GnRHa-accelerated spermatogenesis in the testes of underyearling golden rabbitfish, Siganus guttatus (Bloch)

Golden rabbitfish, Siganus guttatus, do not attain spermiation in the first spawning season after hatching in northern Okinawa, Japan. Nevertheless, underyearling males possess spermatogenic germ cells at the late stages of development, including spermatozoa. The fate of those germ cells is still unknown. This study examined whether prespawning treatment with gonadotropin-releasing hormone agonist (GnRHa) may induce further development and/or maturation of those germ cells in an attempt to obtain expressible milt from underyearling golden rabbitfish. Cholesterol pellets containing three different doses of GnRHa (0 (control), 100 and 200 μg kg^{- 1} body weight) were implanted intramuscularly in underyearling rabbitfish. The implanted fish were examined 2.5 months later. At the end of experiment, mean gonadosomatic index (GSI) was significantly higher in the 200 μg kg^{- 1} group than in the control group. While the testicular histology of about 40% of the control males showed regression from the initial condition, 70-80% of males in both GnRHa-treated groups demonstrated advanced maturation stages, including spermiation. Mean plasma 11-ketotestosterone (11-KT) and testosterone (T) levels in the GnRHa-treated group were significantly higher than in the control group, while no significant changes were shown in 17α,20β-dihydroxy-4-pregnen-3-one (DHP) levels. In conclusion, GnRHa treatment induced the development and maturation of spermatogenic germ cells, leading to spermiation in underyearling male testes, demonstrating that GnRHa treatment could serve as a useful way to induce precocious spermiation in male golden rabbitfish.