Medaka (Oryzias latipes) as a model for hypoosmoregulation of euryhaline fishes

We examined the hypoosmoregulatory ability of a model fish, medaka (Oryzias latipes), in relation to the gill-chloride cells or mitochondrion-rich (MR) cells, and to cortisol. When the medaka were transferred from freshwater (FW) to 30% seawater (SW), muscle water content decreased by 8% after 2 h and normalized within 1 week. Size and density of MR cells in the gill filament increased after 1-2 weeks in SW. Immersion of medaka in FW containing cortisol (10 μg/ml) for 1-2 weeks also doubled the number of MR cells, and abolished the decrease in muscle water content after SW transfer. In SW-adapted medaka, prolactin (PRL; 10 μg/g body weight) injection reduced muscle water content. However, when cortisol (50 μg/g) was injected simultaneously with PRL, cortisol abolished the decrease in muscle water. We concluded that cortisol plays an important role in the SW adaptation of the medaka, whereas PRL may be involved in ion uptake. Medaka seems to be a good model fish useful for the study of osmoregulatory mechanisms in general.