Examination of survival, physiological parameters and immune response in relation to the thermo-resistant heterosis of hybrid oysters derived from Crassostrea gigas and C. angulata

High temperature is an important environmental stressor leading to summer mass mortality of oysters. Significant survival heterosis in summer were observed in hybrids between two phylogenetically closely-related oysters, Crassostrea gigas and C. angulata, but an explicit understanding of hetemsis has been lacking. Here, we investigated the survival performance, oxygen consumption rate (OR) and enzyme activity (i.e., SOD, CAT and MDA) in C. gigas (GG), C. angulata (AA) and their hybrids C. gigas female x C. angulata male (GA) and C. angulata female x C. gigas male (AG) under acute heat stress containing five levels: 22, 26, 30, 34 and 38 degrees C. Compared with parental strains, the hybrid strains exhibited higher cumulative survival rate at all temperatures, indicating hetemsis for thermal tolerance. Cox regression analysis showed that hybrid strains challenged with heat shock exhibited lower hazard ratio and longer expected lifetime. Moreover, OR in hybrid strains were significantly higher (P < 0.05) than those in parental strains under 34 and 38 degrees C. Arrhenius break-point temperature (ABT) and temperature coefficient (Q/O) revealed that hybrids had broader temperature range of aerobic metabolism and lower sensitivity to rising temperature. In addition, the activities of SOD and CAT of hybrid strains were significantly higher than the parental strains at 34 and 38 degrees C-level treatment after 6 h and at 34 degrees C-level treatment after 3 h, whereas the MDA content were reduced, indicating their stronger antioxidant capacity. Accordingly, the enhanced aerobic capacity and antioxidant ability of hybrids under high temperature may contribute to the improvement of thermal tolerance. Our findings can facilitate our understanding of the physiological and immune mechanisms underlying thermo-resistant hetemsis and facilitate the breeding of thermo-resistant oyster varieties.