Hybridization improved stress resistance in the Pacific oyster: Evidence from physiological and immune responses

Recently, mass mortality of cultured Pacific oyster (Crassostrea gigas) has occurred frequently, which has aroused our attention to the stress tolerance of oyster. Hybridization is regarded as an effective way to produce genetic improvement in aquaculture, which can effectively introduce improved characteristics to the hybrids. In the present study, we evaluated the physiological and immune responses at different temperatures (16-36 degrees C) and salinities (15-35 psu) in two strains of C. gigas, a fast-growing strain "Haida No. 1" (HH) and an inbreeding line with orange shell color (OO), and their reciprocal hybrids HO (HH female x OO male), OH (OO female x HH male). Oxygen consumption rate (OCR), ammonia-N excretion rate (AER), superoxide dismutase activity (SOD), catalase activity (CAT) and contents of malondialdehyde (MDA) were determined on day 10 of the temperature and salinity exposure. Results showed higher stress resistance ability of the hybrids than their parents against environment challenge. For physiological parameters, the OCR of OH strain increased in the experimental temperature range, while the maximum values of HO, HH and OO strains were 2.013, 2.193 and 1.994 mg g-1 h-1 respectively at 31 degrees C; OCR and AER of OH strain was significantly higher than that of the other strains at lower salinity (P < 0.05). For immune parameters, the overall SOD and CAT activity of hybrids were lower than that of the other strains under temperature treatments. Besides, the overall MDA content of OH strain was lowest at experimental temperature and salinity, and the other three strains' MDA level ranked as: OO > HH > HO. Overall, the results suggest that environment changes could significantly affect the physiological and immune status of oyster, and hybrids may be more resistant to environment stresses than their parents. This study provides physiological and immune evidences for interpreting the stress resistant heterosis in this oyster hybrid system, which could help us in a better understanding and utilization of heterosis in oyster aquaculture.