The impact of marine heatwaves and hypoxia on behavior and physiology of Manila clam Ruditapes philippinarum

High temperature and hypoxia are important environmental stressors in marine ecosystems that often co-occur, yet their combined effects on marine organisms remain poorly understood. Moreover, the behavior of bivalves has frequently been overlooked in previous studies. This study focuses on investigating the combined effects of high temperature and hypoxia on survival, behavioral characteristics, and physiological responses in the Manila clam (Ruditapes philippinarum). We used three temperature levels (24, 28, and 32 °C) and four dissolved oxygen (DO) concentrations (1, 2, 4, and 6 mg/L) in a two-factor orthogonal experimental design to expose Manila clams to a variety of temperature-DO combinations. The results showed that the combined stress of high temperature and hypoxia significantly affected Manila clam survival, with median lethal times reduced to 107-228 h. Manila clams employed behavioral and physiological responses simultaneously to cope with these combined stresses, including valve opening/closing, digging behavior, activation of antioxidant defense systems, and regulation of respiratory metabolism and energy use. The behavioral responses of the Manila clam were very rapid, even preceding physiological responses, and different behavioral responses were used to cope with different stressors. Changes in behavior, oxygen consumption rates, ammonia excretion rates, and key enzyme activities indicated that co-occurring stresses altered the clams' original responses and frequently caused disorganized responses, ultimately reducing their adaptive capacity. Therefore, the combined effects of high temperature and hypoxia proved more destructive than either stressor alone. Importantly, global warming will not occur in isolation as it can lead to local oxygen deficiencies, so these stressors should be considered together. This study enhances our understanding of behavioral adaptations in bivalves, improves our knowledge of their response strategies under the combined effects of multiple environmental stressors, and provides new insights into the behavioral ecological of bivalves.