Hypothermal stress-induced salinity-dependent oxidative stress and apoptosis in the livers of euryhaline milkfish, Chanos chanos

Abstract Water temperature is one of the key environmental factors affecting the physiological responses of fish, hence it becomes a significant concern for aquaculture fisheries. The euryhaline milkfish, Chanos chanos, one of the major aquaculture species in Southeast Asia, can be cultured in ponds of different salinities ranging from fresh water (FW) to sea water (SW). Being a tropical species, high mortality occurs in cultured milkfish after a cold snap in winter, leading to a huge economic loss. In the present study, we studied salinity effects on mechanisms underlying hypothermal tolerance in milkfish using various molecular and physiological approaches. At low temperatures, FW milkfish were found to have poorer cold-tolerance ability and more oxidative stress than SW milkfish. In addition, milkfish hepatocytes exhibited apoptosis induced via an extrinsic or intrinsic pathway, indicating the source of oxidative stress. This study further revealed that under low-temperature acclimation, more hepatocytes were shrunk and showed positive response to terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining in FW milkfish than in SW milkfish. The caspase-3 activity was up-regulated, which, in turn induced apoptosis via an extrinsic pathway in the hepatocytes of FW milkfish under hypothermal stress. Although the enzymatic antioxidant mechanisms were up-regulated to neutralize oxidative stress, apoptosis was still observed in the hepatocytes of FW milkfish under hypothermal stress. On the other hand, the hepatocytes of SW milkfish under identical hypothermal conditions showed no redox imbalance and apoptotic responses. Overall, this study suggests a hypothermal FW environment induces oxidative stress and apoptosis in hepatocytes and impairs liver functions that may lead to cold intolerance in milkfish.