The black flounder is the only member of the flatfish family, or Pleuronectidae, that is a truly freshwater species. Other members of the family, such as the yellowbelly flounder, occasionally wander into the lower reaches of rivers, but do not usually stay there. As their name implies, the flatfishes are indeed flat, and have adopted a habit of laying on their sides right down on the substrate. Both eyes are on their dorsal or upper side to improve their field of view.
Because of their unusual shape, flounders are unlikely to be confused with other fish species except other flounders. The black flounder is easily distinguished from other flounders by the colouration; the top of the fish is usually dark-coloured with numerous, obvious brick-red spots. Flounders can grow to about 450 mm in length, although 200–300 mm fish are more common.
The black flounder is found throughout New Zealand but is unique to this country. They have not been reported from Chatham or Stewart Island. They are primarily a coastal species, although they can penetrate well inland if the river gradient is not too steep; specimens have been recorded more than 100 km inland in some river systems.
Little is known about the life cycle of the black flounder. The larvae are undoubtedly marine, but where and when spawning takes place is a mystery. Black flounder are a carnivorous species and probably eat a variety of bottom dwelling insects and molluscs. They are also known to feed on whitebait during the spring migration.