V-type H+-ATPase and Na+,K+-ATPase in the gills of 13 euryhaline crabs during salinity acclimation

Because of their diverse habitats, crabs are excellent experimental species to study owing to the morphological changes and physiological adaptation that occur during their terrestrial invasion. Their hemolymphic osmoregulation in brackish water is crucial for a successful terrestrial invasion. Crabs can actively uptake or excrete ions upon salinity change, and the gills play a major role among the osmoregulatory organs. Several enzymes are involved in the osmoregulatory process, including Na+, K+-ATPase and V-type H+-ATPase (V-H+-ATPase). Na+, K+-ATPase is the driving force in establishing an ion gradient across the epithelial cell membrane in marine crabs. It has been reported that the osmoregulatory mechanisms in freshwater crabs are different from those in marine ones, suggesting that the driving force may come from V-H+-ATPase by generating the H+ ion gradient to facilitate the ion flow. Thirteen crab species from two families were used in this study. These crabs lived in five different habitats, including marine, intertidal, bimodal, freshwater and terrestrial habitats. The distribution of V-H+-ATPase in the 13 euryhaline crabs was revealed by histochemistry. V-H+-ATPase was localized in the apical region in crabs that could survive in the freshwater environment. We found that the freshwater and terrestrial crabs with stable Na+, K+-ATPase activity during salinity changes tended to have an apical V-H+-ATPase, whereas the intertidal ones with varying Na+, K+-ATPase activity showed a cytoplasmic V-H+-ATPase distribution. Finally, in Uca formosensis, a crab that had stable Na+,K+-ATPase activity, a significant difference in V-H+-ATPase activity between salinities was found. In conclusion, the hypothesis that V-H+-ATPase plays a crucial role in the freshwater adaptation of crabs is supported by our systemic investigation on 13 euryhaline crabs.