Physiological and molecular responses to hypoxia stress in Manila clam Ruditapes philippinarum

Hypoxia has become one of the major environmental problems in the aquaculture industry. As one of the most commercially important bivalves, Manila clam Ruditapes philippinarum may be suffering substantial mortality attributable to hypoxia. The physiological and molecular responses to hypoxia stress in Manila clam were evaluated at two levels of low dissolved oxygen: 0.5 mg/L (DO 0.5 mg/L) and 2.0 mg/L (DO 2.0 mg/L). With the prolongation of hypoxia stress, the mortality rate was 100% at 156 h under DO 0.5 mg/L. In contrast, 50% of clams survived after 240 h of stress at DO 2.0 mg/L. After the hypoxia stress, some severe structural damages were observed in gill, axe foot, hepatopancreas tissues, such as cell rupture and mitochondrial vacuolization. For the hypoxia-stressed clams, the significant rise and decline of enzyme activity (LDH and T-AOC) was observed in gills, in contrast to the reduction of glycogen content. Furthermore, the expression levels of genes related to energy metabolism (SDH, PK, Na⁺/K⁺-ATPase, NF-κB and HIF-1α) was significantly affected by the hypoxia stress. It is therefore suggested that the short-term survival of clams under hypoxia may be dependent on stress protection by antioxidants, energy allocation, and tissue energy reserves (such as glycogen stores). Despite this, the prolongation of hypoxia stress at DO 2.0 mg/L may cause the irreversible damages of cellular structures in clam tissues, eventually leading to the death of clams. We therefore support the hypothesis that the extent of hypoxia impacts on marine bivalves may be underestimated in the coastal areas.