Physiology and behaviour of eastern oysters (Crassostrea virginica) and soft-shell clams (Mya arenaria) under hypoxic and heatwave conditions

Coastal marine environments are becoming increasingly degraded, particularly in urbanized areas where anthropogenic nutrient loading and climate change are contributing to the rise of extreme events such as heatwaves and hypoxia. These extreme events can result in adverse effects and mass mortalities of marine life, especially sessile animals such as bivalves. Bivalves are important economically but are also important contributors to ecosystem functioning via habitat creation, water filtration, and nutrient and carbon cycling. This study aims to characterize how eastern oysters (Crassostrea virginica) and soft-shell clams (Mya arenaria) physiologically and behaviourally respond to repeated hypoxia during a heatwave. This eight-day study aimed to mimic field conditions using water pumped directly from the estuary into a dock-side mobile lab. Water temperature and dissolved oxygen were manipulated to match observations of heatwave and hypoxic conditions in the study area (Prince Edward Island, Canada). The physiological and behavioural responses of oysters suggest that they perform better under high temperatures (27 °C) as they maintain metabolic rate while increasing feeding. Oysters were also able to tolerate repeated periods of hypoxia (<2 mg O₂ L^-1) during a heatwave by maintaining metabolic rate and reducing feeding costs. Conversely, soft-shell clams experienced stress under the induced heatwave and showed signs of metabolic depression. Repeated hypoxia during the heatwave resulted in elevated physiological responses in clams, suggesting elevated energy requirements under these stressors. This study provides insight into the vulnerability of these species under climate change scenarios at northern latitudes, with applicability to aquaculture and fisheries management.