Intertidal Exposure Modulates Time-Integrated Heat Tolerance of the Eastern Oyster Crassostrea virginica

Andrew R Villeneuve , Brittany M Jellison, Easton R White 

The rise of unprecedented heatwaves globally has caused an increase in mass mortality events, motivating the need for accurate predictions of population declines. Predicting organismal function under fluctuating thermal regimes is a central challenge in thermal biology, particularly in intertidal systems where organisms experience rapid shifts between submerged and aerial exposure. Here, we developed thermal death time (TDT) curves for the eastern oyster Crassostrea virginica under both immersed and emersed conditions to construct exposure-specific thermal tolerance landscapes (TTLs). Using heart rate monitoring to determine time to death, we found significant differences in thermal tolerance between immersion and emersion. At lower lethal temperatures, submerged oysters survived longer than oysters in air, but this relative tolerance flips at temperatures above 37 °C. We then integrated these TTLs into a dynamic survival model that accounted for tidal cycles, allowing us to predict cumulative survival of oysters in the intertidal zone. After calibrating the model to account for field acclimation, our model predictions corroborated the survival of oysters in the field, demonstrating the utility of exposure-specific TTLs in forecasting survival outcomes. Simulated warming of 2 °C in air, water, and both exposures revealed particular sensitivity of oysters to atmospheric heatwaves — survival dropped by 12.9 – 13.8% across modelled sites when only air temperatures were increased. Our findings highlight the importance of considering exposure type in thermal tolerance assessments and provide a framework for predicting organismal responses to thermal stress in fluctuating environments.