Effects of salinity on haemolymph osmolality, gill Na+/K+ ATPase and antioxidant enzyme activities in the male mud crab Scylla olivacea (Herbst, 1796)

The mud crab Scylla olivacea is a well-known commercially and ecologically important species that inhabits intertidal mangroves. These mud crabs spend juvenile and adult stages in mangroves where they can experience daily fluctuations in salinity. They are almost certainly adapted to deal with salinity challenges. Although S. olivacea has a wide salinity tolerance, there is a lack of studies on its osmoregulatory responses. Antioxidant enzymes play crucial roles to counteract oxidative stress in various tissues of aquatic animals. However, salinity changes affecting antioxidant responses are not well studied in tropical decapod crustaceans. Therefore, this study aims to characterize the osmoregulatory and antioxidant responses of the male S. olivacea when exposed to salinity changes. Haemolymph osmolality, Na+/K+ ATPase and antioxidant enzyme activities in gills were analysed after they were exposed to various salinities. The results showed that S. olivacea is a hyper-hypoosmoregulator. The salinity had greater effects on Na+/K+ ATPase and antioxidant enzyme activities in the posterior gills than the anterior gills. Notably, the modifications of gill SOD activity were detected only in the posterior gills, which function highly in active ion uptake. These findings may imply some associations between osmoregulatory and antioxidant responses during salinity challenges. The increase in gill SOD activities be a response for coping with elevated ROS levels due to high metabolic demands for osmoregulation under salinity stress. Moreover, the crabs exhibited some modified activities of gill CAT in several salinity levels. Overall, the results demonstrated that S. olivacea were well-adapted to the salinity changes based on their osmoregulatory and antioxidant responses. These physiological adaptations may allow these crabs to disperse in some areas of mangroves. The salinity range of 15-25 psu would likely be the recommended optimal range for adult S. olivacea, which may enhance growth and minimize oxidative stress in gills of crabs, especially in aquaculture conditions.