Effects of transport stress on immune response, physiological state, and WSSV concentration in the red swamp crayfish Procambarus clarkii

Transport is an essential part of the aquaculture and research of the main freshwater aquaculture crayfish Procambarus clarkii in China. However, transport is often accompanied by a low survival rate. Assessing the physiological state of P. clarkii before and after transport may discover the cause of this high mortality rate. In this study, ice-cold and exposed-to-air transport methods were compared using an array of parameters, including relative expression level of heat shock protein 70 (HSP70), content of serum glucose and cortisol, immune parameters (enzyme and immune-related genes), and white spot syndrome virus (WSSV) concentration were investigated to understand the physiological state of P. clarkii before and after transport, as well as the cause of dying crayfish on days 5 and 7 after transport stress. Histological sections of hepatopancreas, gills, and intestines reflected pathological changes. The survival rate of crayfish with ice-cold transport was significantly higher than that with exposed-to-air transport, and mortality peaked at 3–9 days after transport stress. A prolonged response to oxidative stress and short-term immunosuppression was present after transport, and the trend of the WSSV concentration in the hepatopancreas was similar to the mortality rate of P. clarkii. The contents of serum glucose and cortisol, antioxidant enzymes and immune-related indexes, and the concentration of WSSV in hepatopancreas of dying crayfish were significantly higher than those of vibrant crayfish on the 5th and 7th days after transport. The hepatopancreas, intestines, and gills of dying crayfish had varying degrees of damage, and the hepatopancreas and intestines were severely damaged. The results suggested that the death of P. clarkii after transport stress is caused by oxidative stress, the imbalance of reactive oxygen species regulation, and decreased WSSV resistance, which eventually led to irreversible tissue damage. The increase of WSSV in the body of crayfish might be the direct cause of crayfish death.