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The effect of all‐trans retinoic acid (atRA) on pigmentation and skeletal formation of Japanese flounder Paralichthys olivaceus was investigated. Five groups of flounder larvae were fed live food enriched with 0.5 mL docosahexaenoic acid (DHA) 38G oil containing 10 mg of atRA/10 L of culture medium during different developmental stages; that is, A–B, C–D, E–F, G–H, and I. The control group was fed live food enriched with only DHA 38G oil. Flounder that were fed live food enriched with atRA during the A–B stages showed albinism, and mandible and severe caudal defects (albinism 75.7%, lower jaw defects 75%, caudal defects 100%). Occurrence of these abnormalities in other groups was 0%, 1–4%, and 4.5–10.7%, respectively. Administration of atRA during the A–B stages also caused a high number of vertebrae mainly in the caudal area. Moreover, additional abdominal vertebrae had formed in 25% of fish that were fed live food enriched with atRA in the A–B stages. These results indicate that the effect of atRA is dependent on the developmental stage of flounder larvae and they also suggest that morphological and color abnormalities in flounder were induced by atRA accumulated in live food (rotifers 13 mg/g; Artemia 1.6 mg/g), especially during the A–B stages.

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