Asymmetrical development of bones and soft tissues during eye migration ofmetamorphosing Japanese flounder, Paralichthys olivaceus

The symmetrical body of flatfish larvae dramatically changes into an asymme trical form after metamorphosis. Eye migration results in the most signific ant asymmetrical development seen in any vertebrate. To understand the mech anisms involved in eye migration, bone and cartilage formation was observed during metamorphosis in laboratory-reared Japanese flounder, Paralichthys olivaceus, by using whole-body samples and histological sections. Most of t he hard tissues of the cranium (parasphenoid, trabecular cartilage, supraor bital canal, and supraorbital bar) exist symmetrically in the larval period before metamorphosis and develop by twisting in the same direction as that in which the eye migrates. An increase in skin thickness beneath the eye w as observed only on the blind side at the beginning of eye migration, this was the first definitive difference between the right and left sides of the body. The pseudomesial bar, a peculiar bone present only in flatfishes, de veloped from this thick skin and grew dorsad. Novel sac-like structures wer e found and named retrorbital vesicles. The retrorbital vesicle of the blin d side grew larger and faster than that of the ocular side when the right e ye moved most dramatically, whereas no difference was observed between the volume of right and left connective tissue in the head. The asymmetrical pr esence and growth of the pseudomesial bar together with inflation of the re trorbital vesicle on the blind side may be responsible for right eye migrat ion during metamorphosis in the Japanese flounder.