Growth, energetics and the cortisol-hepatic glucocorticoid receptor axis of medaka (Oryzias latipes) in various salinities

We examined growth of euryhaline Japanese medaka (Oryzias latipes) after transfer to freshwater or seawater from isotonic saline. Growth was unaffected by the different salinities for 1week, but the body weight increase and BMI of fish kept in freshwater for 2-3weeks were significantly higher than those in the isotonic controls. These results may reflect the usual habitat of this species. To assess the basis for the difference in growth, energetics and the hepatic stress axis were evaluated 1week after the transfer. Unexpectedly, despite the higher growth rate, the rate of routine oxygen consumption was significantly higher in freshwater. Plasma cortisol levels in freshwater were significantly higher than those in seawater, and the mRNA levels of the glucocorticoid receptor (GR1) in the liver were significantly lower in freshwater and seawater, compared to that in isotonic saline. Branchial Na⁺/K⁺-ATPase activities were also reduced significantly in freshwater and seawater, compared to that in isotonic saline. The higher levels of hepatic GR1 expression and branchial Na⁺/K⁺-ATPase activity in isotonic salinity than those in freshwater and seawater for 1week may account for the lower growth rate under the isotonic condition. After 3weeks, however, the Na⁺/K⁺-ATPase activity in seawater was significantly higher than that in freshwater. No significant difference in growth rate between freshwater and seawater groups indicates that medaka is a good model for studies of hypo- and hyperosmotic adaptations, since osmoregulation is not strongly associated with size and growth.