Pleuronectiformes

(vertebrate zoology) The flatfishes, an order of actinopterygian fishes distinguished by the loss of bilateral symmetry.

View more Animal Life videos

One of the most distinctive orders of actinopterygian fishes, also called Heterosomata, composed of the flatfishes: halibut, plaice, flounders, soles, tonguesoles, and their allies. The striking feature of the group is the loss of bilateral symmetry (see illustration), a characteristic of almost all vertebrates. Like the ancestors of pleuronectiforms, young flatfishes are symmetrical and swim upright; early in life, however, one eye migrates across the top of the skull to lie on the same side as the other eye. This transformation is associated with deformation of the skull bones and nerves, a change in position so that the fish lies on one (the blind) side, partial or complete depigmentation of the blind surface, and sometimes modification and development of asymmetry in paired fins, dentition, squamation, visceral anatomy, and other structures. See also Actinopterygii.

Fourspot flounder (Paralichthys oblongus), of the Pleuronectiformes. 1 In. = 2.5 cm. (After G. B. Goode, Fishery Industries of the United States, sect. 1, 1884)