Sensitivity of the metamorphic events and morphogenesis of Japanese flounder Paralichthys olivaceus during larval development to thyroxine

Abstract

An experiment was conducted to determine the impact of exogenous thyroid hormone on metamorphic events and morphogenesis of larval Japanese flounder Paralichthys olivaceus. Six groups of 600 larvae each were immersed in 10 nmol thyroxine-sodium salt (T₄) during different developmental stages: A and B, C and D, E and F, C to F, and A to F. T₄ treatment during stages E and F resulted in premature metamorphosis including not only advanced settlement but also advanced eye migration of the juvenile fish. T₄ treatments also resulted in rapid absorption of the dorsal fin rays during stages C to F. As a result, metamorphosis to the bottom juvenile was induced by 10 nmol T₄. It was also found that the rate of metamorphosis was proportional to the T₄ immersion period. These results confirm that T₄ plays a major role in metamorphic events and morphogenesis during the early life stages of flounder. Thus, this study suggests that sensitivity to T₄ depends on the larval organ and the larval developmental stage.