Note: Information is dated. Do not rely on it.Flounder. One of the flat fishes, family Pleuronectidae, characterized by a flattened oval or elliptical body which is whitish beneath and dark colored above. In the very young the eyes grow one on each side of the head and the body is vertical in the water like a sunfish. As it develops, the head becomes twisted so that both eyes are on the upper side of the head, while the body now becomes horizontal instead of vertical. Many of the flounders are important food fishes. They are found in cold, temperate, and tropical seas.