How does exposing oysters to warmer temperatures and tidal inundation in the lab affect their response to a simulated marine heatwave?

Background/Question/Methods

Climate change threatens ecologically important foundation species, such as oysters. Marine heatwaves (MHWs) pose a physiological challenge for oysters and their inhabitation of tidal environments increases their risk of desiccation. One coping strategy is to ‘stress harden’ oysters prior to a MHW. For this study, we used Pacific (Magellana gigas) and Kumamoto oysters (Crassostrea sikamea), which were introduced to California estuaries in the 20th century, and the native Olympia oyster (Ostrea lurida), whose populations are declining throughout the U.S. west coast. To initiate stress hardening, we exposed oysters to a combination of temperature (15°C vs 21°C) and tidal treatments (cyclical vs constant immersion) for two weeks in the lab. We then exposed the oysters to a simulated MHW at one of four temperatures (15°C, 18°C, 21°C, and 24°C) for 72 hours. The three highest temperatures represent the upper 2% of seawater temperatures they experience and thus would be a stressful heatwave. We measured shell growth and mortality weekly during the experiment. Afterwards, we performed glycogen and carbonylated protein assays to assess energy levels and immune response, respectively.

Results/Conclusions

We found differences among species and among lab exposure treatments, suggesting that stress hardening may only be an effective strategy under certain conditions. Specifically, Ostrea lurida and Magellana gigas were less sensitive than Crassostrea sikamea to stress hardening conditions. The former two species exhibited greater proportional growth and less mortality during the stress hardening phase than C. sikamea. Additionally, all species experienced greater growth in the 21°C stress hardening treatments than 15°C treatments, irrespective of tidal treatment. However, all oysters that experienced cyclical immersion during the stress hardening phase were smaller than their counterparts in the constant immersion treatment. Overall, it appears that stress hardening through temperature, but not necessarily tidal exposure, can provide a boost to oyster species that may allow them to better endure a marine heatwave.