Metabolic depression explains differences in vulnerability to low oxygen between two species of temperate marine bivalves

Nutrient enrichment and rising temperatures increasingly generate summer hypoxia in temperate estuaries, potentially impacting the physiology and survival of sessile marine bivalves. In the western Atlantic ocean, eastern oysters, Crassostrea virginica, and blue mussels, Mytilus edulis, often co-occur in these habitats, but their comparative resilience to extended hypoxia and anoxia is uncertain. In this study, the effects of hypoxia and anoxia on the physiological response and survival of oysters and mussels were examined under summer eutrophic conditions. Survival, respiration, and pumping rates were determined in both species under three treatments: normoxia (>5 mg O₂ L^-1), hypoxia (∼2 mg O₂ L^-1), and anoxia (<0.5 mg O₂ L^-1). On day 25, when all bivalves in the anoxic treatments had perished, the normoxic and hypoxic treatments were switched to anoxia to further explore the long-term effects of prior hypoxia on survival. Respiration rate could not be carried out under anoxia, and feeding was negligible; accordingly, detailed analyses were limited to the comparison of normoxia vs. hypoxia. Both species reduced pumping rate under hypoxia and further suppressed it to negligible levels under anoxia; however, only oysters reduced respiration rate, indicating metabolic depression. Oysters survived longer than mussels in all treatments. Moreover, oysters pre-exposed to hypoxia survived anoxia as long as normoxic controls, suggesting the physiological strategy adopted by oysters during hypoxia did not have major long-term effects on survival when further exposed to anoxia. Overall, these findings suggest that metabolic depression, rather than feeding reduction, underpins superior low-oxygen tolerance of C. virginica relative to M. edulis, implying that eastern oysters will better withstand low-oxygen episodes expected in eutrophic coastal waters under climate change scenarios. As the frequency and duration of low-oxygen events will increase in a warmer ocean, we provide preliminary support that species that can effectively engage metabolic depression under periods of high temperature and low-oxygen conditions may be more resilient to climate change.