Seasonal Variations and Maturity Stages in Relation to Differences in Serum Levels of Gonadal Steroids, Vitellogenin, and Thyroid Hormones in the Common Dentex (Dentex dentex)

Abstract

Seasonal variations in serum concentrations of 17β-estradiol (E2), vitellogenin (Vg), testosterone (T), 11 ketotestosterone (11-KT), and thyroid hormones (T4, l-thyroxine; and T3, 3,5,3′-triiodo-l-thyronine) were investigated during the first, second, and third reproductive cycles in intensively reared populations of common dentex, Dentex dentex, and correlated with gonadal development and spawning. In females, there were baseline E2 values (<0.10 ng/ml) and negligible Vg concentrations during the postspawning and pregametogenesis period (June to December), and these increased thereafter to peak during the spawning period. Maximum T3 and T4 serum concentrations were found around spawning. There was a positive correlation during vitellogenesis and final maturation between Vg and T3 (r2=0.366). In addition, Vg and T3 concentrations were statistically higher in the stages of vitellogenesis and final maturation than at the other stages (P<0.001). Minimum T3 and T4 concentrations (October) coincided with the decrease in water temperature and the associated decrease in the daily feeding rate and the specific growth rate. In males, as in females, seasonal changes in serum levels of T and 11-KT were well correlated with gonadal development. The presence of males in the stage of completed spermiogenesis in December coincided with the surge in both androgens and this increase lasted until the end of the spawning period. There were no significant differences in serum T3 and T4 levels among the maturity stages. The observed seasonal changes in serum gonadal steroids and Vg reflected the pattern of oocyte development and the spawning behavior of common dentex and were typical of the patterns described in most multiple spawners studied to date. Thyroid hormones may enhance early ovarian development and stimulate vitellogenesis in female dentex.