View source for Flatfish

{{short description|Order of fishes}}
{{for-multi|the album by Flook|Flatfish (album)|the move in shogi|Flatfish (shogi)}}
{{Automatic taxobox
| fossil_range = {{Fossil range|Late Paleocene|Recent|ref=<ref name=EoF/>}}
| image = Pleuronectes platessa.carrelet02.jpg
| image_caption = Plaice (''[[Pleuronectes platessa]]''), the first [[Binomial nomenclature|named species]] of flatfish
| taxon = Pleuronectoidei
| synonyms = * Heterosamata <small>Jordan & Evermann, 1896</small>
* Pleuronectiformes <small>Regan, 1910</small>
* Soleiformes <small>Regan, 1910</small>
* Polynemoidei <small>Regan, 1909</small>
* Pleuronectoideo <small>Girard ''et al'', 2020</small>
| authority = [[Cuvier]], 1817<ref>{{Cite book |last=Scudder |first=Samuel Hubbard |url=https://books.google.com/books?id=LVk0AQAAMAAJ |title=Nomenclator Zoologicus: An Alphabetical List of All Generic Names that Have Been Employed by Naturalists for Recent and Fossil Animals from the Earliest Times to the Close of the Year 1879 ... |date=1882 |publisher=U.S. Government Printing Office |language=en}}</ref>
| type_species = ''[[Pleuronectes platessa]]''
| type_species_authority = [[Carl Linnaeus|Linnaeus]], [[10th edition of Systema Naturae|1758]]
| subdivision_ranks = Families
| subdivision = See text
}}

'''Flatfish''' are a group of [[ray-finned fish]] belonging to the [[Order (biology)|suborder]] '''Pleuronectoidei''' of the order [[Carangiformes]]. Their collective [[common name]] is due to their habit of lying on one side of their laterally-compressed body (flattened side-to-side) upon the [[seafloor]]; in this position, both eyes lie on the side of the head facing upwards, while the other side of the head and body (the "blind side") lies on the [[Substrate (biology)|substrate]]. This loss of [[Bilateral symmetry|symmetry]], a [[Autapomorphy|unique adaptation]] in [[vertebrate]]s, stems from one eye "migrating" towards the other during the juvenile's [[Metamorphosis#Fish|metamorphosis]]. Due to interspecific variation, some species tend to face their left side upward, some their right side, and others face either side upward; members of [[Pleuronectidae]] lie on their left side, with eyes on the right, [[Paralichthyidae]] lie on their right side, with eyes on the left,<ref>{{cite book |isbn=978-0544859937|title=[[The Ancestor's Tale: A Pilgrimage to the Dawn of Life]]|last1=Dawkins|first1=Richard|author-link1=Richard Dawkins| last2=Wong|first2=Yan| author-link2=Yan Wong| year=2016| p=398|publisher=Houghton Mifflin Harcourt }}</ref> while the "[[Primitive (phylogenetics)|primitive]]" genus ''[[Psettodes]]'' may develop into "right-facing" or "left-facing" individuals.

They are one of the most [[Species diversity|diverse]] groups of [[demersal fish]]. Their [[crypsis|cryptic coloration and habits]], a form of [[camouflage]], conceals them [[Anti-predator adaptation|from potential predators]]. Many species are of interest for [[fisheries]].

==Common names==
[[File:FMIB 36294 Fleuronectidae Family 1Flounder 2Turbot 3Plaice 4Sole 5Dab.jpeg|thumb|Illustration of several common European flatfish species]]
There are a multitude of [[List of fish common names|common names for flatfish]], as they are a widespread group of fish and important [[food fish]] across the world. The following are common flatfish names in [[English language|English]]:
{{div col|colwidth=14em}}
* [[Brill (fish)|Brill]]
* [[Dab (fish)|Dab]]
* [[Flounder]]
* [[Halibut]]
* [[Megrim]]
* [[Plaice]]
* [[Sanddab]]
* [[Sole (fish)|Sole]]
* [[Tonguefish]]
* [[Turbot]]
{{div col end}}
As these are merely common names, they do not conform with the "natural" relationships that are recovered through scientific studies of [[Morphology (biology)|morphology]] or [[Genetic analysis|genetics]]. As examples, the three species consistently called "halibut" are themselves part of the [[right-eye flounder]] family, while the [[spiny turbot]]s are not at all closely related to "true" turbot, but are consistently recovered in a "primitive" or [[Basal (phylogenetics)|basal position]] at the base of flatfish [[phylogenetic tree]]s.

==Distribution==
Flatfishes are [[Cosmopolitan distribution|found in oceans worldwide]], ranging from the [[Arctic]], through the [[tropics]], to [[Antarctica]]. [[Species diversity]] is centered in the [[Indo-West Pacific]] and declines following both [[latitudinal]] and [[longitude|longitudinal]] gradients away from this centre of diversity.<ref>{{Cite journal|last1=Campbell|first1=Matthew A.|last2=Chanet|first2=Bruno|last3=Chen|first3=Jhen-Nien|last4=Lee|first4=Mao-Ying|last5=Chen|first5=Wei-Jen|date=2019|title=Origins and relationships of the Pleuronectoidei: Molecular and morphological analysis of living and fossil taxa|journal=Zoologica Scripta|language=en|volume=48|issue=5|pages=640–656|doi=10.1111/zsc.12372|s2cid=202856805|issn=1463-6409|url=https://hal.science/hal-03971070/file/Campbell%20et%20al%202019%20Zool%20Scripta.pdf }}</ref> Most species are found in depths between 0 and {{convert|500|m|ft|abbr=on}}, but a few have been recorded from depths in excess of {{convert|1500|m|ft|abbr=on}}. None have been confirmed from the [[abyssal]] or [[hadal zone]]s of the [[deep sea]]; a reported observation of a flatfish from the [[Bathyscaphe Trieste]]'s dive into the [[Mariana Trench]] (at a depth of almost {{convert|11|km|ft|abbr=on}}) has been questioned by [[ichthyologist]]s, and recent authorities do not recognize it as valid.<ref>Jamieson, A.J., and Yancey, P. H. (2012). ''[http://www.biolbull.org/content/222/3/171.full On the Validity of the Trieste Flatfish: Dispelling the Myth.]'' [[The Biological Bulletin]] 222(3): 171-175</ref> Among the deepwater species is ''[[Symphurus thermophilus]]'', a tonguefish which congregates around "ponds" of [[sulphur]] at [[hydrothermal vent]]s on the seafloor; no other flatfish is known from hydrothermal vent ecosystems.<ref>Munroe, T.A.; and Hashimoto, J. (2008). ''A new Western Pacific Tonguefish (Pleuronectiformes: Cynoglossidae): The first Pleuronectiform discovered at active Hydrothermal Vents.'' Zootaxa 1839: 43–59.</ref>

Conversely, many species will enter [[brackish]] or [[Freshwater ecosystem|fresh water]], and a smaller number of soles (families [[Achiridae]] and [[Soleidae]]) and tonguefish ([[Cynoglossidae]]) are entirely [[Freshwater fish|restricted to fresh water]].<ref>Duplain, R.R.; Chapleau, F; and Munroe, T.A. (2012). ''A New Species of Trinectes (Pleuronectiformes: Achiridae) from the Upper Río San Juan and Río Condoto, Colombia.'' Copeia 2012 (3): 541-546.</ref><ref>Kottelat, M. (1998). ''Fishes of the Nam Theun and Xe Bangfai basins, Laos, with diagnoses of twenty-two new species (Teleostei: Cyprinidae, Balitoridae, Cobitidae, Coiidae and Odontobutidae).'' Ichthyol. Explor. Freshwat. 9(1):1-128.</ref><ref>Monks, N. (2007). ''[http://brackishfaq.webspace.virginmedia.com/Projects/FAQ/4h.html Freshwater flatfish, order Pleuronectiformes.] {{Webarchive|url=https://web.archive.org/web/20140815222521/http://brackishfaq.webspace.virginmedia.com/Projects/FAQ/4h.html |date=2014-08-15 }}'' Retrieved 18 May 2014</ref>

