Osmotic and ionic regulation of the prawn Macrobrachium rosenbergii (De Man) adapted to varying salinities and ion concentrations

1. 1. Osmotic and ionic regulation of certain key ions (Na⁺, Cl^-, K⁺, Ca²⁺, Mg²⁺) were determined in Macrobrachium rosenbergii maintained for long periods in freshwater, 15‰ and 24‰ diluted sea-water and in waters of varying ion compositions (three different continental brackish water sources). 2. 2. Blood osmotic concentrations were regulated hyperosmotically in a range of 450-500 mOsm/1 with the exception at 24‰ sea-water where the blood osmotic concentration rose to 636 mOsm/l. 3. 3. Na⁺ and Cl^- blood concentrations were maintained hyperionic in all media except for 24‰ sea-water in which both ions were hypoionic. 4. 4. Blood K⁺ and Ca²⁺ concentrations were hyperionic in all treatments while Mg²⁺ was maintained as a rule hypoionic. 5. 5. In freshwater, M. rosenbergii produced urine with a urine/blood ratio of 0.02 and 0.024 for Na⁺ and Cl^- respectively suggesting hypo-osmotic urine. 6. 6. In 15‰ sea-water the U B ratio is approximately 1-suggesting iso-osmotic urine. 7. 7. It is concluded that Macrobrachium rosenbergii has the ability to regulate and maintain its hemolymph osmolality and ionic composition (Na⁺, Cl^-, K⁺, Ca²⁺ and Mg²⁺) in media varying from freshwater to concentrations approaching iso-osmoticity. 8. 8. Changes in relative concentrations of various ions in media-typical of saline waters having a nonmarine origin-seem to have no significant influence on the hemolymph composition in prawns that managed to survive and grow in them.