Changes in tissue thyroxine level of metamorphosing Japanese flounder Paralichthys olivaceus reared at different temperatures.

Abstract

Effects of thyroid hormones and rearing temperature on Japanese flounder metamorphosis were studied by measuring tissue concentration of thyroxine (T4) by radioimmunoassay. Larvae reared at 15 or 22°C of various developmental stages were quickly frozen and stored at about -32°C until assayed. Non-detectable values of T4 were obtained for pre-metamorphic larvae at both 15 and 22°C. The T4 level increased to a maximum of 16.2ng/g during late metamorphosis, and then decreased slightly. Higher rearing temperature accelerated growth and metamorphosis and also induced earlier increase in tissue T4 levels. Comparison of larvae of similar developmental stages reared in different temperatures revealed no significant difference in T4 levels. These results indicate that thyroid hormones play an important role in Japanese flounder metamorphosis.