The annual reproductive cycle of the female golden rabbitfish, Siganus guttatus, was investigated by histological observation of ovaries and by immunological detection of plasma steroid hormone and vitellogenin (VTG) concentrations. The ovarian development was histologically
divided into three periods, the immature (August–April), the pre-spawning (May), and the spawning (June–July) periods. In the immature period ovaries were characterized by oocytes at the perinucleolus stage. Vitellogenic oocytes appeared in some ovaries from the prespawning period,
and became predominant in the spawning period. In the spawning period, empty follicles were also observed in some ovaries. Plasma levels of steroid hormones, estradiol-17β(E2), testosterone (T), 17α,20β-dihydroxy-4-pregnen-3-one (DHP) and 17α,20β,21-trihydroxy-4-pregnen-3-one
(20β-S), and VTG rose markedly in the spawning period, but remained low (or not detectable) during the pre-spawning and immature periods. The present results suggest that changes in the plasma steroid hormone and VTG levels are correlated to the annual ovarian activity of S. guttatus
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