Salinity‐induced changes in gill Na, K‐ATPase activity in the mud fiddler crab, Uca pugnax

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Abstract
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.