Cortisol is necessary for seawater tolerance in larvae of a marine teleost the summer flounder

Larval-stage summer flounder (Paralichthys dentatus) were immersed in the corticosteroid-receptor blocker RU486 to test the effects of cortisol deficiency on salinity tolerance. Premetamorphic larvae held at 10 (near isosmotic) or 30 (hyperosmotic) parts per thousand (‰) seawater survived well over 5 d in 0, 0.012, or 0.12 μM RU486. However, at concentrations of 1.2 or 3.6 μM RU486, mortality was significantly greater for larvae in 30‰ compared to larvae in 10‰. In a separate experiment, the ability of RU486 to inhibit tolerance to hyperosmotic medium (30‰) was confirmed; immersion at 1.2 μM RU486 induced mortality of larvae in the metamorphic climax stage held at 30‰, but not 0 or 10‰. Mortality due to RU486 in pre- or prometamorphic stage larvae was prevented by concurrent immersion in cortisol at concentrations approximately 10-200 times greater than RU486, indicating that the action of RU486 was specific to antagonism of cortisol. The efficacy of 1.2 μM RU486 in reducing survival in 30‰ was found to be stage-dependent and exhibited the following hierarchy for fastest time to 50% mortality: prometamorphosis > metamorphic climax > premetamorphosis. In a 5-d pretreatment of pre- or prometamorphic larvae by immersion in 20 μM cortisol and/or 0.12 μM RU486 at 30‰, only RU486 had a limited effect on decreasing survival when larvae were challenged with abrupt exposure to 50‰. In total, the results evidence for the first time a necessary role for cortisol in seawater tolerance of a larval marine teleost.