Fish

Flatfish

Pleuronectiformes · also called flounder, sole, plaice

Not Evaluated carnivore Ocean Coasts & shorelines

A flatfish looks like a fish that has been pressed. It lies on its side on the sea floor, both eyes staring up from one surface, and it got that way by remodelling its own skull as a youngster, walking one eye over the top of its head to join the other.

Overview

Flatfish are the order Pleuronectiformes, around 800 species — the flounders, soles, plaice, halibut, turbot, and dab. An adult flatfish lies flat on the sea floor on its blind underside, with both eyes on the pigmented upper side, and it ambushes worms, shellfish, shrimp, and small fish from the bottom. They live on soft sea beds in every ocean, from estuaries to the deep sea.

Rebuilding the head

A flatfish hatches as a normal, upright, symmetrical larva with one eye on each side. As it grows, one eye migrates across the skull to the other side, the head twists, and the young fish settles onto the bottom for life. Families divide into right-eyed (plaice, sole, halibut) and left-eyed (turbot, brill).

Vanishing on the bottom

The upper side can change colour and pattern within minutes to match sand, gravel, or mud, and the fish flicks a layer of sediment over its back so only the raised eyes show. The blind underside stays pale.

Status

Flatfish as a group are Not Evaluated. Many are common and support large fisheries, but several key stocks are overfished, and the Atlantic halibut, the largest flatfish at nearly 5 m, is Endangered.

Flatfish: common questions

Why do flatfish have both eyes on one side?

Because they lie flat on the sea floor on one side of the body, and an eye on the down side, pressed into the sand and in shadow, would be useless and vulnerable. A flatfish is not born this way. It hatches as a normal, upright, bilaterally symmetrical fish larva that swims in open water with one eye on each side. As it develops, one eye gradually migrates around or over the top of the skull to the other side, connective tissue and bone remodel, the mouth and fins shift, and the young fish settles onto the bottom lying on its now-blind side, with both eyes looking up. From then on it can watch for predators and prey above it while remaining almost invisible against the sea floor.

What is the difference between a flounder, a sole, a plaice, and a halibut?

They are all flatfish, differing mainly in family, shape, size, and which side the eyes are on. Plaice, dab, and halibut are right-eyed flounders in the family Pleuronectidae, with plaice a diamond-shaped fish marked with orange spots, and halibut a large, elongated, powerful predator. True soles, family Soleidae, are right-eyed too but have a small, twisted mouth and a rounded, tongue-shaped body, and the famous Dover sole is one of them. Turbot and brill are left-eyed fish in the family Scophthalmidae, roundish and scaleless. In North America the word flounder is used broadly for many of these fish, and fluke is another name for the summer flounder, so common names vary a lot by region.

How does a flatfish hide?

In two ways at once. The first is colour, since the upper side has pigment cells that the fish controls through its nervous system, letting it change its overall shade and even reproduce a coarse pattern of spots and blotches to match whatever it is lying on, all within a few minutes. Experiments have shown flatfish reproducing a rough checkerboard pattern from the tank floor. The second is cover, because a settling flatfish shuffles its body down into soft sediment and uses a rippling motion of the fins to throw a thin layer of sand or mud over its back, so that only the two bulging eyes and perhaps the edge of the body remain visible. The combination makes a resting flatfish extremely hard to see.

Are flatfish good to eat, and are they sustainable?

Flatfish are among the most prized food fish, with firm, white, mild flesh, and plaice, sole, flounder, halibut, and turbot are all major commercial species. Sustainability varies by species and area. Some flatfish stocks are well managed and in good shape, while others are overfished, and the fishing method matters, since much flatfish is caught by beam trawling and dredging, which disturb the sea floor. The Atlantic halibut is a cautionary case, having been fished down so far that wild catches are tightly restricted and the fish on sale is nearly all farmed. Checking a reliable seafood guide for the specific species and its origin is the practical way to choose.

Sources & further reading

Written by HideShark Editorial Team · reviewed by HideShark Science Desk. Published November 1, 2026. Last updated November 1, 2026.

Quick facts

Scientific name: Pleuronectiformes
Diet: Bottom-living animals ambushed from the sea floor, including worms, clams and other shellfish, shrimp and crabs, brittlestars, and small fish, with the large species such as halibut and turbot becoming active fish hunters
The eye that moves: A flatfish hatches as an ordinary upright, symmetrical larva with one eye on each side, then over days to weeks one eye migrates across the top or front of the skull to join the other, the skull twists, and the fish tips onto what is now its blind underside for life
Left-eyed and right-eyed: Flatfish families split by which side the eyes end up on, so a plaice, sole, dab, and halibut are right-eyed, lying on the left side, while a turbot, brill, and many tropical flounders are left-eyed, and a few species come both ways
A camouflage that changes in minutes: The upper side carries colour cells the fish can adjust within minutes to match sand, gravel, mud, or even a checkerboard laid in an aquarium, and it flicks a shower of sand over its back so only the raised eyes show
The blind side stays pale: The underside, pressed against the sea floor and in shadow, keeps its pale larval colouring and lacks the pattern of the top, and it carries most of the pale fin and the mouth twisted round toward the bottom
The biggest is Endangered: The Atlantic halibut, the largest flatfish, can reach nearly 5 m and over 300 kg, but centuries of trawling reduced it so severely that it is listed as Endangered, and most on sale now is farmed
A puzzle Darwin discussed: The lopsided, half-finished look of a flatfish, with a twisted skull and both eyes crowded onto one side, was raised by critics of natural selection as hard to explain in gradual steps, and Darwin and later biologists answered it by pointing to fossil flatfish with eyes caught partway across the head
Range: Sandy, muddy, and gravelly sea floors of every ocean, from the shoreline and estuaries to the deep sea and from the tropics to polar waters, with a few species entering fresh water; most live on the continental shelf.
Conservation status: Not Evaluated ≈ Not assessed as an order of around 800 species; many flatfish are common and support large fisheries, but a number of important stocks, including Atlantic halibut, common sole in some seas, and several tropical species, are fished at or beyond sustainable limits or are already depleted in the wild