Process of true ambicoloration in larval and juvenile Japanese flounder Paralichthys olivaceus:: An ultrastructural study

This study investigated the process of true ambicoloration in larval Japanese flounder Paralichthys olivaceus induced by retinoic acid. Larvae at around F-G stages (TL 8.0 +/- 0.75 mm) were immersed in 50 nM all-trans retinoic acid for 5 days and reared until 60 days post hatch (dph) in order to develop true ambicoloration. Skin of fish was sampled every 4 days after immersion and subjected to ultrastructural analysis by transmission electron microscope. Melanophages started to be observed in the skin on the blind side of control fish at 30 dph. However, there were no melanophages in fish treated with at RA at 30 dph. In addition, larval type melanophores were observed under the subepidermal collagenous lamella in the skin on the blind side of fish treated with atRA. Only chromatophores existed in skin on the blind side of the control fish at 40 dph. However, all three kinds of chromatophore existed on both sides of the skin of fish treated with atRA at 40 dph. These results suggested that the developmental process of pigmentation on the blind side of fish showing true ambicoloration mimicked that on the ocular side.