Flatfish larvae hatch bilaterally symmetrical and swim upright in the plankton like any other fish. In Atlantic halibut, metamorphosis rewrites that body plan. The left eye moves upward and around the dorsal midline until both eyes sit on the right side, cranial bones and connective tissue remodel to accommodate the shift, and pigmentation develops on the eyed side while the blind side stays pale.
The fish then settles and swims on its side, with the former left flank against the seabed. Atlantic halibut are almost always dextral, meaning right-eyed, which is a species-level convention: some other flatfish groups are consistently left-eyed, and a few show both.
Metamorphosis is also the hard part of rearing halibut in captivity, since incomplete eye migration and failed pigmentation are common and produce unsaleable fish. Precisely how thyroid hormone signalling coordinates the eye movement with the skeletal remodelling, and why the process is so easily disrupted by temperature and nutrition, is still being worked out. The species is assessed as Endangered.