==Description==
[[File:Pseudopleuronectes americanus.jpg|thumb|left|[[Winter flounder]]; Pleuronectidae]]
The most obvious characteristic of the flatfish is their [[asymmetry]], with both eyes lying on the same side of the head in the adult fish. In some families, the eyes are usually on the right side of the body ([[dextral]] or right-eyed flatfish), and in others, they are usually on the left ([[Sinistral and dextral|sinistral]] or left-eyed flatfish). The primitive [[spiny turbot]]s include equal numbers of right- and left-sided individuals, and are generally less asymmetrical than the other families.<ref name=EoF/> 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.
{{Multiple image
| image1            = ShimaUS.jpg
| caption1          = ''[[Zebrias zebra]]''; Soleidae
| image2            = Peacock Flounder Bothus mancus in Kona.jpg
| alt2              = Four frames of the same peacock flounder, a sand colored flatfish with a pattern of blue rings, taken a few minutes apart which shows its ability to change colors to match its surroundings. The last photo shows it buried under sand with only its eyes visible
| caption2          = This sequence of photos shows an individual [[peacock flounder]] (Bothidae) changing its coloration over different substrates.
| direction   = vertical
}}
The surface of the fish facing away from the sea floor is pigmented, often serving to [[camouflage]] the fish, but at times displaying striking patterns. Some flatfishes are also able to change their pigmentation to match the background using their [[chromatophore]]s, in a manner similar to some [[cephalopods]]. The side of the body without the eyes, facing the seabed, is usually colourless or very pale.<ref name=EoF>{{cite book |editor=Paxton, J.R. |editor2=Eschmeyer, W.N.|author1=Chapleau, Francois  |author2=Amaoka, Kunio|year=1998|title=Encyclopedia of Fishes|publisher= Academic Press|location=San Diego|page= xxx|isbn= 0-12-547665-5 |no-pp=true}}</ref>

In general, flatfishes rely on their camouflage for avoiding predators, but some have [[aposematic trait]]s such as conspicuous [[Eyespot (mimicry)|eyespots]] (e.g., ''[[Microchirus ocellatus]]'') and several small tropical species (at least ''[[Aseraggodes]]'', ''[[Pardachirus]]'' and ''[[Zebrias]]'') are poisonous.<ref name=RandallHawaii/><ref>Elst, R. van der (1997) ''A Guide to the Common Sea Fishes of South Africa.'' {{ISBN|978-1868253944}}</ref><ref>Debelius, H. (1997). ''Mediterranean and Atlantic Fish Guide.'' {{ISBN|978-3925919541}}</ref> Juveniles of ''[[Soleichthys maculosus]]'' [[mimic]] toxic [[flatworm]]s of the genus ''[[Pseudobiceros]]'' in both colours and swimming pattern.<ref>Practical Fishkeeping (22 May 2012) ''[http://www.practicalfishkeeping.co.uk/content.php?sid=5061 Video: Tiny sole mimics a flatworm.] {{Webarchive|url=https://web.archive.org/web/20140517213218/http://www.practicalfishkeeping.co.uk/content.php?sid=5061 |date=2014-05-17 }}'' Retrieved 17 May 2014.</ref><ref>[[Australian Museum]] (5 November 2010). ''[http://australianmuseum.net.au/BlogPost/Fish-Bits/This-week-in-Fish-Flatworm-mimic-and-shark-teeth This week in Fish: Flatworm mimic and shark teeth.] {{Webarchive|url=https://web.archive.org/web/20130226172618/http://australianmuseum.net.au/blogpost/Fish-Bits/This-week-in-Fish-Flatworm-mimic-and-shark-teeth |date=2013-02-26 }}'' Retrieved 17 May 2014.</ref>  Conversely, a few [[octopus]] species have been reported to mimic flatfishes in colours, shape and swimming mode.<ref>Hanlon, R.T.; Warson, A.C.; and Barbosa, A. (2010). ''A "Mimic Octopus" in the Atlantic: Flatfish Mimicry and Camouflage by Macrotritopus defilippi.'' [[The Biological Bulletin]] 218(1): 15-24</ref>

Flatfishes range in size from the sand flounder ''[[Tarphops oligolepis]]'', measuring about {{convert|6.5|cm|in|abbr=on}} in length,<ref>{{FishBase|Tarphops|oligolepis}}</ref> and weighing {{convert|2|g|oz|abbr=on}},<ref name=EoF/> to the [[Hippoglossus|''Hippoglossus'' halibuts]], with the [[Atlantic halibut]] measuring up to {{convert|4.7|m|ft|abbr=on}} long,<ref>{{FishBase|Hippoglossus|hippoglossus}}</ref> and the [[Pacific halibut]] weighing up to {{convert|363|kg|lb|abbr=on}}.<ref>{{FishBase|Hippoglossus|stenolepis}}</ref><ref name=EoF/>

Many species such as flounders and spiny turbots [[Predatory fish|eat smaller fish]], and have well-developed teeth. These species sometimes hunt in the [[midwater]], away from the bottom, and show fewer [[Derived (phylogenetics)|"extreme" adaptations]] than other families. The [[Sole (fish)|sole]]s, by contrast, are almost exclusively bottom-dwellers (more strictly [[Demersal fish|demersal]]), and feed on [[benthic invertebrate]]s. They show a more extreme asymmetry, and may lack teeth on one side of the jaw.<ref name=EoF/>

===Development===
[[File:Platichthys flesus - en.pdf|thumb|left|European flounder, like other flatfish, experience an eye migration during their lifetime.]]
Flatfishes lay eggs that hatch into [[Juvenile fish|larvae]] resembling typical, symmetrical, fish. These are initially elongated, but quickly develop into a more rounded form. The larvae typically have protective [[Spine (zoology)|spine]]s on the head, over the gills, and in the [[Fish fin|pelvic and pectoral fins]]. They also possess a [[swim bladder]], and do not dwell on the bottom, instead dispersing from their hatching grounds as [[ichthyoplankton]].<ref name=EoF/> Bilaterally symmetric fish such as [[goldfish]] maintain balance using a system within their [[inner ear]]s which involves the [[otolith]], but larval and metamorphizing flatfish require visible light (such as [[sunlight]]) to properly orient themselves.<ref name="NOVA"/>

The length of the [[plankton]]ic stage varies between different types of flatfishes, but through the influence of [[thyroid hormones]],<ref>{{cite journal |last1=Schreiber |first1=Alexander M. |title=Flatfish: an asymmetric perspective on metamorphosis |journal=Curr Top Dev Biol |date=2013 |volume=103 |pages=167–94 |doi=10.1016/B978-0-12-385979-2.00006-X |pmid=23347519}}</ref> they eventually begin to [[Metamorphosis|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.<ref>{{cite book |last1=Bao |first1=Baolong |title=Flatfish Metamorphosis |date=2022 |publisher=Springer |location=Singapore |isbn=978-981-19-7858-6 |edition=1 |doi=10.1007/978-981-19-7859-3 }}</ref><ref name="NOVA" >{{cite web |last1=Jabr |first1=Feris |title=The Improbable—but True—Evolutionary Tale of Flatfishes |url=https://www.pbs.org/wgbh/nova/article/flatfish-evolution/ |website=pbs.org |date=7 May 2014 |publisher=PBS NOVA |access-date=17 July 2025}}</ref>

