The black flounder (Rhombosolea retiaria) is the only member of the flatfish family, or Pleuronectidae, that is a truly freshwater species. The black flounder is unique to New Zealand and is the only freshwater flounder (of the right-eyed family) in the whole world.
They are primarily a coastal species, but also live in estuaries, lowland lakes and the lower reaches of rivers. They can penetrate well inland if the river gradient is not too steep. They occupy a variety of substrates, from silty to cobbly bottoms, and can live in rivers and streams of different flow rates.
The adults are predatory carnivores – camouflaging themselves on the bottom then ambushing small fish. They are also known to feed on whitebait during the spring migration. Little is known about the life cycle of the black flounder except that adults migrate to sea in winter, where they likely spawn.