DAILY SPAWNING AND DEVELOPMENT OF SENSITIVITY TO GONADOTROPIN AND MATURATION-INDUCING STEROID IN THE OOCYTES OF THE BAMBOOLEAF WRASSE, PSEUDOLABRUS-JAPONICUS

The cycle of oocyte development of the bambooleaf wrasse, Pseudolabrus japonicus, was studied to elucidate the endocrinological mechanism of oocyte maturation in a marine teleost. A single female reared with tw o males spawned every day for 17 days in captivity, indicating that th is species is a daily spawner. Ovarian histology revealed that germina l vesicle migration of the largest oocytes progressed from 12:00 to 3: 00 h, and germinal vesicle breakdown (GVBD) was completed at 6:00 h. O vulation and spawning occurred between 6:00 and 9:00 h. The effectiven ess of human chorionic gonadotropin (HCG) and 17 alpha,20 beta-dihydro xy-4-pregnen-3-one (17,20 beta-P), which is one of the most potent ste roidal inducers of GVBD in bambooleaf wrasse oocytes, in inducing fina l oocyte maturation was examined at eight different times of the day. The responsiveness of the oocyte to HCG and steroid differed at differ ent times of the day. The GVBD could be induced by HCG but not 17,20 b eta-P at 9:00 h. Between 12:00 and 18:00 h, not only HCG but also 17,2 0 beta-P induced GVBD. Both GVBD and ovulation spontaneously occurred between 0:00 and 6:00 h without any hormonal treatment. These results clearly showed that the oocyte of the bambooleaf wrasse possessed a di urnal maturation cycle. Responsiveness of oocytes to HCG appeared earl ier than responsiveness to 17,20 beta-P. This suggests that sensitivit y to 17,20 beta-P is induced by gonadotropic hormone (GTH).