Flatfish, any of an order (Hetero somata) of plate-shaped fish with both eyes on one side of the head. A flatfish starts life as a normal-looking fish, but after a few days, one eye starts moving around the head, the dorsal fin grows along the head, and the mouth becomes twisted. The fish then lies on its side on the bottom. The underside stays pale, and the top becomes pigmented and can change color to match the background. Some of the 500 species are important food fishes, including flounder, halibut, sole, and turbot.