Development of sensitivity to maturation-inducing steroids and gonadotropins in the oocytes of the tobinumeri-dragonet, Repomucenus beniteguri, Callionymidae (Teleostei).

The effectiveness of steroids and gonadotropins (GtHs) in inducing final oocyte maturation was examined at different times of the day (0100, 0500, 0700, 0900, 1100, 1300, 1700, 2100 hr) in a daily spawning marine teleost, the tobinumeri-dragonet, Repomucenus beniteguri. The responsiveness of oocytes to GtHs and steroids was different at various times of day. The sensitivity of oocytes to hormones was apparent only during a certain period of the day (0100-0900 hr). Germinal vesicle breakdown (GVBD) could be induced by GtHs but not by steroids at 0100 hr. At 0500 hr, not only GtHs but also steroids at higher doses induced GVBD and ovulation. Oocytes underwent GVBD and ovulation at low doses of steroids at 0900 hr. GVBD and ovulation spontaneously occurred at 1100 and 1300 hr without any hormonal treatment. In addition, diameter and histological changes of ovarian oocytes were investigated. Ovarian oocytes were smaller than 420 microns, and only one oocyte group existed in distribution of oocyte frequency at 2100 and 0100 hr. With the increase in size of oocytes, two distinct groups appeared at 0500 hr. Larger oocytes completed GVBD at 1100 hr. Ovulation occurred between 1300 and 1700 hr and oviposition was completed between 1700 and 2100 hr daily. These results clearly show that the oocyte of the dragonet possesses a daily maturation rhythm. Responsiveness of oocytes to GtHs appeared earlier than responsiveness to steroids. This suggests that sensitivity to steroids is induced by GtH.