tonguefish, any of the small marine flatfishes of the family Cynoglossidae, found in the tropics, especially in Asia. Tonguefish are flattened, drop-shaped flatfish with small eyes, both on the left side of the head, and with long dorsal and anal fins that join with the tail fin. Most tonguefish grow no longer than about 30 cm (12 inches). Some are used as food, but most, because of their small size, are of no commercial value.