Profiles of thyroid hormones and mRNA expression for thyroid stimulating hormone in Japanese eel during downstream migration

To understand the roles of thyroid hormones in downstream migration of anguillid eels, we examined the profiles of serum thyroid hormones (thyroxine, T4 and triiodothyronine, T3) and thyroid stimulating hormone β-subunit (TSHβ) mRNA expression in relation to habitat use, silvering, and seasonal changes. Female Japanese eels, Anguilla japonica, were collected in the brackish Hamana Lake and its freshwater rivers from July to December. The habitat use history of the eels was determined by otolith analysis of strontium and calcium concentrations. Thyroid hormone levels were measured by enzyme immunoassay and expression levels of TSHβ mRNA were determined by real-time quantitative PCR analysis. There were no consistent differences in the thyroid hormones or TSHβ expression levels between different habitats or seasonality. In addition, there were no or weak significant differences in thyroid hormones levels and TSHβ mRNA expression levels between migrant and other eels, although thyroid hormones have been shown to regulate endocrine control of downstream migratory behavior in some fishes. These findings suggest that thyroid hormones have play a minor role in downstream migration of anguillid eels