===Hybrids===
[[Hybrid (biology)|Hybrids]] are well known in flatfishes. The Pleuronectidae have the largest number of reported hybrids of marine fishes.<ref name=hybridsole>Garrett, D.L.; Pietsch, T.W.; Utter, F.M.; and Hauser, L. (2007). ''The Hybrid Sole Inopsetta ischyra (Teleostei: Pleuronectiformes: Pleuronectidae): Hybrid or Biological Species?'' American Fisheries Society 136: 460–468</ref> Two of the most famous [[intergeneric hybrid]]s are between the [[European plaice]] (''Pleuronectes platessa'') and [[European flounder]] (''Platichthys flesus'') in the [[Baltic Sea]],<ref>[[Food and Agriculture Organization of the United Nations]]: ''[http://www.fao.org/fishery/species/2550/en Platichthys flesus (Linnaeus, 1758).]''. Retrieved 18 May 2014</ref> and between the [[English sole]] (''Parophrys vetulus'') and [[starry flounder]] (''Platichthys stellatus'') in [[Puget Sound]]. The offspring of the latter species pair is popularly known as the hybrid sole and was initially believed to be a valid species in its own right.<ref name=hybridsole/>

==Evolution==
Flatfishes have been cited as dramatic examples of [[evolutionary adaptation]]. In ''[[The Blind Watchmaker]]'', [[Richard Dawkins]] explains the flatfishes' evolutionary history as such:

<blockquote>...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.<ref>{{cite book|last = Dawkins|first = Richard|author-link = Richard Dawkins|title = The Blind Watchmaker|publisher = Penguin Books|year = 1991|location = London|page=92|isbn = 0-14-014481-1}}</ref></blockquote>

Scientists have been proposing since the 1910s that flatfishes evolved from more "typical" [[Percoidea|percoid]] ancestors.<ref name="Regan_1910">Regan C.T. (1910). "The origin and evolution of the Teleostean fishes of the order Heterosomata". ''Annals and Magazine of Natural History'' '''6'''(35): p. 484-496. [https://doi.org/10.1080/00222931008692879 doi.org/10.1080/00222931008692879]</ref> The fossil record indicated that flatfishes might have been present before the [[Eocene]], based on fossil [[otolith]]s resembling those of modern pleuronectiforms dating back to the [[Thanetian]] and [[Ypresian]] stages (57-53 million years ago).<ref name="Schwarzhans1999">Schwarzhans W. (1999). "A comparative morphological treatise of recent and fossil otoliths of the order Pleuronectiformes". ''Piscium Catalogus. Otolithi Piscium 2''. [[doi:10.13140/2.1.1725.5043]]</ref> Despite this, the origin of the unusual morphology of flatfishes was enigmatic up to the 2000s, with earlier researchers having suggested that it came about as a result of [[saltation (biology)|saltation]] rather than gradual evolution through [[natural selection]], because a partially migrated eye was considered to have been [[maladaptive]]. 
[[File:Amphistium.JPG|thumb|Specimen of ''[[Amphistium]]''.]]
This started to change in 2008 with a study on the two fossil fish genera; ''[[Amphistium]]'' and ''[[Heteronectes]]'', which dated to about 50 million years ago. These genera retain [[Primitive (phylogenetics)|primitive]] features not seen in modern types of flatfishes, such as their heads being less asymmetric than modern flatfishes, retaining one eye on each side of their heads, although the eye on one side is closer to the top of the head than on the other.<ref name="Friedman_2008">Friedman M. (2008). "The evolutionary origin of flatfish asymmetry". ''Nature'' '''454'''(7201): p. 209–212. [[doi:10.1038/nature07108]]</ref><ref name="NaEvo">{{cite news | url=http://news.nationalgeographic.com/news/2008/07/080709-evolution-fish.html  | archive-url=https://web.archive.org/web/20080711035445/http://news.nationalgeographic.com/news/2008/07/080709-evolution-fish.html  | archive-date=July 11, 2008  | title=Odd Fish Find Contradicts Intelligent-Design Argument  | publisher=[[National Geographic Society|National Geographic]] |date= July 9, 2008 | access-date =2008-07-17}}</ref> The more recently described fossil genera ''[[Quasinectes]]'' and ''[[Anorevus]]'' have been proposed to show similar morphologies and have also been classified as "[[stem group|stem]]-pleuronectiforms".<ref name="Bannikov&Zorzin_2019">Bannikov A.F. & Zorzin R (2019). [https://museodistorianaturale.comune.verona.it/media/_Musei/_StoriaNaturale/_Allegati/Biblioteca/Studi%20Bolca/Vol.%2019(2019)/5-15_Bannikov-Zorzin.pdf "A new genus and species of ''incertae sedis'' percomorph fish (Perciformes) from the Eocene of Bolca in northern Italy, and a new genus for ''Psettopsis latellai'' Bannikov, 2005"]. ''Studi e ricerche sui giacimenti terziari di Bolca'': p. 5-15.</ref><ref name="Bannikov&Zorzin_2020">Bannikov A.F. & Zorzin R. (2020). [https://www.researchgate.net/publication/344613829 "A new genus and species of percomorph fish ("stem pleuronectiform") from the Eocene of Bolca in northern Italy"]. ''Miscellanea Paleontologica'' '''17''': p. 5–14</ref> Such findings lead palaeontologist Matt Friedman to conclude that the evolution of flatfish morphology "happened gradually, in a way consistent with evolution via natural selection—not suddenly [saltationally] as researchers once had little choice but to believe."<ref name="NaEvo"/>

To explain the survival advantage of a partially migrated eye, it has been proposed that primitive flatfishes like ''Amphistium'' rested with the head propped up above the seafloor (a behaviour sometimes observed in modern flatfishes), enabling them to use their partially migrated eye to see things closer to the seafloor.<ref name="Janvier 2008 p169-170">Janvier P. (2008). "Squint of the fossil flatfish". ''Nature'' '''454'''(7201): p. 169–170</ref> While known basal genera like ''Amphistium'' and ''Heteronectes'' support a gradual acquisition of the flatfish morphology, they were probably not direct ancestors to living pleuronectiforms, as fossil evidence (for example ''[[Eobothus]]'') indicate that most flatfish lineages living today were present in the [[Eocene]] and [[Sympatry|contemporaneous with them]].<ref name="Friedman_2008" /> It has been suggested that the more primitive forms were eventually outcompeted.<ref name="NaEvo"/>

