Effects of emersion and re-submergence on key physiological responses in the semi-terrestrial crab Episesarma mederi (H. Milne Edwards, 1853)

Emersion elicits diverse physiological responses in semi-terrestrial crustaceans, thereby shaping their ecological distribution and behaviour. Here, we examined the physiological responses associated with emersion tolerance in Episesarma mederi (H. Milne Edwards, 1853). During 24 h of emersion and 12 h of subsequent re-submergence, the time course of changes in selected key physiological responses was determined. The results showed that rates of water loss were relatively high during the initial phase of emersion and declined thereafter. Haemolymph osmolality increased after 24 h of emersion, reflecting concentration effects associated with water loss. Following re-submergence, haemolymph osmolality gradually decreased and reached a steady state after 6 h. Oxy-haemocyanin levels remained constant during both emersion and recovery. Haemolymph ammonia levels and free amino acid profiles in leg muscle did not vary during emersion and re-submergence. Relatively high ammonia excretion was detected within the first 10 min following re-submergence. These findings suggest that E. mederi is unlikely to rely on substantial ammonia detoxification via temporary storage as free amino acids under the conditions tested. Branchial chamber fluids may play roles in storing ammonia, and stabilizing osmotic balance during emersion. This study offers valuable insights into physiological responses in E. mederi, highlighting environmental adaptations to support its semi-terrestrial lifestyle.