Endocrine profiles during the short reproductive cycle of an autumn-spawning bitterling, Acheilognathus rhombea

When autumn-spawning bitterling, Acheilognathus rhombea (known as the kanehira bitterling in Japan) were held under a constant short photoperiod (10 hr light-14 hr dark) at 25 degrees during their natural breeding season, the females spawned regularly at intervals of about 5 days. During each 5-day period, the oocytes passed through three distinct physiological stages: vitellogenesis (3 days), maturation (1 day), and spawning (1 day). During the maturational stage, germinal vesicle breakdown (GVBD) was first observed at 18 hr and was complete by 21 hr. Ovulation occurred at 24 hr. Steroid measurements made on plasmas collected from fish throughout their reproductive cycle showed that estradiol-17 beta levels were about 12 ng/ml at the beginning of the vitellogenic stage and about 5 ng/ml during the other stages. Testosterone levels were about 2 ng/ml during the vitellogenic stage, but fell to about 0.5 ng/ml during maturation and spawning. Progesterone levels were very low throughout the cycle and showed little variation. Both 17 alpha-hydroxyprogesterone and 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one levels were both very low (0.5 ng/ml) up until 12 hr on Day 4 (maturation), at which time they increased rapidly. The levels reached peaks of 4 and 7 ng/ml, respectively, at 18 hr and reached basal levels again at 8 hr the following day. GtH-like glycoprotein levels also began to increase at 12 hr on Day 4, but peaked later, at the time of ovulation (24 hr).