Impact of nighttime hypoxia on ark shell Scapharca broughtonii mortality on a semi-enclosed embayment seabed

The ark shell Scapharca broughtonii population has decreased markedly in recent years in Nanao Bay, a semi-enclosed embayment in Japan. Continuous measurement of dissolved oxygen (DO) at the bottom of Nanao Bay was performed, and S. broughtonii survival rates were investigated experimentally and in situ. In summer the DO concentration showed diel variation, with generally sufficient oxygen during the day but concentrations < 2 mg/l at night. In rearing experiments to investigate their physiological state, ark shells were placed in a tank designed to produce hypoxic conditions at night. Blood succinate levels in the ark shells increased, and all individuals were dead after 35 days. The results of the study indicate that the main cause of mortality of S. broughtonii on the seabed in Nanao Bay is hypoxia at night.