Photograph by Joel Sartore, National Geographic Photo Ark
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A hogchoker, Trinectes maculatus, is seen at the Gulf Specimen Marine Lab and Aquarium in Florida.

Photograph by Joel Sartore, National Geographic Photo Ark

What is a flatfish?

No misnomers here. Flatfish are exactly what they sound like: fish with thin, oval or diamond-shaped bodies that are lie flat on the seafloor.

There are 822 known species in 16 families, and they reside in oceans, estuaries, and freshwater environments in nearly every part of the globe. Well-known commercial fish, including flounder, halibut, sole, and turbot, are flatfish.

Flatfish range in color from a speckled brown, black, and beige, like a black sea turbot; or spotted, like the blue-and-yellow peacock flounder. Most species live in highly diverse tropical and subtropical oceans.

How the fish become flat

Flatfish begin life symmetrically, as regular-looking fish with eyes on either side of their head. About a month into their development, the fry undergo a metamorphosis: Their skull shape begins to change, and one eye begins to migrate over the fish’s head to join the other. Their pigment also changes, making them light on their belly side and dark on their back—a type of camouflage called countershading.

Once this change is complete, the fish will settle onto the seafloor, its dark back blending into its surroundings and its eyes well positioned for spotting predators or prey.

Camouflage and reproduction

Flatfish are masters of camouflage, thanks to their ability to mimic the various colors of the ocean floor. The fish expand and retract their chromatophores—pigment-containing cells—to quickly change color if threatened or stalking prey. They can also bury themselves quickly in the sand, protruding their independently moving eyes to keep watch without being seen.

This works well for the ambush predator, which lie in wait until their prey—often fish or crustaceans—come within striking distance. Smaller species like the two-foot-long Greenland turbot will eat crabs, squid, and fish, while larger species, like the six-foot-long Atlantic halibut, will chase after cod, haddock, or even lobsters.

Reproduction

Male and female peacock flounders approach each other with pectoral fins raised. The male then positions himself under the female, and the pair rise up from the seafloor during the release of eggs and sperm. Mating in the eyed flounder, which range throughout the Atlantic, is similar, but males keep harems of females that live in their own areas within the male’s territory.

Conservation status

Of the more than 120 flatfish species listed on the International Union for the Conservation of Nature’s Red List of Threatened Species, most are either of least concern, meaning their populations are stable, or there isn’t sufficient data to assess their numbers.

However the IUCN lists the Atlantic halibut, a fish popular for seafood and recreational angling, as endangered due to overfishing.

Flounders are masters of disguise