Branchial Na+−K+-ATPase activity during osmotic adjustments in two freshwater euryhaline teleosts,tilapia (Sarotherodon mossambicus) and orange chromid (Etroplus maculatus)

Effects of osmotic stress on the branchial Na⁺–K⁺-ATPase activity in two freshwater euryhaline teleosts (the tilapia Sarotherodon mossambicus Peters and the orange chromid Etroplus maculatus Bleeker) were studied. Direct transfer from fresh water to salt water with a salinity of 35 S caused extensive mortality in tilapia. Of the remaining four saltwater concentrations (4.375, 8.75, 17.5, and 26.25 S), the higher two increased the enzyme activity significantly in the first week without affecting it further during the remaining seven weeks of acclimation. The lower two concentrations failed to elicit any significant change. In the case of orange chromid, all three salt-water concentrations (4.375, 8.75, and 17.5 S) which the fish survived produced a more significant and extensive change in the enzyme activity, which continued to rise during the entire acclimation period. Employing a procedure of gradual transfer, tilapia and orange chromid were successfully adapted to saltwater concentrations of up to 61.25 and 35 S, respectively. In this experiment, almost each successive increase in salinity elevated the enzyme activity further, but the total change in both species was considerably less extensive than might be expected from the direct transfer study. When the freshwater fish were transferred to identical concentrations of pure NaCl or whole ocean salt, the former produced a relatively greater change in the enzyme activity in tilapia. No such difference was seen in orange chromid. During reacclimation of the saltwater-adapted fish to fresh water, the enzyme activity began to decrease immediately in both species, but remained above the freshwater control levels at the end of eight weeks.