Flatfish

Flatfish are a group of ray-finned fish in the suborder Pleuronectoidei, part of the order Carangiformes. They get their name from how they rest on the seafloor: lying on one side of their flattened body, with both eyes on the side facing up. This asymmetry—rare among vertebrates—develops when one eye moves toward the other during the fish’s juvenile stage. Depending on the species, some flatfish lie on their left side (eyes on the right), some on their right side (eyes on the left), and a few, like the primitive genus *Psettodes*, can end up facing either way. They are a highly diverse group of bottom-dwelling fish, and their camouflaged bodies help hide them from predators. Many flatfish are important for fishing.

Common names for flatfish vary widely, since they are found in many regions and are often eaten. These names don’t always match the fish’s true evolutionary relationships. For example, the three species called “halibut” belong to the right-eyed flounder family, while “spiny turbots” are not closely related to true turbot and sit near the base of the flatfish family tree.

Flatfish live in oceans all over the world, from the Arctic to the tropics to Antarctica. Most species are found in the Indo-West Pacific, with diversity dropping off as you move away from that region. They typically live in water depths between 0 and 500 meters, though a few have been recorded deeper than 1,500 meters. None have been confirmed from the abyssal or hadal zones; a reported sighting from the Mariana Trench is considered unreliable by experts. One deepwater species, *Symphurus thermophilus*, lives around hydrothermal vents, the only flatfish known from such environments. Many flatfish also enter brackish or fresh water, and some families—Achiridae, Soleidae, and Cynoglossidae—include species that live only in freshwater.

The most obvious feature of adult flatfish is their asymmetrical head, with both eyes on the same side. In some families, the eyes are usually on the right (dextral); in others, on the left (sinistral). The primitive spiny turbots have equal numbers of right- and left-eyed individuals and are less asymmetrical overall. Flatfish also have eyes that can protrude, an adaptation for life on the seabed, and their dorsal fin extends onto the head. The side facing up is pigmented, often for camouflage, and some flatfish can change color to match their surroundings using chromatophores. The blind side, facing the seabed, is usually pale or colorless.

Flatfish rely mainly on camouflage to avoid predators, but some have warning features like eyespots or are poisonous (e.g., *Aseraggodes*, *Pardachirus*, and *Zebrias*). Juveniles of *Soleichthys maculosus* mimic toxic flatworms in color and swimming style. A few octopus species have been seen mimicking flatfish. Flatfish range greatly in size: the sand flounder *Tarphops oligolepis* reaches about 6.5 cm and 2 grams, while Atlantic halibut can grow to 4.7 meters and Pacific halibut up to 363 kilograms. Many species, like flounders and spiny turbots, eat smaller fish and have well-developed teeth; they sometimes hunt in open water and are less extremely adapted than other flatfish. Soles, in contrast, stay close to the bottom, feed on invertebrates, show more extreme asymmetry, and may lack teeth on one side of the jaw.

Flatfish lay eggs that hatch into symmetrical, typical fish larvae. These larvae are initially elongated but soon become rounder, with protective spines on the head, gills, and fins. They have a swim bladder and drift as plankton rather than staying on the bottom. Unlike symmetrical fish, which use inner-ear otoliths for balance, larval and metamorphosing flatfish need visible light to orient themselves. The time they spend as plankton varies by species, but eventually, triggered by thyroid hormones, they begin to metamorphose.