| Summary: | In the present study, we investigated the function of thyroid hormones (TH) in visual remodeling during Japanese flounder (<i>Paralichthys olivaceus</i>) metamorphosis through cellular molecular biology experiments. Our results showed that the expression of the five opsin genes of the flounder were highest in eye tissue and varied with the metamorphosis process. The expression of <i>rh</i>1, <i>sws</i>2<i>aβ</i> and <i>lws</i> was positively regulated by exogenous TH, but inhibited by thiourea (TU) compared to the control group. In addition, there was a significant increase in <i>sws</i>2<i>aβ</i> and <i>lws</i> in the rescue experiments performed with TU-treated larvae (<i>p <</i> 0.05). Meanwhile, T3 levels in flounder larvae were increased by TH and decreased by TU. Based on the differences in the expression of the three isoforms of the thyroid hormone receptor (TR) (Trαa, Trαb and Trβ), we further hypothesized that T3 may directly or indirectly regulate the expression of <i>sws</i>2<i>aβ</i> through Trαa. This study demonstrates the regulatory role of TH in opsins during flounder metamorphosis and provides a basis for further investigation on the molecular mechanisms underlying the development of the retinal photoreceptor system in flounders.
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