ANNUAL CHANGES IN OOCYTE DEVELOPMENT AND SERUM VITELLOGENIN LEVEL IN THE RABBITFISH SIGANUS-CANALICULATUS (PARK) IN OKINAWA, SOUTHERN JAPAN

Annual changes in oocyte development in the ovary and serum vitellogen in, an egg yolk precursor, were investigated in the rabbitfish, Siganu s canaliculatus, in Okinawa, southern Japan. The progression of oocyte development was histologically divided into eight stages; the chromat in-nucleolus, peri-nucleolus, oil-droplet, primary yolk, secondary yol k, tertiary yolk, migratory nucleus and maturation stages. The oocytes laden with yolk first appeared in March. From April through June, the stage composition of oocytes was characterized by a high percentage o f oocytes advanced beyond the primary yolk stage and was correlated wi th the increase of gonado-somatic index (GSI), Post-ovulatory follicle s could be observed in the ovaries in May, The GSI values gradually de creased after June and were maintained at low levels from October thro ugh February. During these months, the ovaries were occupied by the im mature oocytes at the chromatin-and peri-nucleolus stages. Vitellogeni n, which was measured immunologically with rabbit anti-rabbitfish vite llogenin antiserum, changed in accordance well with the monthly fluctu ation of GSI as well as development of oocytes laden with yolk. These results suggest that S. canaliculatus starts vitellogenesis in March a nd spawns several times in the following three months in Okinawa.