more intense and variable storm events predicted to accompany global climate change therefore raise concerns for habitat suitability for ecologically important species such as the eastern oyster. To better understand environmental limitations, oysters of all life stages were exposed to a range of salinities (0-40) and temperatures (25 and 30°C) to test physiological tolerance to the combined effects of osmotic and thermal stresses. Elevated temperatures (30°C) amplified negative effects during exposure to salinity extremes at all life stages; however, tolerance to extremes increased with developmental stage (gametes < embryos < larvae < spat < adult). Gradual changes allow for a wider tolerance in juvenile oysters (spat) compared to acute changes, and short-term reprieves during low salinity exposure improved survival rates for adults. Overall, the present study found a threshold salinity of 15 for polyhaline oyster populations and highlights the importance of both rate of change and temperature as critical components of salinity tolerance. Additionally, salinities <10 during the summer months could result in negative population effects, especially if extreme low salinity occurs during peak reproduction when pelagic gametes, embryos, and larvae are most vulnerable to environmental stresses. This work will benefit population models and inform resource management decisions regarding the timing of controlled freshwater releases.