Annual changes in testicular activity and plasma steroid hormones in the golden rabbitfish Siganus guttatus (Bloch).

SUMMARY: Characteristics in annual reproductive activity of the male golden rabbitfish, Siganus guttatus, were examined by observing testicular histology and measuring plasma levels of steroid hormones, testosterone (T), 11-ketotestosterone (11-KT), and 17α,20β-dihydroxy-4-pregnen-3-one (DHP) by enzyme immunoassay (EIA). The testicular development was divided into four periods: immature (November–March); pre-spawning (April–May); spawning (June–July); and post-spawning (August–October). The immature period was associated with low gonadosomatic index (GSI) values, and the testicular lobules were exclusively occupied by spermatogonia. In the pre-spawning period the testes were characterized by a new generation of spermatogenic cells. Spermatozoa appeared in some testes at the beginning of that period, and became dominant in the testes and the sperm duct in the spawning period. In the post-spawning period, only spermatogonia were observed in the testis, but residual spermatozoa were still observed in the seminal lobules and the sperm duct. Plasma T, 11-KT and DHP levels simultaneously increased during the pre-spawning period, rose markedly at the spawning period, and remained low from the post-spawning to immature periods, suggesting that these changes apparently were related to the annual testicular activity in S. guttatus.