Osmotic and ionic regulation in polar fishes

Serum ion concentrations increase in marine fishes and decrease in freshwater species as environmental temperature is lowered. Measurements from Antarctic fishes at temperatures near the freezing point of sea water show some of the highest serum ion concentrations and osmolarities found in anisosmotic fishes. Three species of Antarctic fishes were subjected to hyperosmotic and hypoosmotic conditions to determine to what extent these fish osmoregulate, and if intracellular isosmotic regulation serves as a means of compensation for the high serum osmolarities and ion concentrations observed in Antarctic fishes. The results indicate that Trematomus borchgrevinki Boulenger, T. nicolai Boulenger, and T. bernacchii Boulenger can effectively osmoregulate over a range of 500 to 1750 mOsm. Although these fish experience little change in environmental salinity under normal conditions, this finding indicates that the mechanisms responsible for anisosmotic regulation are intact in these fish. Intracellular ion concentrations and free amino acid determinations show that free amino acids do not serve as intracellular osmotic effectors within the muscles of T. borchgrevinki or T. bernacchii. It appears that osmotic balance is achieved by higher intracellular sodium and potassium concentrations in comparison with temperate water marine fishes.