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Temporal range: Paleocene–Recent
A camouflaged flatfish.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Pleuronectiformes

Suborder Psettodoidei
    Psettodidae (spiny turbots)
Suborder Pleuronectoidei
    Scophthalmidae (turbots)
    Bothidae (lefteye flounders)
    Pleuronectidae (righteye flounders)
    Paralichthyidae (large-tooth flounders)
    Achiropsettidae (southern flounders)
    Samaridae (crested flounders)
Suborder Soleoidei
    Soleidae (true soles)
    Achiridae (American soles)
    Cynoglossidae (tonguefishes)

A flatfish is a member of the order (Pleuronectiformes) of ray-finned demersal fishes, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migrating through and around the head during development. Some species face their left sides upward, some face their right sides upward, and others face either side upward.

Many important food fish are in this order, including the flounders, soles, turbot, plaice, and halibut. There are over 400 species. Some flatfish can camouflage themselves on the ocean floor.


Flatfish are asymmetrical, with both eyes lying on the same side of the head

The most obvious characteristic of the flatfish is its asymmetry, with both eyes lying on the same side of the head in the adult fish. In some families, the eyes are always on the right side of the body (dextral or right-eyed flatfish), and in others, they are always on the left (sinistral or left-eyed flatfish). The primitive spiny turbots include equal numbers of right- and left-sided individuals, and are generally less asymmetrical than the other families.1 Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on the seabed (benthos), and the extension of the dorsal fin onto the head.

The surface of the fish facing away from the sea floor is pigmented, often serving to camouflage the fish, but sometimes with striking coloured patterns. Some flatfishes are also able to change their pigmentation to match the background, in a manner similar to a chameleon. The side of the body without the eyes, which faces the seabed, is usually colourless or very pale.1

The flounders and spiny turbots eat smaller fish, and have well-developed teeth. They sometimes seek prey in the midwater, away from the bottom, and show fewer extreme adaptations than other families. The soles, by contrast, are almost exclusively bottom-dwellers, and feed on invertebrates. They show a more extreme asymmetry, and may lack teeth on one side of the jaw.1

Flatfishes range in size from Tarphops oligolepis, measuring about 4.5 cm (1.8 in) in length, and weighing 2 g (0.071 oz), to the Atlantic halibut, at 2.5 m (8.2 ft) and 316 kg (697 lb).1

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Species and species groups


Flatfishes lay eggs that hatch into larvae resembling typical, symmetrical, fish. These are initially elongated, but quickly develop into a more rounded form. The larvae typically have protective spines on the head, over the gills, and in the pelvic and pectoral fins. They also possess a swim bladder, and do not dwell on the bottom, instead dispersing from their hatching grounds as plankton.1

The length of the planktonic stage varies between different types of flatfishes, but eventually they begin to metamorphose into the adult form. One of the eyes migrates across the top of the head and onto the other side of the body, leaving the fish blind on one side. The larva also loses its swim bladder and spines, and sinks to the bottom, laying its blind side on the underlying surface.


Fossil of Amphistium

In 2008, a 50-million-year-old fossil, Amphistium, was identified as an early relative of the flatfish and transitional fossil.2 In a typical modern flatfish, the head is asymmetric, with both eyes on one side of the head. In Amphistium, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top of the head.3 The researchers concluded, "the change happened gradually, in a way consistent with evolution via natural selection—not suddenly, as researchers once had little choice but to believe."2

Flatfishes have been cited as dramatic examples of evolutionary adaptation. Richard Dawkins, in The Blind Watchmaker, explains the flatfishes' evolutionary history thus:

…bony fish as a rule have a marked tendency to be flattened in a vertical direction…. It was natural, therefore, that when the ancestors of [flatfish] took to the sea bottom, they should have lain on one side…. But this raised the problem that one eye was always looking down into the sand and was effectively useless. In evolution this problem was solved by the lower eye ‘moving’ round to the upper side.4


As food

Flatfish is considered a Whitefish5 because of the high concentration of oils within its liver. Its lean flesh makes for a unique flavor that differs between each individual species. Methods of cooking include grilling, pan-frying, baking and deep-frying.

Timeline of genera

Quaternary Neogene Paleogene Holocene Pleist. Plio. Miocene Oligocene Eocene Paleocene Symphurus Parophrys Isopsetta Eopsetta Chibapsetta Pegusa Lyopsetta Limanda Glyptocephalus Clidoderma Atheresthes Pleuronichthys Paralichthys Monochirus Citharichthys Evesthes Microstomus Microchirus Achiurus Platichthys Paraplagusia Dicologoglossa Lepidorhombus Hippoglossoides Buglossidium Solea Monolene Bothus Arnoglossus Psettodes Citharus Scophthalmus Turahbuglossus Joleaudichthys Imhoffius Eobuglossus Eobothus Amphistium Quaternary Neogene Paleogene Holocene Pleist. Plio. Miocene Oligocene Eocene Paleocene

See also


  1. ^ a b c d e f Chapleau, Francois & Amaoka, Kunio (1998). Paxton, J.R. & Eschmeyer, W.N., ed. Encyclopedia of Fishes. San Diego: Academic Press. xxx. ISBN 0-12-547665-5. 
  2. ^ a b "Odd Fish Find Contradicts Intelligent-Design Argument". National Geographic. July 9, 2008. Retrieved 2008-07-17. 
  3. ^ Matt Friedman (2008). "The evolutionary origin of flatfish asymmetry". Nature Letters 454 (7201): 209–212. doi:10.1038/nature07108. PMID 18615083. 
  4. ^ Dawkins, Richard (1991). The Blind Watchmaker. London: Penguin Books. p. 92. ISBN 0-14-014481-1. 
  5. ^ "Flatfish BBC". 

Further references

  • Sepkoski, Jack (2002). "A compendium of fossil marine animal genera". Bulletins of American Paleontology 364: p.560. Retrieved 2011-05-17. 
  • Gibson, Robin N (Ed) (2008) Flatfishes: biology and exploitation. Wiley.
  • Munroe, Thomas A (2005) "Distributions and biogeography." Flatfishes: Biology and Exploitation: 42-67.

External links

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