Permanent sex inversion in 1-year-old juveniles of the protogynous dusky grouper (Epinephelus marginatus) using controlled-release 17α-methyltestosterone implants

Long-term treatments (April–June 2004) with 17α-methyltestosterone (MT) and/or human chorionic gonadotropin (hCG) were used to induce precocious sex change in 1-year-old juveniles (mean ± SD body weight of 131 ± 25 g) of the protogynous dusky grouper (Epinephelus marginatus). The MT treatment was given once a month using controlled-release implants (1.5 mg per implant, ∼11 mg kg− 1 body weight), whereas hCG was injected intramuscularly once a month (MT/hCG group) or every 2 weeks (hCG group). In vitro mean (± SEM) daily and total MT release from the implants was 22.6 ± 5.4 μg day− 1 implant− 1 and 686 ± 113 μg implant− 1, respectively. In vivo, plasma MT levels of European sea bas (Dicentrarchus labrax) used as model fish (203 ± 41 g body weight) and given a single MT implant were 18.5 ± 0.5 ng ml− 1 one day after implantation, declined to 9.6 ± 2.7 ng ml− 1 by day 11 and remained stable between 7.9 and 5.5 ng ml− 1 until day 32. Plasma MT levels were significantly elevated (2-way ANOVA, P < 0.01) in dusky grouper juveniles in response to the MT treatment, both 6 and 12 weeks after beginning the treatment. On the contrary, no significant differences (2-way ANOVA, P > 0.05) between MT/hCG-treated and control fish were observed in plasma testosterone (T) or 11-keto T (11-KT). Pituitary luteinizing hormone (LH) content was significantly lower in both the MT/hCG and hCG groups, compared to controls (2-way ANOVA, P < 0.001), while plasma LH was below detection limits in all treatments. At the onset of the experiment all but one fish were already differentiated into females, having ovaries with primary oocytes. No signs of vitellogenesis or sex inversion were observed in fish from the control and hCG groups during the study. On the contrary, the MT implants induced complete sex change after 12 weeks, with the gonads organized in lobules and cysts filled with germ cells at all stages of spermatogenesis, including spermatozoa. This sex inversion was permanent and fish underwent complete spermatogenesis again during the following year. The results demonstrate that the produced MT-implants, but not the hCG injections, were highly efficient in inducing both sex inversion and complete spermatogenesis in pre-pubertal dusky grouper at the completion of their second year in life. Production of male fish at such an early age may dramatically reduce by many years the time required for natural puberty, sex inversion and the production of sperm by dusky grouper reared under culture conditions.