Effects of salinity on physiological responses in the sesarmid crab Episesarma mederi (H. Milne Edwards, 1853)

The sesarmid crab Episesarma mederi (H. Milne Edwards, 1853) lives in mid to upper intertidal mangroves, while its physiological responses to salinity are still poorly known. Here, we examined the effects of salinity on osmoregulatory and antioxidant enzyme responses of E. mederi. The crabs were exposed to experimental salinities ranging from 5 to 40 PSU for 96 h under the laboratory conditions. After that, haemolymph osmolality, oxygen consumption rates (OCR), gill Na+/K+ ATPase (NKA) activity and selected antioxidant enzyme activities of the crabs were analysed. Our results showed that E. mederi was a euryhaline hyper-hypo-osmoregulator. Hyperosmoregulation at low salinities was achieved by elevated gill NKA activity. The crabs acclimated to 5 PSU exhibited a significant increase in their OCRs compared to those in the control group (25 PSU). Conversely, the gill NKA activities were unchanged at high salinities. The OCRs increased only at a salinity of 35 PSU under high salinity conditions. Salinity changes can cause modulations in gill catalase (CAT) activity, while the gill superoxide dismutase (SOD) activity was unaffected. This result indicates that the crab gills could experience some degrees of oxidative stress during salinity acclimation. Overall, these physiological abilities can enable E. mederi to thrive in salinity-fluctuating areas.