Editing Flatfish

{{short description|Order of fishes}}
{{for-multi|the album by Flook|Flatfish (album)|the move in shogi|Flatfish (shogi)}}
{{Automatic taxobox
| name = Flatfish
| fossil_range = {{Fossil range|Late Paleocene|Recent|ref=<ref name=EoF/>}}
| image = BonyFlatfish.jpg
| image_caption = Illustration of different flatfish by [[Alfred Brehm|Brehm]] (1865), including [[turbot]], [[sole (fish)|sole]], and [[plaice]]
| taxon = Pleuronectoidei
| subdivision_ranks = Families
| subdivision_ref = <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 |pages=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/ |url-status=dead }}</ref>
| subdivision = : [[Polynemidae]] (threadfins)
: [[Psettodidae]] (spiny turbots)
:Superfamily Citharoidea
:: [[Citharidae]] (largescale flounders)
:Superfamily Pleuronectoidea
:: [[Scophthalmidae]] (turbots)
:: [[Paralichthyidae]] (large-tooth flounders)
:: [[Pleuronectidae]] (righteye flounders)
:: [[Bothidae]] (lefteye flounders)
:Superfamily Soleoidea
:: [[Paralichthodidae]] (measles flounders)
:: [[Poecilopsettidae]] (bigeye flounders)
:: [[Rhombosoleidae]] (rhombosoleids)
:: [[Achiropsettidae]] (southern flounders)
:: [[Samaridae]] (crested flounders)
:: [[Achiridae]] (American soles)
:: [[Soleidae]] (true soles)
:: [[Cynoglossidae]] (tonguefishes)
| 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]]
}}

A '''flatfish''' is a member of the [[Ray-finned fish|ray-finned]] [[demersal fish]] [[Order (biology)|suborder]] '''Pleuronectoidei''', also called the Heterosomata. In many species, both eyes lie on one side of the head, one or the other migrating through or around the head during development. Some species face their left sides upward, some face their right sides upward, and others face either side upward. The most primitive members of the group, the [[Threadfin|threadfins]], do not resemble the flatfish but are their closest relatives.

Many important [[food fish]] are in this order, including the [[flounder]]s, [[sole (fish)|sole]]s, [[turbot]], [[plaice]], and [[halibut]]. Some flatfish can [[camouflage]] themselves on the ocean floor.

==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 are most closely related to the [[Threadfin|threadfins]], which are now also placed in the suborder Pleuronectoidei. Together, the group is most closely related to the [[archerfish]] and [[Beachsalmon|beachsalmons]] within [[Toxotoidei]]. 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 |pages=400 |doi=10.1186/s12864-018-4788-5 |doi-access=free |pmid=29801430 |pmc=5970519 |issn=1471-2164}}</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=9781118342336|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" /><ref name="NGW-2016" /> 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 [[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/>

===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/>

==Distribution==
Flatfishes are 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 longitudinal gradients away from the Indo-West Pacific.<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. An observation of a flatfish from the [[Bathyscaphe Trieste]] at the bottom of the [[Mariana Trench]] at a depth of almost {{convert|11|km|ft|abbr=on}} has been questioned by fish experts, 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, ''[[Symphurus thermophilus]]'' lives congregating around "ponds" of [[sulphur]] at [[hydrothermal vent]]s on the seafloor. No other flatfish is known from hydrothermal vents.<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> Many species will enter [[brackish]] or fresh water, and a smaller number of soles (families [[Achiridae]] and [[Soleidae]]) and tonguefish ([[Cynoglossidae]]) are entirely 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>

==Characteristics==
[[File:Pseudopleuronectes americanus.jpg|thumb|left|Flatfish are asymmetric, with both eyes lying on the same side of the head]]
[[File:Platichthys flesus - en.pdf|thumb|left|European flounder, like other flatfish, experience an eye migration during their lifetime.]]
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 usually on the right side of the body (dextral or right-eyed flatfish), and in others, they are usually on the left (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.
[[File:Polynemus paradiseus Ford 42.jpg|thumb|''[[Polynemus]]'', a threadfin, belongs to the same group as flatfish, but looks completely different]]
The most basal members of the group, the [[Threadfin|threadfins]], do not closely resemble the flatfishes.

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 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|pages= xxx|isbn= 0-12-547665-5 |no-pp=true}}</ref>

In general, flatfishes rely on their camouflage for avoiding predators, but some have [[aposematism|aposematic]] traits 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 flatworms of the genus ''[[Pseudobiceros]]'' in both colours and swimming mode.<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>

The [[flounder]]s 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 [[Sole (fish)|sole]]s, 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.<ref name=EoF/>

Flatfishes range in size from ''[[Tarphops oligolepis]]'', measuring about {{convert|4.5|cm|in|abbr=on}} in length, and weighing {{convert|2|g|oz|abbr=on}}, to the [[Atlantic halibut]], at {{convert|2.5|m|ft|abbr=on}} and {{convert|316|kg|lb|abbr=on}}.<ref name=EoF/>

 {{clear}}

{{common fish}}

==Species and species groups==
{{div col|colwidth=22em}}
* [[Brill (fish)|Brill]]
* [[Common dab|Dab]]
* [[Sanddab]]
* [[Flounder]]
* [[Halibut]]
* [[Megrim]]
* [[Plaice]]
* [[Sole (fish)|Sole]]
* [[Tonguefish]]
* [[Turbot]]
{{div col end}}

==Reproduction==
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]].<ref name=EoF/>

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.

==Origin and evolution==
Scientists have been proposing since the 1910s that flatfishes evolved from [[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> There has been some disagreement whether they are a monophyletic group. Some palaeontologists think that some percomorph groups other than flatfishes were "experimenting" with head asymmetry during the [[Eocene]],<ref name="Bannikov&Zorzin_2019"></ref><ref name="Bannikov&Zorzin_2020"></ref> 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> Many scientists, however, argue that pleuronectiformes are [[monophyletic]].<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> 

The fossil record indicates 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>

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

<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>

[[File:Amphistium.JPG|thumb|left|Fossil of ''[[Amphistium]]''.]]
The origin of the unusual morphology of flatfishes was enigmatic up to the 2000s, and early researchers suggested that it came about as a result of [[saltation (biology)|saltation]] rather than gradual evolution through natural selection, because a partially migrated eye were considered to have been maladaptive. This started to change in 2008 with a study on the two fossil genera ''[[Amphistium]]'' and ''[[Heteronectes]]'', dated to about 50 million years ago. These genera retain primitive features not seen in modern types of flatfishes. In addition, their heads are 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  | url-status=dead  | 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]]''<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> and ''[[Anorevus]]''<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> have been proposed to show similar morphologies and have also been classified as "stem pleuronectiforms". Suchs findings lead Friedman to conclude that the evolution of flatfish morphology "happened gradually, in a way consistent with evolution via [[natural selection]]—not suddenly, 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 indicate that most flatfish lineages living today were present in the [[Eocene]] and contemporaneous with them.<ref name="Friedman_2008"></ref> It has been suggested that the more primitive forms were eventually outcompeted.<ref name="NaEvo"/>

{{clear}}

<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>

==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.

==Timeline of genera==
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</timeline>

==See also==
{{Commons category|Pleuronectoidei}}
* [[Sinistral and dextral]]

==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 |url-status=dead |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]]