Sex change of adult initial-phase male wrasse, Halichoeres trimaculatus by estradiol-17β treatment

Sex steroids are considered major regulators of sex change processes in fish. Estrogen depletion is shown to be crucial for female–male sex change initiation; however, its role in male–female sex change is largely unknown. In the present study, we examined the effects of estradiol-17β (E2) treatments on testes of initial-phase (IP) males of the three-spot wrasse (Halichoeres trimaculatus), which naturally do not undergo male–female sex change. Sexually mature IP males were fed a diet containing E2 (low, 20 μg/g feed; high, 200 μg/g feed) for 6 or 12 weeks, and changes in gonadal structures were examined. Percentage of sex change varied with the dosage of E2 and the duration of treatment. All individuals treated with high-dose E2 for 6 weeks had ovaries with many immature oocytes; whereas 75% of individuals treated with low-dose of E2 for 6 weeks and sampled on the 12th week had ovaries with yolky oocytes and an ovarian cavity indicating a typical mature ovary. No testicular tissue was observed in sex-reversed gonads in both treatment groups. Contrary to the previous assumptions, present results suggest that IP male wrasses have the potential to undergo male–female sex change in response to exogenous estrogen. How the presence or absence of estrogen creates sexual plasticity in gonadal germ and somatic cells remains to be clarified.