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The flatfish are an (Pleuronectiformes) of ray-finned fish, also called the Heterosomata, sometimes classified as a suborder of Perciformes. The name means "side-swimmers" in Greek. 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" side upward, some face their "right" side upward, and others face either side upward.

Many important food fish are in this order, including the flounders, soles, turbot, plaice, and halibut. There are more than 400 species of this order. Some flatfish can camouflage themselves on the ocean floor.

Characteristics

The most obvious characteristic of the flatfish is their asymmetry, with both eyes lying on the same side of the head in the adult fish. In some families, the eyes are a lways on the right side of the body, and in others, they are always on the left. The primitive spiny turbots include equal numbers of right and left sided individuals, and are generally less asymmetrical than the other familes.^[1] 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 colored patterns. Some flatfish are also able to change their pigmentation to match the background, in a similar manner to a chameleon. The side of the body without the eyes, which faces the seabed, is usually colorless or very pale.^[1]

The flounders and spiny turbots eat smaller fish, and have well-developed teeth. They sometimes seek prey in the mid-water, away from the bottom, and show less 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.^[1]

Flatfish range in size from Tarphops oligolepis, measuring about 4.5 centimetres (1.8 in) in length, and weighing 2 grams (0.071 oz), to the Atlantic halibut, at 2.5 metres (8.2 ft) and 316 kilograms (700 lb).^[1]

Reproduction

Flatfish lay eggs that hatch into larvae resembling typical, symmetrical, f ish. 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.^[1]

The length of the planktonic stage varies between different types of flatfish, 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.

Origins

Flatfish have been cited as dramatic examples of evolutionary adaptation. For example, Richard Dawkins in The Blind Watchmaker, explains the flatfish's evolutionary history as:

…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.^[2]

The development of flatfish is thus considered to recapitulate their evolutionary history.^{[citation needed]}

In 2008, scientists discovered "50-million-year-old fossils have revealed an intermediate species between primitive flatfishes (with eyes on both sides of their heads) and the modern, lopsided versions, which include sole, flounder, and halibut."^[3] The research concluded that "the change happened gradually, in a way consistent with evolution via natural selection—not suddenly, as researchers once had little choice but to believe."^[3]

The asymmetric geometry of flatfish has been likened to the cubist paintings of Pablo Picasso, and is often perceived as being "imperfect", "grotesque", "strange", etc. It is likely that the asymmetry contributes to their survival by helping to disguise them on the ocean floor.

Species

Some "species" listed here are groups of species. See individual entries for further lists.

• Brill
• Dab
• Flounder
• Halibut
• Megrim
• Plaice
• Sole
• Tonguefish
• Turbot

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