Diurnal rhythm of oocyte development and plasma steroid hormone levels in the female red sea bream, Pagrus major, during the spawning season

Diurnal maturation rhythm in the female red sea bream, Pagrus major, which spawns every day during the spawning season and has an asynchronous-type ovary containing oocytes at various stages of development, was studied by histological investigation of the ovary. Plasma levels of estradiol-17β and testosterone corresponding to the ovarian maturation rhythm were also investigated. The fish had a diurnal ovarian maturation rhythm in which the most advanced oocytes in the ovary finished germinal vesicle breakdown at 07.00 h, arrived at a mature state at 10.00 h, and ovulation began at 13.00 h. Spawning mainly occurred between 18.00 and 19.00 h. Plasma estradiol-17β levels reached a peak of about 1200 pg/ml in fish with pre-mature stage oocytes at 07.00 h, followed by a sharp drop in ovulated fish at 13.00 h (to about 200 pg/ml); the levels showed a tendency to increase from the time of postovulation to the next germinal vesicle breakdown. Testosterone levels decreased (to about 20 pg/ml) at ovulation time (13.00 h) in parallel with the estradiol-17β levels, but were largely unchanged (70-110 pg/ml) throughout the rest of the ovarian maturation cycle. These results are discussed in relation to other studies on the role of these steroid hormones responsible for the ovarian maturation of teleosts.