Abstract
Both male and female blue crabs were shown to hyperosmoregulate efficiently at low salinities. The posterior gill pairs, particularly numbers six and seven, exhibited the highest specific activity of Na, K‐ATPase in crabs adapted to full‐strength seawater. When the crabs were acclimated to 200 mOs/kg, Na,K‐ATPase activity increased in all gill pairs, with the most pronounced increase in gills six and seven (∼ 80%). Crabs which had been acclimated to waters of 1000, 800, 500, and 200 mOs/kg showed asequential increase in Na, K‐ATPase activity which matched the decrease in medium osmolality. At the lowest salinity, the increase was somewhat greater in males, approximately 26% higher than in females. In crabs adapted to 100% seawater, the highest enzyme activity was localized in the lateral one‐third of the gill lamellae, near the afferent vessel. Acclimation to low salinities was accompanied by a significant increase in Na,K‐ATPase activity in the middle and medial portions of the gill. Abrupt transfer of crabs from 1000 mOs/kg to 200 mOs/kg, or vice versa, resulted in rapid changes in hemolymph osmolality which stabilized within 24 hours. However, the Na,K‐ATPase activity changed gradually and did not reach equilibrium levels until 12 to 18 days,after transfer. These data suggest that Na,K‐ATPase activity was altered primarily by synthesis and degradation, rather than by modulation of the activity of existing enzyme.