The survival and responses of blue mussel Mytilus edulis to 16-day sustained hypoxia stress

The blue mussel Mytilus edulis, which is a worldwide commercial species distributed mainly from the intertidal zone to tens of meters deep, has been previously studied regarding its acute defense responses to air exposure and intermittent hypoxia. However, the effects of sustained hypoxia, such as caused by coastal eutrophication, remain to be explored. In the present study, the critical threshold of dissolved oxygen (DO) for experimental mussels exposed to 16 days of hypoxia was DO 0.7-0.8 mg L-1, below which survival dropped drastically from nearly 80% to <38%. When hypoxia was combined with DO fluctuations or with poor water quality, the threshold rose to an average of DO 1.0 mg L-1, which resulted in less than 80% survival. To find possible clues of physiological stress to account for mortalities, the metabolic rate and enzyme activities of Na+/K+ ATPase, superoxide dismutase, acid phosphatase, and alkaline phosphatase were further recorded.