Pleuronectiformes

(Heterosomata), flatfish, an order of fishes.

The body of Pleuronectiformes is laterally compressed and, in the majority, broad and relatively short. Order Pleuronectiformes is related to order Perciformes, but members of the group are distinguished by their asymmetrical skull structure and by the location of both eyes on one side. The fins of almost all flatfish lack spiny rays. The swim bladder is absent and the body cavity is significantly shortened. The dorsal and anal fins are long, the caudal fin is usually isolated, and the ventral fins are displaced toward the throat, usually with no more than six rays. Flatfish range in length from 7 cm (Arnoglossus kessleri) to 4.5 m (halibut). Members of the order are predominantly marine, although some enter rivers. The adults lie on one side at the bottom of the sea. The blind side is only weakly pigmented.

There are two suborders of Pleuronectiformes. Suborder Pset-todoidei (Psettoidei) consists of one family with a single genus that comprises two species. The body is up to 70 cm long. One of the species, Psettodes erumei, is found in the Indian Ocean and the Red Sea; the other, P. belcheri, lives in the coastal waters of tropical western Africa. Suborder Pleuronectoidei comprises four families: Bothidae (turbot), Pleuronectidae (flounder), Soleidae (soles), and Cynoglossidae (tonguefishes). Members of the latter two families live mainly in tropical waters. Family Pleuronectidae comprises five subfamilies, two of which live in equatorial waters and two in the waters of the southern hemisphere.

The only members of the order found in the USSR are the Pleuronectinae (the subfamily of flounder and related groups), typically bottom marine fish inhabiting coastal and moderate depths (rarely found at depths exceeding 1,000 m). Amphiboreal distribution is characteristic of a number of species. Some species enter the mouths of rivers (for example, the river flounder, the starry flounder, and the arctic flounder).

The eggs of almost all flatfish are pelagic. The larvae are structually symmetrical and have a well-developed swim bladder; they inhabit the middle depths of the water, feeding on small planktonic crustaceans. As the larvae develop, their skulls become increasingly asymmetrical, the eyes migrate to one side of the head, the swim bladder disappears, the body thickens, and the young fish move to the bottom, lying with one side of their bodies on the bottom. Flatfish can quickly change their coloration in response to the color of the background. Adult flatfish feed on benthic invertebrates; some prey on other fish.

The majority of the Pleuronectinae have tasty flesh and are of commercial value. In the USSR the group in the Far East is the most varied (25 species); there are fewer species (nine) in the Barents and White seas. In arctic seas only the arctic flounder (Liopsetta glacialis ) is widely distributed. Along the shores of Kamchatka and in the northern part of the Sea of Japan the principal commercial flatfish are the yellowfin sole (Limanda áspera) and the sohachi flounder (Cleisthenes herzensteini); in the Barents Sea, the plaice (Platessa platessa), the American plaice, and the dab are commercially important. The Black Sea turbot, or kalkan, is caught commercially in the Black Sea. Halibuts are the most important as food.