Changes in serum thyroid hormone levels and thyroid gland activity of artificially maturing female Japanese eel（Anguilla japonica）

In order to evaluate the involvement of the thyroid gland in reproduction, thyroid func-tioning was described in female cultivated Japanese eel (Anguilla japonica ) in progressive stages of sex-ual maturation induced by chum salmon pituitary homogenate (SPH) treatment. Serum thyroid hor-mones, thyroxine (T4) and triiodothyronine (T3) were also measured in each stage. Thyroid gland ac-tivity (epithelial cell height) was high before SPH injection (previtellogenic stage), further increasing at the early vitellogenic stage, thereafter decreasing to late vitellogenic and migratory nucleus stages. Theprofiles of both T3 and T4 changed during vitellogenesis, being high during previtellogenic and earlyvitellogenic stages, and subsequently declining, thus mimicking thyroid gland activity. These resultssuggest that the thyroid has relation with eel ovarian development during artificial maturation.