==Taxonomy==
Due to their highly distinctive morphology, flatfishes were previously treated as belonging to their own order, Pleuronectiformes. However, more recent taxonomic studies have found them to group within a diverse group of nektonic marine fishes known as the [[Carangiformes]], which also includes [[Carangidae|jacks]] and [[billfish]]. Specifically, flatfish have been recovered to be closely related to various groups, such as the [[threadfin]]s (often recovered as a [[sister group]] to flatfish), [[archerfish]], and [[beachsalmon]]s. Due to this, they are now treated as a suborder of the Carangiformes,<ref>{{Cite journal |last1=Girard |first1=Matthew G. |last2=Davis |first2=Matthew P. |last3=Smith |first3=W. Leo |date=2020-05-08 |title=The Phylogeny of Carangiform Fishes: Morphological and Genomic Investigations of a New Fish Clade |url=https://meridian.allenpress.com/copeia/article-abstract/108/2/265/436665/The-Phylogeny-of-Carangiform-Fishes-Morphological |journal=Copeia |volume=108 |issue=2 |pages=265–298 |doi=10.1643/CI-19-320 |issn=0045-8511}}</ref><ref>{{Cite journal |last1=Shi |first1=Wei |last2=Chen |first2=Shixi |last3=Kong |first3=Xiaoyu |last4=Si |first4=Lizhen |last5=Gong |first5=Li |last6=Zhang |first6=Yanchun |last7=Yu |first7=Hui |date=2018-05-25 |title=Flatfish monophyly refereed by the relationship of Psettodes in Carangimorphariae |journal=BMC Genomics |language=en |volume=19 |issue=1 |page=400 |doi=10.1186/s12864-018-4788-5 |doi-access=free |pmid=29801430 |pmc=5970519 |issn=1471-2164}}</ref> as represented in [[Eschmeyer's Catalog of Fishes]].<ref>{{cite web |last1=Fricke |first1=Ron |last2=Fong |first2=Jon David |title=GENERA/SPECIES BY FAMILY/SUBFAMILY IN Eschmeyer's Catalog of Fishes |url=https://researcharchive.calacademy.org/research/ichthyology/catalog/SpeciesByFamily.asp |website=researcharchive.calacademy.org |publisher=California Academy of Sciences |access-date=17 July 2025}}</ref>

=== Classification ===
The following classification is based on ''[[Eschmeyer's Catalog of Fishes]]'' (2025):<ref name=":13">{{Cite web |last1=Fricke |first1=R. |last2=Eschmeyer |first2=W. N. |last3=Van der Laan |first3=R. |date=2025 |title=ESCHMEYER'S CATALOG OF FISHES: CLASSIFICATION |url=https://www.calacademy.org/eschmeyers-catalog-of-fishes-classification |access-date=2025-10-22 |website=California Academy of Sciences |language=en}}</ref>

* Suborder '''Pleuronectoidei'''
** Family [[Polynemidae]] <small>[[Constantine Samuel Rafinesque|Rafinesque]]</small><small>, 1815</small> (threadfins or tassel-fishes)
** Family [[Psettodidae]] <small>[[Charles Tate Regan|Regan]]</small><small>, 1910</small> (spiny turbots)
** Family [[Citharidae]] <small>de Buen</small><small>, 1935</small> (largescale flounders)
** Family [[Scophthalmidae]] <small>Chabanaud</small><small>, 1933</small> (turbots)
** Family [[Cyclopsettidae]] <small>Campbell, Chanet, Chen, Lee & Chen</small><small>, 2019</small> (sand whiffs or large-tooth flounders)
**Family [[Grammatobothidae]] <small>Tongboonkua, Chanet & Chen</small><small>, 2025</small> (small lefteye flounders)<ref name=":1">{{Cite journal |last=Tongboonkua |first=Pakorn |last2=Chanet |first2=Bruno |last3=Chen |first3=Wei-Jen |title=Integrated Molecular and Morphological Analyses Resolve Long-Standing Classification Challenges in the Sinistral Flatfish Family Bothidae (Teleostei: Carangiformes) |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/zsc.70020 |journal=Zoologica Scripta |language=en |volume=n/a |issue=n/a |doi=10.1111/zsc.70020 |issn=1463-6409}}</ref>
** Family [[Monolenidae]] <small>Tongboonkua, Chanet & Chen</small><small>, 2025</small> (deepwater lefteye flounders)<ref name=":1" />
** Family [[Taeniopsettidae]] <small>Amaoka</small><small>, 1969</small><ref name=":1" />
** Family [[Bothidae]] <small>Smitt</small><small>, 1892</small> (lefteye flounders)
** Family [[Paralichthyidae]] <small>Regan</small><small>, 1910</small> (sand flounders)
** Family [[Pleuronectidae]] <small>Rafinesque</small><small>, 1815</small> (righteye flounders)
** Family [[Peppered flounder|Paralichthodidae]] <small>Regan</small><small>, 1920</small> (peppered flounders)
** Family [[Oncopteridae]] <small>[[David Starr Jordan|Jordan]] & Goss</small><small>, 1889</small> (remo flounders)
** Family [[Rhombosoleidae]] <small>Regan</small><small>, 1910</small> (South Pacific flounders)
** Family [[Achiropsettidae]] <small>Heemstra</small><small>, 1990</small> (southern flounders or armless flounders)
** Family [[Achiridae]] <small>Rafinesque</small><small>, 1815</small> (American soles)
** Family [[Samaridae]] <small>Jordan & Goss</small><small>, 1889</small> (crested flounders)
** Family [[Poecilopsettidae]] <small>[[John Roxborough Norman|Norman]]</small><small>, 1934</small> (bigeye flounders)
** Family [[Soleidae]] <small>[[Charles Lucien Bonaparte|Bonaparte]]</small><small>, 1833</small> (soles)
** Family [[Cynoglossidae]] <small>Jordan</small><small>, 1888</small> (tonguefishes)

==== Fossil taxa ====
The following [[Basal (phylogenetics)|basal]] fossil flatfish from the [[Paleogene]] are also known:<ref>{{Cite journal |last=Laan |first=Richard van der |date=2018-10-11 |title=Family-group names of fossil fishes |url=https://europeanjournaloftaxonomy.eu/index.php/ejt/article/view/597 |journal=European Journal of Taxonomy |language=en |issue=466 |doi=10.5852/ejt.2018.466  |doi-access=free|issn=2118-9773}}</ref>

* Genus ?†''[[Anorevus]]'' <small>Bannikov & Zorzin, 2020</small> ([[Ypresian|Early Eocene]] of Italy)<ref name="Bannikov&Zorzin_2019" />
* Genus †''[[Eobothus]]'' <small>[[Charles R. Eastman|Eastman]], 1914</small> (Early Eocene of Italy)
* Genus †''[[Heteronectes]]'' <small>Friedman, 2008</small> ([[Middle Eocene]] of France)
* Genus †''[[Imhoffius]]'' <small>[[Paul Chabanaud|Chabanaud]], 1940</small> (Middle Eocene of France)<ref name=":0">{{Cite journal |last1=Chanet |first1=Bruno |last2=Schultz |first2=Orwin |date=1994 |title=Pleuronectiform fishes from the Upper Badenian (Middle Miocene) of St. Margarethen (Austria) |url=https://www.researchgate.net/publication/230844383 |journal=Ann. Naturhist. Mus. Wien |issue=96 |pages=95–115}}</ref>
* Genus †''[[Keasichthys]]'' <small>Murray & Champagne, 2025</small> ([[Early Oligocene]] of Oregon, US)<ref>{{Cite journal |last1=Murray |first1=Alison M. |last2=Champagne |first2=Donald E. |date=January 2025 |title=A new species of early Oligocene flatfish (Pleuronectiformes) from Oregon, USA |url=https://www.cambridge.org/core/journals/geological-magazine/article/new-species-of-early-oligocene-flatfish-pleuronectiformes-from-oregon-usa/29C984BB97C135EF0ADC1CB242CD5143 |journal=Geological Magazine |language=en |volume=162 |article-number=e22 |doi=10.1017/S0016756825100125 |bibcode=2025GeoM..162E..22M |issn=0016-7568}}</ref>
* Genus †''[[Numidiopleura]]'' <small>Gaudant & Gaudant, 1969</small> (Eocene of [[Tunisia]])<ref name=":0" /><ref>{{Cite journal |last1=Campbell |first1=Matthew A. |last2=Chanet |first2=Bruno |last3=Chen |first3=Jhen-Nien |last4=Lee |first4=Mao-Ying |last5=Chen |first5=Wei-Jen |date=2019 |title=Origins and relationships of the Pleuronectoidei: Molecular and morphological analysis of living and fossil taxa |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/zsc.12372 |journal=Zoologica Scripta |language=en |volume=48 |issue=5 |pages=640–656 |doi=10.1111/zsc.12372 |issn=1463-6409}}</ref>
* Genus ?†''[[Quasinectes]]'' <small>Bannikov & Zorzin, 2019</small> (Early Eocene of Italy)<ref name="Bannikov&Zorzin_2020" />
* Family †[[Amphistium|Amphistiidae]] <small>[[George Albert Boulenger|Boulenger]]</small><small>, 1902</small>
* Family †[[Joleaudichthyidae]] <small>Chabanaud</small><small>, 1937</small>

