Regulation of hemolymph osmolytes and gill Na +/K +-ATPase activities during acclimation to saline media in the freshwater shrimp Macrobrachium olfersii (Wiegmann, 1836) (Decapoda, Palaemonidae)

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To evaluate the mechanisms of long-term osmotic adaptation to saline media, total hemolymph osmolytes, hemolymph sodium and chloride concentrations, and gill Na +/K +-ATPase activities were measured in the freshwater shrimp Macrobrachium olfersii (Wiegman) after acclimation for 10 or 20 days to media of <0.5, 21 and 28‰ salinity. Total hemolymph osmolytes are maintained strongly hyperosmotic to the external medium in low and moderate salinities, becoming slightly hyperosmotic at high salinity. In contrast, however, hemolymph [Na +] and [Cl −] are distinctly hyporegulated at the higher salinities. This difference in response pattern to acclimation apparently results from the presence of free amino acids in the hemolymph as a consequence of the synthesis of intracellular organic osmolytes. The resulting hyperosmotic regulation both avoids water loss from the animal in saline media and provides an osmotic gradient, allowing the uptake of water from the medium for excretion of the salt load. Gill Na +/K +-ATPase activities decrease by ≈35% after acclimation to saline media, suggesting that the rates of the cellular mechanisms responsible for salt uptake in freshwater become reduced, avoiding excessive salt loading. A Na +-ATPase activity is salinity independent. These data are examined with regard to the intrinsic coupling between the physiological alterations taking place in hemolymph osmolytes during acclimation to saline media and the concomitant ultrastructural rearrangements in the salt transporting tissues of the gills, and their possible neurosecretory control mechanisms.