Flatfish
Flatfish, a fish with a body greatly compressed from side to side, and with both eyes on one side of its head. Flatfish differ from rays, which are compressed from back to belly, and have both eyes on top of the head. .
A flatfish is born with its eyes in the normal position, and the young fish swims normally. Before long—within two or three days in some species—one eye begins to move around the head, and the body grows wider and flatter. Finally both eyes are on one side of the head, and the fish swims and lies with this side up. The left eye moves to the right in some species, while the right eye moves to the left in other species.
A flatfish spends most of its time lying on one side on mud, sand, or gravel. The underside usually turns white, while the upper side is pigmented. The color of the upper side changes to match the background.
Most flatfishes live in the sea, but a few inhabit freshwater. Some flatfish are important food fishes.
Flatfishes belong to the order of Pleuronectiformes.