Changes in Serum Steroid Hormones during Ovarian Development in the Captive Common Japanese Conger Conger myriaster (Brevoort)

摘要

Seasonal changes in gonadosomatic index (GSI), ovary and pituitary histology, and serum concentrations of estradiol-17 β (E_2) and testosterone (T) were investigated in the captive common Japanese conger Conger myriaster. The GSI increased from December to June then, after peaking at 4.8 in July, decreased to its initial level by September. Although at the start of the experiment, oocytes were in the oil droplet stage, ovaries spontaneously started vitellogenesis from December to April in accompaniment with the increase in GSI. With the peak in GSI in June, oocytes in most fish were at the vitellogenic stage, but atretic oocytes were observed in the ovaries of some; in August, all showed atretic oocytes. Histochemical observations of the pituitaryrevealed that aldehyde-fuchsin (AF) positive cells in the region of the proximal pars distalis increased with progression of vitellogenesis from December to June, but they decreased with the increase in atretic oocytes. Serum E_2 levels increased to a peak of 3.53 ng/ml at the initiation of vitellogenesis in January, but subsequently decreased to the initial level and remained low during vitellogenesis. Average serum T levels dramatically increased to a peak of 7.93 ng/ml in fish showing early atresia of vitellogenic oocytes in June, followed by a sharp drop to initial levels (3.86 ng/ml) in the late atretic stage.