Background: Invasive mitten crabs of the genus Eriocheir, are catadromous species. As in other decapod crustaceans, their hyperosmoregulation during and after migration into fresh water has been studied, but information about the physiology during seawater acclimation of adults is limited mostly to the dynamics of free amino acids. Therefore, the purpose of this study was to investigate the possible osmo/iono-regulation in adult mitten crabs (E. japonica) during adaptation to seawater.
Methods: Adult crabs collected in fresh water were transferred to 30-ppt seawater for 3 and 10 days, and the osmolality and ionic status of the hemolymph as well as gill Na+/K+-ATPase, which has been implicated in ion transport in various crustaceans, were analyzed.
Results: Analysis of the hemolymph osmolality and Na+ status indicated that adults were able to hypo-regulate these parameters in seawater. On the other hand, the free Ca2+ concentration was two-fold hyper-ionic to seawater in both fresh water and seawater, but relatively maintained compared with Na+ and osmolality in seawater, while complexed calcium (total minus free calcium) increased after 3 days in the hemolymph. Examination of Na+/K+-ATPase activity revealed that seawater acclimation decreased the activity in the most posterior gill (gill number 8), where immunoreactive Na+/K+-ATPase was localized to the basolateral membrane region of gill epithelial cells in fresh water.
Conclusions: Adult mitten crabs can hypo-regulate the hemolymph concentrations of ions and their osmolality during seawater adaptation, partly by reducing the Na+/K+-ATPase activity involved in ion absorption in fresh water and using the hemolymph complexed calcium as an internal reserve.
Keywords: Crustacean, osmoregulation, salinity, gills, Na+/K+-ATPase