HIGH SALINITY ACCLIMATION BY THE PRAWNMACROBRACHIUM ROSENBERGIIUPTAKE OF EXOGENOUS AMMONIA AND CHANGES IN ENDOGENOUS NITROGEN COMPOUNDS

Abstract

The freshwater prawn M. rosenbergii was subjected to a hyperosmotic transfer from 0% to 24% salinity. Changes in NH3 excretion, blood Na+, pH, protein, free amino acids (FAA) and NH3 were monitored for 48 h. Following a rapid reduction in NH3 excretion after transfer, NH3 concentrations in exposure water declined for 24 h. These losses could not be attributed to bacterial growth or aerial diffusion. During the first 24 h, blood Na+ increased from 150 to 280 mM, still far below ambient Na+ concentrations of 366 mM. Blood NH3 decreased nearly 5-fold and protein concentrations in blood declined about 40 g/l. Probably a reversal of normal Na+/NH4+ exchange occurs following a hyperosmotic shock, so that blood Na+ is hyporegulated using exogenous NH4+ as a counter-ion. During the first 24 h after transfer, net NH3 acquired by uptake could be used to increase intracellular NH3 concentrations as a prelude to increased synthesis of FAA. This model may compliment recent evidence of a CL-/HCO3- reversal in fish hyporegulating blood Cl- in seawater.