The abrupt decrease in V-ATPase specific activity in D. pagei posterior gills on transference from fresh water to 21‰ salinity, attaining basal values just after 1 h exposure (see Fig. 2), reveals the action of a very efficient short-term regulatory mechanism, resulting in early inhibition of ion uptake processes. In 21‰ salinity, external Na+ (≈ 300 mmol L− 1) and Cl− (≈ 340 mmol L− 1) concentrations are much higher than those in D. pagei hemolymph in fresh water (200 and 215 mmol L− 1, respectively), generating passive influx to the hemolymph in addition to the active uptake driven by the gill (Na+,K+)- and V-ATPases. When D. pagei is acclimated to 25‰, hemolymph Na+ and Cl− concentrations increase rapidly, reach values similar to those of the external medium after just 48-h exposure, remaining unchanged for 10 days ( Augusto et al., 2007). The rapid reduction in V-ATPase activity after 1-h exposure thus constitutes an important component of the short-term osmoregulatory adjustment in D. pagei, rapidly diminishing total ion uptake.