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.

Male mud fiddler crabs (Uca pugnax) were shown to hyperosmoregulate in 10% and 50% seawater (SW) and to hypoosmoregulate in 150% and 200% SW. Crabs fully immersed in the media did not osmoregulate as well as those that were free to enter or leave the medium at will ("natural" environment). Gill sodium-plus-potassium ATPase (Na,K-ATPase) enzyme specific activity was characterized in crude homogenates. After acclimation (21 days) in 10% SW, all six gill pairs in immersed crabs showed significantly higher enzyme specific activity (ESA) than 100% SW controls; crabs in 50% SW had significantly higher ESA in gills 3 through 6. With one exception, a decrease, acclimation to 150% or 200% SW did not significantly change gill ESA in immersed crabs. In all media, gills 5 and 6 had approximately 75% of the total ESA in the crabs' gills. Crabs in the "natural" environment showed a similar pattern of changes in gill ESA. Thus, changes in gill ESA appear to be important only in hyperosmotic regulation and in active sodium uptake by the gills from dilute media. The time-course of changes in crude homogenate ESA in gills 5 and 6, after transfer of 100% SW crabs to 10% SW, was relatively rapid (significantly increased after 1 day) and correlated well with changes in hemolymph osmolality. Three different microsomal preparations from gills 5 and 6 did not show significantly increased ESA until between 3 and 7 days after transfer. Therefore, gill Na,K-ATPase activity may be altered both by rapid modulation of existing enzyme and by longer term synthesis of new enzyme.

Nitrogen excreted into the urine (<1 mM) has generally been considered a negligible component of total nitrogen output of crustaceans. But concentrations of ammonia >100 mM were found in the urine of laboratory-held Ocypode quadrata, suggesting that this notion might not be applicable to all crustaceans. To address this issue, hemolymph and urine were removed from freshly captured O. quadrata and analyzed for nitrogenous catabolites and major ions. Hemolymph composition was similar to that of other crustaceans, but the urine was acidic ({Xbar} pH = 5.50) and contained ammonia, often at >100 mM. Other nitrogenous catabolites in the urine (urea, amino acids, and uric acid) were much less concentrated: totaling <12 mM on average. The ionic composition of the urine was similar to that of other crustaceans, with the exception that Na was much less concentrated than Cl-. Total osmolality of hemolymph and urine was similar. The Na+/K+ ATPase activity was greater in the antennal glands than in the posterior gills of O. quadrata, suggesting that this enzyme is important for ammonia concentration and Na resorption. This pattern of enzyme activity was not present in two terrestrial brachyurans whose urine contains little ammonia. The evolutionary significance of high ammonia concentrations in the urine of ghost crabs is unclear.

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