Steroid hormone levels and bi-directional sex change in Gobiodon histrio

Sex change in the coral-dwelling goby Gobiodon histrio was induced by placing two adult fish of the same sex on a coral colony. The sex change of individual fish was confirmed using histology, and whole-body concentrations of the gonadal steroids testosterone (T), 11-ketotestosterone (11-KT), and 17β-oestradiol (E2) were examined. The results show that T, 11-KT and E2 occurred in both female and male G. histrio. E 2 concentration in females was twice that in males, while concentrations of T did not differ between the sexes. Contrary to predictions, concentrations of Tand E2 did not differ between fish that changed sex and those that did not. Most samples had 11-KT concentrations below minimum levels of detection (i.e. <0.15 ng ml-1) and were therefore not analysed statistically The results suggest that: (i) specific activation or de-activation of theT-E2 (aromatase) pathway is a probable candidate for mediating serial adult sex change in G. histrio, and (ii) low levels of 11-KT may be important in allowing serial adult sex change in G. histrio. © 2003 The Fisheries Society of the British Isles.