What fish has both eyes on one side of its flattened body as an adult? The answer is the flatfish.
Flatfish are ray-finned fishes adapted to lie on one side on the seafloor. During development, one eye moves around the head to join the other, leaving both eyes on the upward-facing side. The lower side becomes a usually pale “blind side,” while the upper side is often camouflaged against sand, gravel, or reef habitat.
This dramatic change happens during juvenile metamorphosis. Young flatfish begin with a more typical, symmetrical fish shape and swim upright. As they mature, the skull and eyes remodel, the body becomes laterally compressed, and the fish settles into a bottom-dwelling adult lifestyle.
Flatfish are a diverse group, including flounders, soles, halibut, turbots, and plaice. The direction of eye migration varies: some lineages rest on the left side, some on the right, and the primitive genus Psettodes can include individuals facing either way. Thus “flatfish” is the appropriate group-level answer, while halibut and flounder are particular kinds of flatfish.