REGULATION OF HEMOLYMPH OSMOLYTES AND GILL NA+ K(+)ATPASE ACTIVITIES DURING ACCLIMATION TO SALINE MEDIA IN THE FRESH-WATER SHRIMP MACROBRACHIUM-OLFERSII (WIEGMANN, 1836) (DECAPODA, PALAEMONIDAE)/

To evaluate the mechanisms of long-term osmotic adaptation to saline m edia, total hemolymph osmolytes, hemolymph sodium and chloride concent rations, and gill Na+/K+-ATPase activities were measured in the freshw ater shrimp Macrobrachium olfersii (Wiegman) after acclimation for 10 or 20 days to media of < 0.5, 21 and 28 parts per thousand salinity. T otal hemolymph osmolytes are maintained strongly hyperosmotic to the e xternal medium in low and moderate salinities, becoming slightly hyper osmotic at high salinity. In contrast, however, hemolymph [Na+] and [C l-] are distinctly hyporegulated at the higher salinities. This differ ence in response pattern to acclimation apparently results from the pr esence of free amino acids in the hemolymph as a consequence of the sy nthesis 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 m edium for excretion of the salt load. Gill Na+/K+-ATPase activities de crease by approximate to 35% after acclimation to saline media, sugges ting that the rates of the cellular mechanisms responsible for salt up take in freshwater become reduced, avoiding excessive salt loading, A Na+-ATPase activity is salinity independent. These data are examined w ith regard to the intrinsic coupling between the physiological alterat ions taking place in hemolymph osmolytes during acclimation to saline media and the concomitant ultrastructural rearrangements in the salt t ransporting tissues of the gills, and their possible neurosecretory co ntrol mechanisms. (C) 1997 Elsevier Science B.V.