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Why do adult flatfishes have both eyes on the same side of the head?

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Body plan and sexual dimorphism in the common sole (Solea solea) and dusky flounder (Syacium papillosum). — pleuronectiform Body plan and sexual dimorphism in the common sole (*Solea solea*) and dusky flounder (*Syacium papillosum*).

Adult flatfishes have both eyes on the same side of the head because they are specialized benthic (bottom-dwelling) animals that habitually rest on one side of their bodies.12

While larval flatfishes are bilaterally symmetrical with an eye on each side of the head, they undergo a dramatic metamorphosis as they mature. During this process, one eye migrates around or through the head to the other side, allowing the fish to maintain a functional field of vision while lying flat against the substrate. This migration involves significant osteological changes, including the twisting of the skull bones, which results in the unique asymmetry characteristic of the order Pleuronectiformes.13

Once this transformation is complete, the fish settles to the bottom, with the ocular (eyed) side facing upward and often exhibiting pigmentation to mimic the surrounding environment, while the blind side remains unpigmented and rests against the seafloor.132