Osmotic and Ionic Regulation in the Far Eastern Big-Scaled Redfin Tribolodon hakonensis Günther, 1877 (Cypriniformes: Cyprinidae) in the Sea and in Rivers

The values of the osmolality and the contents of sodium, potassium, as well as the contents of the total, free, and bound water fractions in blood serum, muscles, liver, and brain that are characteristic for fish in the sea and in rivers were determined for the far eastern big-scaled redfin, Tribolodon hakonensis . The osmolality associated with the content of inorganic ions (Osm inorg ) was the same in the internal environment (blood serum), in the muscles, liver, and brain of redfins caught in the sea at the gonad maturity stage IV. In fish caught in the sea in pre-spawning period (gonad maturity stage V) and in rivers after spawning (gonad maturity stage VI-II) or in wintering period (gonad maturity stage IV), Osm inorg in muscles, liver, and brain was significantly lower than in the blood serum. To achieve osmotic equilibrium with the internal environment, the lack of electrolytes in body tissues was compensated by accumulation of organic osmolytes. Migration of the redfin from rivers to the sea was associated with a significant increase in Osm inorg in the blood serum, by 9.4–18.7% on average. As an adaptation in response to this increase, muscle tissue cells increased the concentration of sodium and potassium, while liver and brain cells accumulated potassium.