HIGH SALINITY ACCLIMATION BY THE PRAWN MACROBRACHIUM ROSENBERGII UPTAKE OF EXOGENOUS AMMONIA AND CHANGES IN ENDOGENOUS NITROGEN COMPOUNDS

¹ Department of Land, Air and Water Resources, University of California, Davis, California 95616, College of Fisheries, University of Washington, Seattle, WA 98195
² Department of Zoology University of California, Davis, California 95616
³ Department of Zoology University of California, Davis, California 95616, Bodega Marine Laboratory, Bodega Bay, California 94923
⁴ Department of Land, Air and Water Resources, University of California, Davis, California 95616

The freshwater prawn Macrobrachium rosenbergii was subjected to a hyperosmotic transfer from 0to 24 salinity. Changes in ammonia excretion, blood Na⁺, pH, protein, free amino acids (FAA), and ammonia were monitored for 48 h. Following a rapid reduction in ammonia excretion after transfer, ammonia concentrations in exposure water declined for 24 h. These losses could not be attributed to bacterial growth or aerial diffusion. Also during the first 24 h, blood Na⁺ increased from 150 to 280 mM, still far below ambient Na⁺ concentrations of 366 mM. Blood ammonia decreased nearly five-fold and protein concentrations in blood declined about 40 g/l. These data may indicate a reversal of normal Na⁺/NH⁺₄ exchange following a hyperosmotic shock, such that blood Na⁺ is hyporegulated using exogenous NH⁺₄ as a counter-ion. During the first 24 h after transfer, net ammonia acquired by uptake could be used to increase intracellular ammonia concentrations as a prelude to increased synthesis of FAA. This model may compliment recent evidence of a CL^-/HCO^-₃ reversal in fish hyporegulating blood Cl^- in sea water.