Flatfish, common name for bottom-living, primarily marine fishes of class Actinopterygii, order Pleuronectiformes (Heterosomata). Among the flatfishes are some of the most valuable food fishes, eg, halibut, plaice, flounder, turbot and sole.
Distribution and Habitat
About 570 species in 11 families inhabit Continental Shelf waters of world oceans, but a few occur in deep seas and some ascend rivers. Most occur in temperate and tropical seas but some penetrate the Arctic waters. In Canada, 48 species in 3 families occur in coastal waters. The family Pleuronectidae, the right-eyed flounders, with 29 species (including halibut, genus Hippoglossus; plaice, genus Hippoglossoides; and the introduced European flounder in the Great Lakes), is best represented in Canadian waters. The other families are the Bothidae (left-eyed flounders, eg, sand dabs, genus Citharichthys-17 species) and the Cynoglossidae (tonguefishes-2 species).
The loss of bilateral symmetry sets the flatfishes apart from all others. Newly hatched flatfishes swim upright and are symmetrical, but a few days after hatching, one eye begins to migrate across the skull and eventually both eyes are on one side. The dorsal fin grows forward onto the head. Changes in jaw structure, pelvic fins and anatomy also occur. Finally, young flatfish drop to the bottom to lie eyed side uppermost, blind side on the bottom. The left eye migrates in some species; the right in others. Flatfish with both eyes on the right side are said to be right-eyed (dextral); those with both eyes on the left side are left-eyed (sinistral). The eyed side is pigmented; the blind side is usually white. Some flatfishes have a highly developed ability to vary body colouring to match the ocean bottom.
Most flatfishes are small (30-38 cm long) but the halibuts may attain lengths of 267 cm and weights up to 316 kg. The mouth is usually small (except in halibut). The lower jaw is prominent in most species but in true soles (found in European waters) the rounded snout projects beyond the mouth. Dorsal and anal fins are usually soft-rayed and long. Flatfishes are carnivorous, eating various fishes, crustaceans and bottom-dwelling invertebrates.
Significance of Fishery
Flatfishes are caught mainly by otter trawl, Danish seines or longlines; some are angled. In Canada, Atlantic and Pacific halibut (H. hippoglossus and H. stenolepis, respectively) are the most highly esteemed flatfishes and command the highest price on both domestic and foreign markets. The smaller flatfishes, known locally as flounder, sole, yellowtail, dab, witch or plaice, are marketed as fresh or frozen fillets under the name "sole".