Adult flatfish wear both eyes on one side of the head
Flatfish are ray-finned members of Pleuronectoidei that lie on one flank on the seafloor, their bodies flattened side-to-side. As adults both eyes sit on the upward face while the underside stays blind and pale. They rank among the most diverse demersal fishes and feed major fisheries.
Most species live between the shallows and 500 metres, a few below 1,500 metres. Some families put both eyes on the right, others on the left, while the primitive spiny turbots come in equal numbers of each. Chromatophores let some flatfish shift colour to match the bottom, much as some cephalopods do. A few advertise danger instead: Microchirus ocellatus has bold eyespots, several small tropical soles are poisonous, and juvenile Soleichthys maculosus mimic toxic Pseudobiceros flatworms in colour and swimming style.
Diet splits the group. Flounders and spiny turbots have strong teeth, chase smaller fish and sometimes hunt off the bottom, while soles stay on the seabed eating invertebrates and show more extreme lopsidedness, sometimes with teeth on only one side.
Every flatfish starts as a symmetrical larva drifting in the plankton, armed with spines and a swim bladder. Thyroid hormones then trigger metamorphosis: one eye travels over the top of the head, the spines and bladder disappear, and the young fish sinks to lie blind side down. Hybrids are common, and righteye flounders report more crosses than any other marine family, including plaice with flounder in the Baltic.
Richard Dawkins used the migrating eye in The Blind Watchmaker as a showcase of evolution. Fossil ear stones hint at flatfish 57 to 53 million years ago, and a 2008 study of 50-million-year-old Amphistium and Heteronectes found heads only partly asymmetric, one eye per side but one set higher, perhaps used while resting propped above the seafloor. Once given their own order, flatfish now sit inside the Carangiformes beside jacks and billfish.
Source: Flatfish
