Crabs can actively uptake or extract ions to adapt to their diverse habitats. The gills are the main osmoregulatory organ in crabs, and Na+/K+-ATPase (NKA) pump is one of the main enzymes involved in the osmoregulatory process. Three crab species from three different habitats, including Portunus armatus (marine spicies), Macrophthalmus dentipes (estuarial species) and Eriocheir hepuensis (freshwater species) were used in the present study. The aim of the present study was to investigate the distribution and activity of NKA pump in the posterior gill of these species. In this regard, the tissue structure of the posterior gill, Immunolocalization and activity of the NKA pump in the posterior gill were assessed in three selected species. The levels of electrolytes (including Na+, Cl−, K+, Ca2+, Mg2+) in the posterior gill, hemolymph and water of different habitats were also measured. In addition, the osmolality of crab species compared to the water of their habitats was investigated. Based on the results, the gill tissue structure was similar in the studied species. However, the highest and lowest intensity of immunofluorescence staining and the activity of NKA pump were recorded in estuarial and freshwater species, respectively. The osmolality of hemolymph in P. armatus was isosmotic with seawater, whereas, the hemolymph of M. dentipes and E. hepuensis was in hyperosmotic state with ambient water. It can be concluded that, Na+/K+-ATPase plays a more important role in the estuary and seawater adaptation of crabs. On the other hand, Freshwater species are not highly dependent on this enzyme for osmoregulation.
