Opposite-directional sex change in functional female protandrous anemonefish, Amphiprion clarkii: effect of aromatase inhibitor on the ovarian tissue

Introduction: The anemonefish, Amphiprion clarkii, is a protandrous hermaphrodite. Under appropriate social conditions, male fish can become female. Previous studies indicated that estrogens are important regulators of sex change in this fish. However, the mechanism of sexual plasticity in the gonad of this fish is still unknown. To elucidate the mechanisms underlying the sexual plasticity in the ovary of female anemonefish, an aromatase inhibitor (AI, 500 mu g/g diet) was administered to the functional female fish for 80 days. Results: The levels of estradiol-17 beta (E2) in the fish treated with AI were significantly lower than those in the control group. Three out of five fish had ambisexual gonads with active spermatogenic germ cells in the ovarian tissue. However, female fish in the AI-treated group prior to treatment and those in the control group displayed no testicular characteristics in their developed ovaries. This result strongly suggests that germ cells with bipotentiality or spermatogonial cells remain in the functional ovaries of anemonefish following sex change from functional males to functional females. There is a possibility that estrogen depletion due to AI treatment might have caused the opposite-directional sex change from functional female to male in the anemonefish. Conclusions: The anemonefish keeps their high sexual bipotential in the ovary after sex change.