SEASONAL CHANGES OF OESTRADIOL-17b AND TESTOSTERONE CONCENTRATIONS IN THE GONAD OF THE RAZOR CLAM SINONOVACULA CONSTRICTA (LAMARCK, 1818)

Seasonal variations in oestradiol-17b and testosterone levels of the razor clam Sinonovacula constricta from the Yellow River delta in China were investigated in relation to the reproductive cycle from April 2007 to March 2008. Histological analysis indicated that gametogenesis began in June–July when water temperature was 22.0–27.08C and ended during September–October when food was abundant. Oestradiol-17b and testosterone concentrations were determined by enzyme-linked immunosorbent assay in the gonad of S. constricta. Oestradiol-17b concentration in the female gonads was higher than that in the male gonads, while testosterone concentration in the male gonads was higher than that in the female gonads during sexual maturation. A positive correlation between oestradiol-17b and mean oocyte diameter was observed during the study period (r 0.664, P, 0.05), indicating that oestra-diol-17b may play an important role in regulating vitellogenesis. Oestradiol-17b and testosterone increased during sexual maturation in females and males, respectively, and then decreased markedly after spawning. These findings indicated that the fluctuations in the levels of both steroids were closely related to the reproductive cycle, thus oestradiol-17b and testosterone could have a role as endogenous modulators in sex determination, development and maturation of S. constricta