=== Phylogeny ===
[[File:Polynemus paradiseus Ford 42.jpg|thumb|Threadfins such as ''[[Polynemus]]'' have been recovered closer to the primitive spiny turbots than those are to other flatfish, or as a [[sister group]] to a [[monophyletic]] flatfish group]]
There has been some disagreement whether flatfish as a whole are a [[monophyletic group]]. Some palaeontologists think that some percomorph groups unrelated to flatfishes were also "experimenting" with head asymmetry during the [[Eocene]],<ref name="Bannikov&Zorzin_2019" /><ref name="Bannikov&Zorzin_2020" /> and certain molecular studies conclude that the primitive family of [[Psettodidae]] evolved their flat bodies and asymmetrical head independently of other flatfish groups.<ref name="Campbell_et.al_2013">Campbell M.A., Chen W-J. & López J.A. (2013). "Are flatfishes (Pleuronectiformes) monophyletic?". ''Molecular Phylogenetics and Evolution'' '''69'''(3): p. 664-673. [https://doi.org/10.1016/j.ympev.2013.07.011 doi.org/10.1016/j.ympev.2013.07.011]</ref><ref name="Campbell_et.al_2014">Campbell M.A., López J.A., Satoh T.P., Chen W-J. & Miya M. (2014). "Mitochondrial genomic investigation of flatfish monophyly". ''Gene'' '''551'''(2): p. 176-182. [https://doi.org/10.1016/j.gene.2014.08.053 doi.org/10.1016/j.gene.2014.08.053]</ref> The following phylogeny is from Lü ''et al.'' 2021; a whole-[[genome]] analysis using [[concatenated]] sequences of coding sequence (CDS) (codon1 + 2 + 3, GTRGAMMA model; codon1 + 2, GTRGAMMA model) and 4dTV (fourfold degenerate synonymous site, GTRGAMMA model) derived from 1,693 single-copy genes. Notably, Pleuronectiformes is found to be [[polyphyletic]] as seen here:<ref>{{cite journal |date=19 April 2021|title=Large-scale sequencing of flatfish genomes provides insights into the polyphyletic origin of their specialized body plan|author1=Zhenming Lü|author2=Li Gong|author3=Yandong Ren|author4=Yongjiu Chen|author5=Zhongkai Wang|author6=Liqin Liu|author7=Haorong Li|author8=Xianqing Chen|author9=Zhenzhu Li|author10=Hairong Luo|author11=Hui Jiang|author12=Yan Zeng|author13=Yifan Wang|author14=Kun Wang|author15=Chen Zhang|author16=Haifeng Jiang|author17=Wenting Wan|author18=Yanli Qin|author19=Jianshe Zhang|author20=Liang Zhu|author21=Wei Shi|author22=Shunping He|author23=Bingyu Mao|author24=Wen Wang|author25=Xiaoyu Kong|author26=Yongxin Li |journal=Nature Genetics |volume=53|issue=5 |pages=742–751|doi=10.1038/s41588-021-00836-9 |pmid=33875864 |pmc=8110480 }}</ref>
{{clade
|1={{clade
 |label1=[[Polynemidae]]
 |1=''[[Polydactylus sextarius]]''
 |2={{clade
  |label1=[[Toxotidae]]
  |1=''[[Toxotes chatareus]]''
  |label2=Psettodoidei|sublabel2=[[Psettodidae]]|bar2=blue
  |2=''[[Psettodes erumei]]'' }} }}
|label2='''Pleuronectoidei'''
|grouplabel2={{Clade labels|label1=''Pleuronectiformes''|color=black}}
|2={{clade
 |1={{clade hidden|mode=left|expand-text=<!--Pleuronectoidea-->[[Bothidae]], [[Paralichthyidae]], [[Pleuronectidae]], [[Scophthalmidae]]
  |label1=Scophthalmidae
  |1=''[[Scophthalmus maximus]]''
  |2={{clade
   |label1=Bothidae
   |1=''[[Chascanopsetta lugubris]]''
   |2={{clade
    |label1=Paralichthyidae
    |1={{clade
     |1=''[[Paralichthys olivaceus]]''
     |2=''[[Pseudorhombus dupliocellatus]]'' }}
    |label2=Pleuronectidae
    |2=''[[Platichthys stellatus]]'' }} }} }}
 |2={{clade hidden|mode=left|expand-text=<!--Soleoidea-->[[Achiridae]], [[Cynoglossidae]], [[Rhombosoleidae]], [[Soleidae]]
  |1={{clade
   |label1=Soleidae
   |1=''[[Brachirus orientalis]]''
   |label2=Cynoglossidae
   |2={{clade
    |1=''[[Cynoglossus semilaevis]]''
    |2=''[[Paraplagusia blochii]]'' }} }}
  |2={{clade
   |label1=Rhombosoleidae
   |1=''[[Colistium nudipinnis]]''
   |label2=Achiridae
   |2=''[[Trinectes maculatus]]'' }} }} }}|bar2=blue
}}

However, threadfins (Polynemidae) aren't universally found to be nested within the group of flatfish, as recovered by a study of [[ultraconserved element]]s from the threadfin family in Girard ''et al.'' 2022,<ref>{{cite journal|date=12 January 2022|doi=10.1111/jfb.14997|title=Molecular phylogeny of the threadfin fishes (Polynemidae) using ultraconserved elements|journal=Fish Biology|author1=Matthew G. Girard|author2= Matthew P. Davis|author3=Carole C. Baldwin|author4=Agnès Dettaï| author5=Rene P. Martin |author6=W. Leo Smith|volume=100 |issue=3 |pages=793–810 |pmid=35137410 |bibcode=2022JFBio.100..793G |url=https://www.mgirard.fish/files/Girard_et_al_2022.pdf}}</ref> or as represented in the [[World Register of Marine Species]],<ref>{{Cite WoRMS|title=Polynemidae Rafinesque, 1815|id=125551|access-date=17 July 2025|db=FishBase}}</ref> where Pleuronectiformes is retained as a name for the flatfish group.<ref>{{Cite WoRMS|title=Pleuronectiformes|id=10331|access-date=17 July 2025|db=FishBase}}</ref> Numerous scientists continue to argue for a monophyletic group of all flatfish,<ref name="Duarte-Ribeiro et.al.2024">Duarte-Ribeiro E, Rosas-Puchuri U, Friedman M, Woodruff G.C., Hughes L.C., Carpenter K.E., White W.T., Pogonoski J.J., Westneat M, Diaz de Astarloa J.M., Williams J.T., Santos M.D., Domínguez-Domínguez O, Ortí G, Arcila D & Betancur-R R. (2024). "Phylogenomic and comparative genomic analyses support a single evolutionary origin of flatfish asymmetry". ''Nature Genetics'' '''56''': p. 1069-1072. [https://www.nature.com/articles/s41588-024-01784-w doi.org/10.1038/s41588-024-01784-w]</ref> though the debate continues.<ref>{{cite journal |last1=Lü |first1=Zhenming |last2=Li |first2=Haorong |last3=Jiang |first3=Hui |last4=Luo |first4=Hairong |last5=Wang |first5=Wen |last6=Kong |first6=Xiaoyu |last7=Li |first7=Yongxin |title=Reply to: Phylogenomic and comparative genomic analyses support a single evolutionary origin of flatfish asymmetry |journal=Nature Genetics |date=2024-05-27 |volume=56 |issue=6 |pages=1073–1074 |url=https://www.x-mol.net/paper/article/1795282921848369152|doi=10.1038/s41588-024-01783-x|pmid=38802565 |access-date=17 July 2025}}</ref>

