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 parts per thousand 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 mug/ml) for 1-2 weeks also doubled the number of MR cells. and abolished the decrease in muscle water content after SW tr ansfer. In SW-adapted medaka, prolactin (PRL, 10 mug/g body weight) injecti on reduced muscle water content. However, when cortisol (50 mug/g) was inje cted simultaneously with PRL, cortisol abolished the decrease in muscle wat er. 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 g eneral. (C) 2001 Elsevier Science B.V. All rights reserved.