The role of the gills in seawater adaptation inAnguilla dieffenbachii

Summary The mean serum sodium, chloride and potassium concentrations and serum osmotic pressure of freshwaterA. dieffenbachii are 140.4 mmol 1^–1,114.0 mmol 1^–1, 6.66 mmol 1^–1 and 307.7 mOsmol respectively. Gill tissue from freshwater specimens has a water content of 4234 mg g dry wt^–1 (80.9% wet wt), a chloride space of 1852 mg water g dry wt^–1 (35.2% wet wt) and an intracellular volume of 2449 mg water g dry wt (46.0% wet wt). Estimates of the intracellular sodium and potassium concentrations for the gill tissue of freshwater eels gave values of 28.9 mmol kg intracellular water^–1 and 126.5 mmol kg intracellular water^–1 respectively. On transfer of the fish to sea water serum concentrations of sodium and chloride and serum osmotic pressure show rapid initial increases followed by a more gradual decline eventually stabilising at new levels some 100 hours after transfer (Fig. 1). The serum sodium, chloride and potassium concentrations and serum osmotic pressure of seawater-adaptedA. dieffenbaehii are 162.8 mmol 1^–1, 151.0 mmol 1^–1, 6.70 mmol 1^–1 and 376.9 mOsmol respectively.On transfer to sea water the water content and chloride space of the gill tissue is reduced and the intracellular volume is initially decreased but is rapidly restored to its original value (Fig. 2, 3). At the same time intracellular sodium and potassium concentrations are increased but the latter is fairly rapidly restored to pre-transfer levels (Fig. 4).The changes in intracellular potassium concentration can be explained largely by the changes in intracellular volume but intracellular sodium concentrations remain high on transfer because of the increased serum sodium concentration. The initial increases in serum concentrations on transfer to sea water are caused partly by the removal of water and partly by the addition of sodium and chloride ions to the internal body fluids.