Over 800 described species are placed into 16 families.<ref name="ReferenceA">{{Cite journal|last1=Campbell|first1=Matthew A.|last2=Chanet|first2=Bruno|last3=Chen|first3=Jhen-Nien|last4=Lee|first4=Mao-Ying|last5=Chen|first5=Wei-Jen|date=2019|title=Origins and relationships of the Pleuronectoidei: Molecular and morphological analysis of living and fossil taxa|journal=Zoologica Scripta|language=en|volume=48|issue=5|pages=640–656|doi=10.1111/zsc.12372|s2cid=202856805|issn=0300-3256|url=https://hal.science/hal-03971070/file/Campbell%20et%20al%202019%20Zool%20Scripta.pdf }}</ref> When they were treated as an order, the flatfishes are divided into two suborders, Psettodoidei and Pleuronectoidei, with > 99% of the species diversity found within the Pleuronectoidei.<ref>{{Cite book|title=Fishes of the world|author-first1=Joseph S. |author-last1=Nelson|author-first2=Terry C. |author-last2=Grande|author-first3=Mark V. H. |author-last3=Wilson|isbn=978-1-118-34233-6|oclc=958002567|date = 2016-03-28|publisher=John Wiley & Sons }}</ref> The largest families are [[Soleidae]], [[Bothidae]] and [[Tonguefish|Cynoglossidae]] with more than 150 species each. There also exist two monotypic families ([[Paralichthodidae]] and [[Oncopteridae]]). Some families are the results of relatively recent splits. For example, the [[Achiridae]] were classified as a subfamily of Soleidae in the past, and the [[Samaridae]] were considered a subfamily of the Pleuronectidae.<ref name="RandallHawaii">Randall, J. E. (2007). ''Reef and Shore Fishes of the Hawaiian Islands.'' {{ISBN|1-929054-03-3}}</ref><ref name="CopperChapleau">Cooper, J.A.; and Chapleau, F. (1998). ''Monophyly and intrarelationships of the family Pleuronectidae (Pleuronectiformes), with a revised classification.'' Fish. Bull. 96 (4): 686–726.</ref> The families [[Paralichthodidae]], [[Poecilopsettidae]], and [[Rhombosoleidae]] were also traditionally treated as subfamilies of Pleuronectidae, but are now recognised as families in their own right.<ref name="CopperChapleau" /><ref name="Nelson-2006">{{Cite book
 | edition = 4 | publisher = John Wiley & Sons | isbn = 978-0-471-25031-9 | last = Nelson | first = J. S. | title = Fishes of the World | location = Hoboken, NJ | date = 2006| title-link = Fishes of the World }}</ref><ref name="NGW-2016">{{cite book |title=Fishes of the World |edition=5th |author1=J. S. Nelson |author2=T. C. Grande |author3=M. V. H. Wilson |year=2016 |page=752 |publisher=Wiley |isbn=978-1-118-34233-6 |url=https://sites.google.com/site/fotw5th/ |access-date=2018-05-24 |archive-date=2019-04-08 |archive-url=https://web.archive.org/web/20190408194051/https://sites.google.com/site/fotw5th/ }}</ref> The [[Paralichthyidae]] has long been indicated to be paraphyletic, with the formal description of [[Cyclopsettidae]] in 2019 resulting in the split of this family as well.<ref name="ReferenceA" /> The following is the [[maximum likelihood phylogenetic tree]] from Campbell ''et al.'' 2019, which was obtained by analyzing seven [[protein-coding]] genes. This study erected two new families to resolve the previously non-monophyletic status of Paralichthyidae and the [[Rhombosoleidae]]:<ref name="ReferenceA"/>
{{clade
|1=''[[Centropomus]]''
|2={{clade
 |1=''[[Lates]]''
 |2={{clade
  |1=''[[Psettodes]]''
  |label2='''Pleuronectoidei'''
  |2={{clade
   |label1=Citharoidea|sublabel1=[[Citharidae]]
   |1={{clade hidden|mode=left|id=3
    |1=''[[Brachypleura]]''
    |2={{clade
     |1=''[[Lepidoblepharon]]''
     |2={{clade
      |1=''[[Citharoides]]''
      |2=''[[Citharus]]'' }} }} }}
   |2={{clade
    |label1='''Pleuronectoidea'''
    |1={{clade hidden|mode=left|expand-text=[[Scophthalmidae]], [[Pleuronectidae]], [[Paralichthyidae]], [[Cyclopsettidae]], [[Bothidae]]|id=17
     |1={{clade
      |label1=[[Scophthalmidae]]
      |1={{clade
       |1=''[[Scophthalmus]]''
       |2=''[[Lepidorhombus]]'' }}
      |2={{clade
       |1={{clade
        |label1=[[Pleuronectidae]]
        |1={{clade
         |1={{clade
          |1=''[[Pleuronichthys]]''
          |2=''[[Hypsopsetta]]'' }}
         |2={{clade
          |1=''[[Limanda]]''
          |2={{clade
           |1=''[[Eopsetta]]''
           |2=''[[Hippoglossus]]'' }} }} }} }}
        |label2=[[Paralichthyidae]]
        |2={{clade
         |1=''[[Xystreurys]]''
         |2={{clade
          |1=''[[Pseudorhombus]]''
          |2=''[[Paralichthys]]'' }} }} }} }}
       |2={{clade
        |label1=[[Cyclopsettidae]]
        |1={{clade
         |1=''[[Cyclopsetta]]''
         |2={{clade
          |1=''[[Etropus]]''
          |2=''[[Citharichthys]]'' }} }}
        |label2=[[Bothidae]]
        |2={{clade
         |1=''[[Chascanopsetta]]''
         |2={{clade
          |1=''[[Taeniopsetta]]''
          |2={{clade
           |1={{clade
            |1=''[[Engyprosopon]]''
            |2={{clade
             |1=''[[Arnoglossus]]''
             |2=''[[Parabothus]]'' }} }}
           |2={{clade
            |1=''[[Bothus]]''
            |2={{clade
             |1=''[[Laeops]]''
             |2={{clade
              |1=''[[Kamoharaia megastoma|Kamoharaia]]''
              |2={{clade
               |1=''[[Neolaeops microphthalmus|Neolaeops]]''
               |2=''[[Japonolaeops dentatus|Japonolaeops]]'' }} }} }} }} }} }} }} }} }}
    |label2='''Soleoidea'''
    |2={{clade hidden|mode=left|expand-text=[[Achiridae]], [[Rhombosoleidae]], [[Achiropsettidae]], [[Samaridae]], [[Poecilopsettidae]], [[Soleidae]], [[Cynoglossidae]], a few monotypic families|id=20
     |1={{clade
      |label1=[[Achiridae]]
      |1={{clade
       |1=''[[Gymnachirus]]''
       |2=''[[Trinectes]]'' }}
      |2={{clade
       |label1=Paralichthodidae
       |1=''[[Paralichthodes]]''
       |2={{clade
        |label1=Oncopteridae
        |1=''[[Oncopterus]]''
        |2={{clade
         |label1=[[Rhombosoleidae]]
         |1=''[[Rhombosolea]]''
         |label2=[[Achiropsettidae]]
         |2=''[[Mancopsetta]]'' }} }} }} }}
     |2={{clade
      |label1=[[Samaridae]]
      |1={{clade
       |1=''[[Samariscus]]''
       |2={{clade
        |1=''[[Plagiopsetta]]''
        |2=''[[Samaris]]'' }} }}
      |2={{clade
       |label1=[[Poecilopsettidae]]
       |1={{clade
        |1=''[[Nematops]]''
        |2=''[[Poecilopsetta]]'' }}
       |2={{clade
        |label1=[[Soleidae]]
        |1={{clade
         |1={{clade
          |1=''[[Aseraggodes]]''
          |2=''[[Pardachirus]]'' }}
         |2={{clade
          |1={{clade
           |1=''[[Pegusa]]''
           |2=''[[Solea (fish)|Solea]]'' }}
          |2={{clade
           |1=''[[Brachirus]]''
           |2=''[[Zebrias]]'' }} }} }}
        |label2=[[Cynoglossidae]]
        |2={{clade
         |1=''[[Symphurus]]''
         |2={{clade
          |1=''[[Paraplagusia]]''
          |2=''[[Cynoglossus]]'' }} }} }} }} }} }} }}
}} }} }} }}

