Effect of thyroid hormones on the stage-specific pigmentation of the Japanese flounder Paralichthys olivaceus

Abstract

Three experiments were carried out to clarify the effect of thyroid hormones on the pigmentation of larval Japanese flounder Paralichthys olivaceus. The first two experiments were conducted to investigate the critical concentration of thyroxine (T-4) which causes abnormal pigmentation, and the third was to determine the sensitive stage of larval development at which albinism is induced by exogenous T-4. The larvae treated with over 10 nM T-4 showed a significant increase of albinism in Experiments 1 (concentration: 0, 1, 10, 50 nM) and Experiment 2 (concentration: 0, 1, 5, 10 nM). in Experiment 3, seven groups were immersed in 10 nM T-4 at specific stages: between A and B, C and D, E and F, G and I, C and F and A and F. The groups of hormonal treatment at stages E and F (prometamorphosis) produced more than 90% albinism, indicating that differentiation of adult-type melanophores took place at this point. Another 4 groups of larvae were separately treated with levels of 10 nM T-4 at stages A and B, C and D, E and F and G to I. T-4 content of larvae increased corresponding to the developmental stage 180 ng/g-wet weight in A and B, 740 in C and D, and 1350 in E and F, but decreased to 130 in stage G to I (n=3, p<0.01). Changes in the body content of T-4 were reflected in the groups with higher incidence of albinism. The present experiment has revealed the T-4 sensitive period for the induction of albinism in Japanese flounder. The absence of normal coloration in the juveniles after T-4 treatment clearly indicates the involvement of the thyroid in pigmentation at the early stages of development.