flatfish, common name for any member of the unique and widespread order Pleuronectiformes containing over 500 species (including the flounder, halibut, plaice, sole, and turbot), 130 of which are American. Flatfishes are common in both the Atlantic and Pacific; many are important food and game fishes. All flatfishes have an unusual flattened body form well suited to life on the bottom. The development of the young flatfish recapitulates to some degree the probable evolutionary process. The newly hatched transparent larvae are bilaterally symmetrical, but soon the characteristic compression of the body develops and one eye
to the other side of the head—either the left or the right, depending on the species. Changes occur also in the skeletal and digestive systems; adults have only one dorsal and one anal fin, both without spines. The underside of the flatfish is pale and the top is colored to match the environment; some species, especially the flounders, are able to change their pigmentation. Flatfishes are divided into three groups: the soles, families Soleidae, Cynoglossidae, and Achiridae; and the flounders (including the halibuts and others), families Achiropsettidate, Bothidae, Citharidae, Paralichthyidae, Pleuronectidae, Samaridae, and Scophthalmidae, and the spiny turbots, family Psettodidae.
The American soles, of which there are several Atlantic and one Pacific species, have small, close-set eyes and small, twisted mouths with few or no teeth. They prefer warm, shallow water with a sandy or muddy bottom and are generally too small and bony for food. The hogchoker, or broad sole, and the tonguefish, family Cynoglossidae, are most common. The European species Solea solea, a 2-ft (61-cm) flatfish found from the Mediterranean to the North Sea, is a valuable food fish, the source of filet of sole (in the United States filet of sole is usually flounder).
The flounders are much larger fishes, including the fluke (Paralichthys), the halibut (Hippoglossus), the dab (Limanda), and the plaice (Pleuronectes). The smooth flounder is found on muddy bottoms in cold, shallow northern waters. The winter flounder (Pseudopleuronectes americanus) is an important food and game fish, taken in large numbers by trawlers. Like other flounders it migrates in winter to deeper waters to breed. It belongs to the righteye flounder family, Pleuronectidae. Similar is the summer flounder (Paralichthys dentatus), of the large-tooth flounder family, Paralyichthyidae, called fluke by fishermen, common from Maine to the Carolinas. The closely related California flounder, or California halibut (P. californicus), is prized sport fish found off California that weighs up to 50 lb (23 kg). The starry flounder, more brightly colored than its drab relatives, is a common Pacific species found from mid-California N to Alaska and W to Asia. Flounders feed on worms, crustaceans, and other small bottom invertebrates.
The European plaice is an important food fish, as is the American plaice. The American plaice is common at depths of from 20 to 100 fathoms on muddy or sandy bottoms, where it feeds on sea urchins, sand dollars, and other bottom life and grows to 30 in. (76.2 cm) and 14 lb (6.4 kg).
The halibuts are the largest flatfishes and are of great commercial importance. The Atlantic and the Pacific halibuts, Hippoglossus hippoglossus and H. stenolepis, respectively, are very similar, with large mouths and sharp, strong teeth. They feed voraciously on other fish and are found in colder waters. The maximum weight of a halibut is 600 lb (270 kg), but the usual specimens caught offshore at 100 to 400 fathoms weigh from 20 to 100 lb (9–45 kg); the male is generally much smaller than the female.
The commercially valuable tribe of European flatfishes called turbots is represented in American waters by a single species, Scophthalmus aquosus, commonly called the windowpane flounder, found on the Atlantic coast from Maine to the Carolinas. It is much smaller than its European cousins, rarely weighing over 2 lb (.9 kg), whereas the European turbots may reach 30 lb (13.5 kg).
Flatfishes are classified in the phylum Chordata, subphylum Vertebrata, class Actinopterygii, order Pleuronectiformes.