Any of about 300 species of flatfish es (order Pleuronectiformes).
When born, the flounder is bilaterally symmetrical, with an eye on each side, and it swims near the sea's surface. After a few days, it begins to lean to one side, and the eye on that side migrates to what eventually becomes the top side. With this development comes changes in bones, nerves, and muscles, and the underside loses its colour. As an adult, the flounder lives on the sea bottom with the eyed side on top.
Flounder ( Platichthys )
F. Greenaway
Natural History Photographic Agency/EB Inc.