Human chorionic gonadotropin (hCG) induces gonad reversal in a protogynous fish, the bluehead wrasse, Thalassoma bifasciatum (Teleostei, Labridae).

Female to male successive hermaphroditism (protogyny) is common in several groups of marine fish. Thalassoma bifasciatum, the bluehead wrasse (Labridae), found in the reefs of the Caribbean normally undergoes sex reversal after receiving behavioral cues. This report deals with the successful use of human chorionic gonadotropin (hCG) in inducing gonad reversal in this species. Eighty percent (n = 40) of the treated fish showed signs of reversal in 1-6 weeks; only 11% (n = 54) of the control (nontreated) group showed signs of reversal during the same period. The number of fish undergoing reversal increased with the length of the treatment period, 55% after 1 week, 100% after 6 weeks. A bluehead color pattern, typical of the terminal male phase, also appeared more frequently in the longer treated groups. To determine the efficacy of hCG in inducing gonad reversal, morphological criteria for reversal had to be established. For the majority of treated fish, the presence of both spermatogenic cysts and degenerating oocytes was sufficient to label these gonads unambiguously as undergoing reversal. However, at the extremes of this process, i.e., onset (early) and endpoint (late) stages, ambiguity could arise in identifying a gonad as undergoing reversal, and, therefore, key criteria were established for these stages. In our hands, the most consistent and reliable criterion for the early stage reversal was appearance of male germ line cells, clusters of "spermatogonial-like" cells. For the late stage, recognizable remnants of late-state oocytes had to be present. Some details of the histological changes that occur during early, middle, and late stages of reversal are also described. It is suggested that these results with hCG shed new light on the endocrine control of gonad reversal in T. bifasciatum.