The [[Taxonomy (biology)|taxonomy]] of some groups is in need of a review.  The last [[monograph]] covering the entire order was [[John Roxborough Norman]]'s ''Monograph of the Flatfishes'' published in 1934. In particular, ''[[Tephrinectes sinensis]]'' may represent a family-level lineage and requires further evaluation e.g.<ref>{{Cite journal|last=Hoshino|first=Koichi|date=2001-11-01|title=Monophyly of the Citharidae (Pleuronectoidei: Pleuronectiformes: Teleostei) with considerations of pleuronectoid phylogeny|journal=Ichthyological Research|volume=48|issue=4|pages=391–404|doi=10.1007/s10228-001-8163-0|bibcode=2001IchtR..48..391H |s2cid=46318428|issn=1341-8998}}</ref>  [[New species]] are described with some regularity and [[undescribed species]] likely remain.<ref name=RandallHawaii/>

==Timeline of genera==
<timeline>
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 from: -65.5   till:  -55.8    color:paleocene  text:[[Paleocene]]
 from: -55.8   till:  -33.9    color:eocene  text:[[Eocene]]
 from: -33.9   till:  -23.03    color:oligocene  text:[[Oligocene]]
 from: -23.03    till: -5.332    color:miocene    text:[[Miocene]]
 from: -5.332    till: -2.588    color:pliocene    text:[[Plio.]]
 from: -2.588    till: -0.0117   color:pleistocene    text:[[Pleist.]]
 from: -0.0117    till: 0    color:holocene    text:[[Holocene|H.]]

 bar:eratop
 from: -65.5   till:  -23.03    color:paleogene  text:[[Paleogene]]
 from: -23.03    till: -2.588    color:neogene    text:[[Neogene]]
 from: -2.588    till: 0   color:quaternary    text:[[Quaternary|Q.]]

PlotData=
 align:left fontsize:M mark:(line,white) width:5 anchor:till align:left

 color:eocene bar:NAM1  from:-55.8    till:-33.9 text:[[Amphistium]]
 color:eocene bar:NAM2  from:-55.8    till:-33.9 text:[[Eobothus]]
 color:eocene bar:NAM3  from:-55.8    till:-33.9 text:[[Eobuglossus]]
 color:eocene bar:NAM4 from:-55.8    till:-33.9 text:[[Imhoffius]]
 color:eocene bar:NAM5 from:-55.8   till:-33.9 text:[[Joleaudichthys]]
 color:eocene bar:NAM6 from:-55.8    till:-33.9 text:[[Turahbuglossus]]
 color:eocene bar:NAM7 from:-55.8    till:0 text:[[Scophthalmus]]
 color:eocene bar:NAM8  from:-55.8    till:0 text:[[Citharus]]
 color:eocene bar:NAM9  from:-55.8    till:0 text:[[Psettodes]]
 color:eocene bar:NAM10  from:-37.2 till:0 text:[[Arnoglossus]]
 color:oligocene bar:NAM11 from:-33.9 till:0 text:[[Bothus]]
 color:oligocene bar:NAM12 from:-33.9 till:0 text:[[Monolene]]
 color:oligocene bar:NAM13 from:-33.9 till:0 text:[[Solea]]
 color:oligocene bar:NAM14  from:-28.4 till:0 text:[[Buglossidium]]
 color:oligocene bar:NAM15 from:-28.4 till:0 text:[[Hippoglossoides]]
 color:oligocene bar:NAM16  from:-28.4 till:0 text:[[Lepidorhombus]]
 color:miocene bar:NAM17  from:-23.03 till:0 text:[[Dicologoglossa]]
 color:miocene bar:NAM18  from:-23.03 till:0 text:[[Paraplagusia]]
 color:miocene bar:NAM19 from:-23.03 till:0 text:[[Platichthys]]
 color:miocene bar:NAM20 from:-15.97 till:0 text:[[Achiurus]]
 color:miocene bar:NAM21 from:-15.97 till:0 text:[[Microchirus]]
 color:miocene bar:NAM22 from:-15.97 till:0 text:[[Microstomus]]
 color:miocene bar:NAM23 from:-11.608   till:-5.332 text:[[Evesthes]]
 color:miocene bar:NAM24 from:-11.608   till:0 text:[[Citharichthys]]
 color:miocene bar:NAM25 from:-11.608   till:0 text:[[Monochirus]]
 color:miocene bar:NAM26 from:-11.608   till:0 text:[[Paralichthys]]
 color:miocene bar:NAM27 from:-11.608   till:0 text:[[Pleuronichthys]]
 color:pliocene bar:NAM28 from:-5.332    till:0 text:[[Atheresthes]]
 color:pliocene bar:NAM29 from:-5.332 till:0 text:[[Clidoderma]]
 color:pliocene bar:NAM30 from:-5.332 till:0 text:[[Glyptocephalus]]
 color:pliocene bar:NAM31 from:-5.332 till:0 text:[[Limanda]]
 color:pliocene bar:NAM32 from:-5.332 till:0 text:[[Lyopsetta]]
 color:pliocene bar:NAM33 from:-5.332    till:0 text:[[Pegusa]]
 color:pleistocene bar:NAM34 from:-2.588    till:-0.0117 text:[[Chibapsetta]]
 color:pleistocene bar:NAM35 from:-2.588    till:0 text:[[Eopsetta]]
 color:pleistocene bar:NAM36 from:-2.588    till:0 text:[[Isopsetta]]
 color:pleistocene bar:NAM37 from:-2.588    till:0 text:[[Parophrys]]
 color:pleistocene bar:NAM38 from:-2.588    till:0 text:[[Symphurus]]

