Flatfish

Flatfish are a group of mostly saltwater, carnivorous, bottom-dwelling fish in which both eyes are located the same side of the head. The under side of a flatfish is white while the upper side with the two eyes may be brightly colored. Many of these fish can change color to match their surroundings, making them hard to detect. When flatfish hatch, the eyes are located normally on each side of the head. However, when a young flatfish reaches a length of about 0.8 in (2 cm), one eye moves close to the other eye, and the mouth is twisted. Many species of flatfish, such as halibut, sole, and turbot, are popular food fish and are commercially valuable.

The flatfish family Pleuronectidae includes mainly right-sided species (i.e. both eyes are found on the right side of the head), although there are some left-sided species. The largest flatfish is the Atlantic halibut (Hippoglossus hippoglossus), which is found on the European and North American sides of the North Atlantic in Arctic and sub-Arctic waters. The halibut is especially prolific north of Scotland and in the northern North Sea. This species may reach a length of about 7 ft (2.1 m) and a weight of 720 lb (325 kg). It is brown, dark green or blackish on the eyed side.

The Pacific halibut (Hippoglossus stenolepis) is somewhat smaller and slimmer than its Atlantic relative and is found on both sides of the Pacific Ocean. It is greenish brown and may reach a weight of about 440 lb (200 kg).

Some species of flatfish are considerably smaller. For example, the common or winter flounder (Pseudopleuronectes americanus), found in shallow coastal waters of the Atlantic Ocean from Georgia to Labrador, reaches about 1 ft (30 cm) long. The American plaice or rough dab (Hippoglossoides platessoides) reaches a length of 2 ft (60 cm) and a weight of 4 lb (1.8 kg). This reddish or brownish fish is found in the Atlantic Ocean from Massachusetts to the cold waters of Europe. The larger European plaice (Pleuronectes platessa) reaches 3 ft (90 cm) in length and weighs about 20 lb (9 kg).

A windowpane flounder (Scophthalmus aquosus) on the sea floor in the Gulf of Maine. Photograph by Andrew J. Martinez. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.

The more than 100 species of sole (family Soleidae) have a thin body with a downward curved mouth. Of all the flatfish, soles demonstrate the most efficient adaptation to a bottom-living environment. They possess small, paired fins, and the dorsal and anal fins are considerably extended. Unlike the flatfish in the family Pleuronectidae, soles prefer more southern waters, and some are found in the tropics. Soles are found in the Mediterranean Sea and in the Atlantic Ocean extending northward to the North Sea. The most well-known species in this family is the European or Dover sole (Solea solea). It may reach a weight of 3 lb (1.4 kg) and a length of 20 in (50 cm).

The lefteye flounders are classified in the family Bothidae. One species in this family, the summer flounder (Paralichthys dentatus), is found in the Atlantic Ocean from Maine to Florida. The southern flounder (P. lethstigma) is found in the Gulf of Mexico. The turbot (Scophthalmus maximus), another member of the family Bothidae, has a thick, diamond-shaped body, and may weigh more than 44 lb (20 kg). It is found in the Mediterranean and in the European side of the Atlantic Ocean to the southern part of the North Sea.