Many major food fish are in this order, including flounder, soles, turbot, plaice, and halibut. Some flatfish, commonly known by this name, can actively camouflage themselves on the ocean floor.
More than 700 species are in the 11 families. The largest families are Bothidae, Cynoglossidae, Paralichthyidae, Pleuronectidae and Soleidae, with more than 100 species each (the remaining families have less than 50 species each). Some families are the result of relatively recent fractures. For example, the Achiridae were classified as a subfamily of Soleidae in the past, and the Samaridae were considered a subfamily of the Pleuronectidae. The Pleuronectidae can be further divided, as there are those who classify them into Paralichthodinae, Poecilopsettinae, and Rhombosoleinae families rather than subfamilies.
The taxonomy of some groups needs revision, as the last monograph covering the entire order was John Roxborough Norman's Monograph on Flatfish published in 1934. New species are described with some regularity and there are probably undescribed species left.
There are also hybrids, known in flatfish. The Pleuronectidae, of marine fish, have the highest number of hybrids reported. Two of the most famous intergeneric hybrids are found between plaice (Plleurchectys platessa) and flounder (Platichthys flesus) in the Baltic Sea, and between English sole Parophrys vetulus and Platichthys stellatus in Puget Sound. The offspring of the latter pair of species is popularly known as the single hybrid plant and was initially believed to be a valid species in its own right.
Flatfish are found in oceans throughout the world, ranging from the Arctic, through the tropics, to Antarctica. Most species are found at depths between 0 and 500 m (1600 ft), but some have been recorded from depths greater than 1,500 m (4,900 ft). It has not confirmed any of the zones abyssal or hadal. Among the deep-sea species, Symphurus thermophilus lives in congregation around sulfur pools at hydrothermal vents on the seabed. No other flatfish are known from hydrothermal vents. Many species will enter brackish or fresh water, and fewer plants (families Achiridae and Soleidae) and tongue (Cynoglossidae) are totally restricted to fresh water.
