Annual changes in ovarian histology, plasma steroid hormones and vitellogenin in the female golden rabbitfish, Siganus guttatus (Bloch)

The annual reproductive cycle of the female golden rabbitfish, Siganus gutt atus, was investigated by histological observation of ovaries and by immuno logical detection of plasma steroid hormone and vitellogenin (VTG) concentr ations. The ovarian development was histologically divided into three perio ds, the immature (August-April), the pre-spawning (May), and the spawning ( June-July) periods. In the immature period ovaries were characterized by oo cytes at the peri-nucleolus stage. Vitellogenic oocytes appeared in some ov aries from the pre-spawning 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-17 beta (E-2), testos terone (T), 17 alpha ,20 beta -dihydroxy-4-pregnen-3-one (DHP) and 17 alpha ,20 beta ,21-trihydroxy-4-pregnen-3-one (20 beta -S), and VTG rose markedl y in the spawning period but remained low (or not detectable) during the pr e-spawning and immature periods. The present results suggest that changes i n the plasma steroid hormone and VTG levels are correlated to the annual ov arian activity of S. guttatus.