PlotData=
 align:center textcolor:black fontsize:M mark:(line,black) width:25

 bar:period
 from: -65.5   till:  -55.8    color:paleocene  text:[[Paleocene]]
 from: -55.8   till:  -33.9    color:eocene  text:[[Eocene]]
 from: -33.9   till:  -23.03    color:oligocene  text:[[Oligocene]]
 from: -23.03    till: -5.332    color:miocene    text:[[Miocene]]
 from: -5.332    till: -2.588    color:pliocene    text:[[Plio.]]
 from: -2.588    till: -0.0117   color:pleistocene    text:[[Pleist.]]
 from: -0.0117    till: 0    color:holocene    text:[[Holocene|H.]]

 bar:era
 from: -65.5   till:  -23.03    color:paleogene  text:[[Paleogene]]
 from: -23.03    till: -2.588    color:neogene    text:[[Neogene]]
 from: -2.588    till: 0   color:quaternary    text:[[Quaternary|Q.]]
</timeline>

==Relation to humans==
{{common fish}}
{{Expand section|date=July 2025|small=no}}

===Fishing and aquaculture===
Flatfish are commonly fished using [[bottom trawl]]s.<ref>{{cite journal |last1=Segura-Garcia |first1=Iris |last2=Soe |first2=Sabai |last3=Tun |first3=NYO-NYO |last4=Box |first4=Stephen |title=Beyond Bycatch: The Species Diversity of Tonguesole (Pleuronectiformes: Cynoglossidae) in Coastal Fisheries of the Tanintharyi Region, Southern Myanmar |journal=Asian Fisheries Science |date=2021 |volume=34 |pages=23–33 |doi=10.33997/j.afs.2021.34.1.003 |url=https://www.asianfisheriessociety.org/publication/downloadfile.php?id=1347&file=Y0dSbUx6QXhORGsxTWpNd01ERTJNVGN4TkRVMU1qa3VjR1Jt |access-date=17 July 2025|doi-access=free }}</ref><ref>{{cite journal |last1=Zhang |first1=Jinyong |last2=Cui |first2=Xiaoyu |last3=Lin |first3=Lin |last4=Liu |first4=Yuan |last5=Ye |first5=Jinqing |last6=Zhang |first6=Weiyue |last7=Li |first7=Hongjun |title=Unraveling Fish Community Diversity and Structure in the Yellow Sea: Evidence from Environmental DNA Metabarcoding and Bottom Trawling |journal=Animals |date=30 April 2025 |volume=15 |issue=9 |page=1283 |doi=10.3390/ani15091283 |pmid=40362097 |pmc=12070852 |doi-access=free }}</ref> Large species such as the halibuts are specifically targeted by fisheries, resulting in heavy fishing pressures and [[bycatch]].<ref>{{cite journal |last1=Szymkowiak |first1=Marysia |last2=Marrinan |first2=Sarah |last3=Kasperski |first3=Stephen |title=The Pacific Halibut, Hippoglossus stenolepis, and Sablefish, Anoplopoma fimbria, Individual Fishing Quota Program: A Twenty-year Retrospective |journal=Marine Fisheries Review |date=2020 |volume=82 |issue=1–2 |pages=1–16 |doi=10.7755/MFR.82.1-2.1 |url=https://spo.nmfs.noaa.gov/sites/default/files/pdf-content/mfr821-21.pdf |access-date=17 July 2025}}</ref><ref>{{cite journal |title=Collaborative Pacific Halibut, Hippoglossus stenolepis, Bycatch Control by Canada and the United States |journal=Marine Fisheries Review |volume=66 |url=https://spo.nmfs.noaa.gov/sites/default/files/pdf-content/MFR/mfr662/mfr6624.pdf |access-date=17 July 2025}}</ref><ref>{{cite journal |last1=Figus |first1=Elizabeth |last2=Criddle |first2=Keith R. |title=Characterizing preferences of fishermen to inform decision-making: A case study of the Pacific halibut (Hippoglossus stenolepis) fishery off Alaska |journal=PLOS |date=March 1, 2019 |volume=14 |issue=3 |article-number=e0212537 |doi=10.1371/journal.pone.0212537 |pmid=30822324 |pmc=6396916 |bibcode=2019PLoSO..1412537F |doi-access=free }}</ref> Some species are [[aquaculture]]d, such as the tonguefish ''[[Cynoglossus semilaevis]]''.<ref>{{Cite journal |last1=Hu |first1=Yuanri |last2=Li |first2=Yangzhen |last3=Li |first3=Zhongming |last4=Chen |first4=Changshan |last5=Zang |first5=Jiajian |last6=Li |first6=Yuwei |last7=Kong |first7=Xiangqing |date=December 2020 |title=Novel insights into the selective breeding for disease resistance to vibriosis by using natural outbreak survival data in Chinese tongue sole (Cynoglossus semilaevis) |url=https://linkinghub.elsevier.com/retrieve/pii/S0044848620314496 |journal=Aquaculture |language=en |volume=529 |article-number=735670 |doi=10.1016/j.aquaculture.2020.735670|bibcode=2020Aquac.52935670H |s2cid=224900889 |url-access=subscription }}</ref><ref>{{Cite journal |last1=Li |first1=Yangzhen |last2=Hu |first2=Yuanri |last3=Yang |first3=Yingming |last4=Zheng |first4=Weiwei |last5=Chen |first5=Changshan |last6=Li |first6=Zhongming |date=January 2021 |title=Selective breeding for juvenile survival in Chinese tongue sole (Cynoglossus semilaevis): Heritability and selection response |url=https://linkinghub.elsevier.com/retrieve/pii/S0044848620314848 |journal=Aquaculture |language=en |volume=531 |article-number=735901 |doi=10.1016/j.aquaculture.2020.735901|bibcode=2021Aquac.53135901L |s2cid=224878642 |url-access=subscription }}</ref>

===As food===
Flatfish is considered a [[Whitefish (fisheries term)|whitefish]]<ref>{{cite web|title=Flatfish BBC|url=https://www.bbc.co.uk/food/flatfish}}</ref> because of the high concentration of oils within its liver. Its lean flesh makes for a unique flavor that differs from species to species. Methods of cooking include grilling, pan-frying, baking and deep-frying.
<gallery mode="packed">
File:Pleuronectes platessa.jpg|The [[European plaice]] is the principal commercial flatfish in Europe.
File:Lined sole.jpg|[[American sole]]s are found in both freshwater and marine environments of the Americas.
File:Alaska 2007 071.jpg|[[Halibut]] are the largest of the flatfishes, and provide lucrative fisheries.
File:Psetta maxima Luc Viatour.jpg|The [[turbot]] is a large, left-eyed flatfish found in sandy shallow coastal waters around Europe.
File:Flatfish-lefteyed-flounder.jpg|Flatfish (left‐eyed flounder)
</gallery>

==References==
{{Reflist|28em}}

==Further reading==
* {{cite journal|last=Sepkoski |first=Jack |title=A compendium of fossil marine animal genera |journal=Bulletins of American Paleontology |volume=364 |page=560 |year=2002 |url=http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=611&rank=class |access-date=2011-05-17 |archive-url=https://web.archive.org/web/20110723131237/http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=611&rank=class |archive-date=July 23, 2011 }}
* 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==
*[http://www.dfo-mpo.gc.ca/science/publications/uww-msm/articles/plaice-plie-eng.htm Information on Canadian fisheries of plaice]

{{Flatfish}}
{{Commercial fish topics}}
{{Actinopterygii}}

{{Taxonbar|from=Q59577}}
{{Authority control}}

[[Category:Commercial fish]]
[[Category:Pleuronectoidei| ]]
[[Category:Articles which contain graphical timelines]]
[[Category:Extant Paleocene first appearances]]
[[Category:Asymmetry]]
[[Category:Flatfish]]