Flatfish: Types, Names and Why Both Eyes Are on One Side

A flatfish is a bottom-living fish whose adults have both eyes on the same side of the head; flounder, sole, halibut, plaice and turbot are all flatfish. Wikipedia counts over 800 described species in 16 families, the largest being Soleidae, Bothidae and Cynoglossidae, and National Geographic gives 822 species.

Flatfish live in oceans from the Arctic through the tropics to Antarctica, mostly between the surface and 500 m, and a few species enter fresh water (Wikipedia). They lie on the sea floor on their blind side, with the eyed side facing up and coloured to match the bottom; many can change colour to match their surroundings (Wikipedia).

Flatfish do not start out flat. The larvae swim upright with an eye on each side like other fish. National Geographic says that about a month into development one eye begins to move, and Wikipedia describes it migrating across the top of the head onto the other side of the body. The fish then turns to lie on its blind side.

Which eye moves depends on the family (Wikipedia):

• Right-eyed (dextral): Pleuronectidae, the righteye flounders, lie on their left side with both eyes on the right. This family includes plaice and halibut.
• Left-eyed (sinistral): Paralichthyidae, the large-tooth flounders, lie on their right side with both eyes on the left. The summer flounder is one of them.

This is a useful first check when identifying one. Hold the fish eyed side up with the dorsal fin at the top and the belly at the bottom, and note whether the head points left or right.

Wikipedia lists ten common English names used for flatfish. Each covers a group of species rather than one fish:

• Brill
• Dab
• Flounder
• Halibut
• Megrim
• Plaice
• Sanddab
• Sole
• Tonguefish
• Turbot

These names are used loosely, so a fish sold as sole in one country may be a flounder in another. Scientists group them as the suborder Pleuronectoidei (Wikipedia); older sources call the group the order Pleuronectiformes.

Flatfish span a huge size range. The smallest, the sand flounder Tarphops oligolepis, is about 6.5 cm (2.6 in); the largest, the Atlantic halibut, reaches 4.7 m (15 ft) (Wikipedia). National Geographic gives a range of one inch to 15 feet, weights up to 615 pounds, and lifespans from about a year to 60 years depending on species.

Smaller flatfish eat crabs, squid and fish, while the largest take cod, haddock and even lobsters (National Geographic).

Many flatfish look alike, so fisheries agencies publish regional keys. The Washington Department of Fish and Wildlife lists 14 flatfish species in the state's marine waters: arrowtooth flounder, butter sole, curlfin sole, Dover sole, English sole, flathead sole, northern rock sole, Pacific sanddab, petrale sole, rex sole, southern rock sole, sand sole, speckled sanddab and starry flounder.

Its guide tells anglers to check three features on the eyed side, beyond eye position:

• Anal spine: a hard, sharp spine pointing forward in front of the anal fin
• Lateral line: its shape, a row of modified scales with pore-like openings
• Accessory dorsal branch: an extra branch of the lateral line near the base of the dorsal fin

Photograph the whole eyed side flat, then close-ups of the lateral line over the pectoral fin and of the mouth. FishLens can suggest a match from a photo. Check local rules on species, size and bag limits before keeping one, and see our guide to key features to identify fish.

There is no official ranking; taste is personal. What sources say about flatfish as food: Wikipedia classes them as whitefish with lean flesh whose flavour differs from species to species, names the European plaice as the main commercial flatfish in Europe and says halibut support lucrative fisheries. National Geographic lists flounder, halibut, sole and turbot as well-known commercial flatfish.

Sustainability varies a lot. National Geographic says more than 120 flatfish species are on conservation lists and that the Atlantic halibut is listed as endangered because of overfishing. If you buy flatfish, check where and how it was caught; if you catch it yourself, check local regulations and fish consumption advisories for that water.

Research and drafting for this page were assisted by AI and checked line by line against the sources listed below. Every number names the organisation it comes from, and where sources disagree we say so instead of picking one silently. Prices are given with the date we checked them (25 September 2026); they change, so confirm before you rely on them. If you find an error, tell us and we will correct it.