Temporal range: Paleocene–Recent 
|A camouflaged flatfish.|
A flatfish is a member of the order (Pleuronectiformes) of ray-finned demersal fishes, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migrating through and around the head during development. Some species face their left sides upward, some face their right sides upward, and others face either side upward.
The most obvious characteristic of the flatfish is its asymmetry, with both eyes lying on the same side of the head in the adult fish. In some families, the eyes are always on the right side of the body (dextral or right-eyed flatfish), and in others, they are always on the left (sinistral or left-eyed flatfish). The primitive spiny turbots include equal numbers of right- and left-sided individuals, and are generally less asymmetrical than the other families. Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on the seabed (benthos), and the extension of the dorsal fin onto the head.
The surface of the fish facing away from the sea floor is pigmented, often serving to camouflage the fish, but sometimes with striking coloured patterns. Some flatfishes are also able to change their pigmentation to match the background, in a manner similar to a chameleon. The side of the body without the eyes, which faces the seabed, is usually colourless or very pale.
The flounders and spiny turbots eat smaller fish, and have well-developed teeth. They sometimes seek prey in the midwater, away from the bottom, and show fewer extreme adaptations than other families. The soles, by contrast, are almost exclusively bottom-dwellers, and feed on invertebrates. They show a more extreme asymmetry, and may lack teeth on one side of the jaw.
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Species and species groups
Flatfishes lay eggs that hatch into larvae resembling typical, symmetrical, fish. These are initially elongated, but quickly develop into a more rounded form. The larvae typically have protective spines on the head, over the gills, and in the pelvic and pectoral fins. They also possess a swim bladder, and do not dwell on the bottom, instead dispersing from their hatching grounds as plankton.
The length of the planktonic stage varies between different types of flatfishes, but eventually they begin to metamorphose into the adult form. One of the eyes migrates across the top of the head and onto the other side of the body, leaving the fish blind on one side. The larva also loses its swim bladder and spines, and sinks to the bottom, laying its blind side on the underlying surface.
In 2008, a 50-million-year-old fossil, Amphistium, was identified as an early relative of the flatfish and transitional fossil. In a typical modern flatfish, the head is asymmetric, with both eyes on one side of the head. In Amphistium, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top of the head. The researchers concluded, "the change happened gradually, in a way consistent with evolution via natural selection—not suddenly, as researchers once had little choice but to believe."
…bony fish as a rule have a marked tendency to be flattened in a vertical direction…. It was natural, therefore, that when the ancestors of [flatfish] took to the sea bottom, they should have lain on one side…. But this raised the problem that one eye was always looking down into the sand and was effectively useless. In evolution this problem was solved by the lower eye ‘moving’ round to the upper side.
The European plaice is the principal commercial flatfish in Europe.
American soles are found in both freshwater and marine environments of the Americas.
Halibut are the largest of the flatfishes, and provide lucrative fisheries in Canadian and US waters.
The turbot is a large, left-eyed flatfish found in sandy shallow coastal waters around Europe.
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Flatfish is considered a Whitefish because of the high concentration of oils within its liver. Its lean flesh makes for a unique flavor that differs between each individual species. Methods of cooking include grilling, pan-frying, baking and deep-frying.
Timeline of genera
- Chapleau, Francois & Amaoka, Kunio (1998). Paxton, J.R. & Eschmeyer, W.N., ed. Encyclopedia of Fishes. San Diego: Academic Press. xxx. ISBN 0-12-547665-5.
- "Odd Fish Find Contradicts Intelligent-Design Argument". National Geographic. July 9, 2008. Retrieved 2008-07-17.
- Matt Friedman (2008). "The evolutionary origin of flatfish asymmetry". Nature Letters 454 (7201): 209–212. doi:10.1038/nature07108. PMID 18615083.
- Dawkins, Richard (1991). The Blind Watchmaker. London: Penguin Books. p. 92. ISBN 0-14-014481-1.
- "Flatfish BBC".
- Sepkoski, Jack (2002). "A compendium of fossil marine animal genera". Bulletins of American Paleontology 364: p.560. Retrieved 2011-05-17.
- Gibson, Robin N (Ed) (2008) Flatfishes: biology and exploitation. Wiley.
- Munroe, Thomas A (2005) "Distributions and biogeography." Flatfishes: Biology and Exploitation: 42-67.