Product Code Database
Flatfish are ray-finned fish belonging to the suborder Pleuronectoidei and historically the order Pleuronectiformes (though this is now disputed). 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) lays on the substrate. This loss of symmetry, a Autapomorphy in , stems from one eye "migrating" towards the other during the juvenile's metamorphosis; due to variation, some species tend to face their left side upward, some their right side, and others face either side upward.This is an example. They are one of the most speciose groups of demersal fish, though their crypsis, a form of camouflage, conceals them from potential predators.
As these are merely common names, they do not conform with the "natural" relationships that are recovered through scientific studies of morphology or Genetic analysis. As examples, the three species consistently called "halibut" are themselves part of the right-eye flounder family, while the are not at all closely related to "true" turbot, but are consistently recovered in a "primitive" or basal position at the base of flatfish phylogenetic trees.
Conversely, many species will enter brackish or fresh water, and a smaller number of soles (families Achiridae and Soleidae) and tonguefish (Cynoglossidae) are entirely Freshwater fish.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.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.Monks, N. (2007). Freshwater flatfish, order Pleuronectiformes. Retrieved 18 May 2014
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 , in a manner similar to some cephalopods. The side of the body without the eyes, facing the seabed, is usually colourless or very pale. (1998). 9780125476652, Academic Press. ISBN 9780125476652
In general, flatfishes rely on their camouflage for avoiding predators, but some have such as conspicuous eyespots (e.g., Microchirus ocellatus) and several small tropical species (at least Aseraggodes, Pardachirus and Zebrias) are poisonous.Elst, R. van der (1997) A Guide to the Common Sea Fishes of South Africa. Debelius, H. (1997). Mediterranean and Atlantic Fish Guide. Juveniles of Soleichthys maculosus mimic toxic of the genus Pseudobiceros in both colours and swimming pattern.Practical Fishkeeping (22 May 2012) Video: Tiny sole mimics a flatworm. Retrieved 17 May 2014.Australian Museum (5 November 2010). This week in Fish: Flatworm mimic and shark teeth. Retrieved 17 May 2014. Conversely, a few octopus species have been reported to mimic flatfishes in colours, shape and swimming mode.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
Flatfishes range in size from the sand flounder Tarphops oligolepis, measuring about in length, and weighing , to the Hippoglossus, with the Atlantic halibut measuring up to long, and the Pacific halibut weighing up to .
Many species such as flounders and spiny turbots Predatory fish, and have well-developed teeth. These species sometimes huny in the midwater, away from the bottom, and show fewer "extreme" adaptations than other families. The soles, by contrast, are almost exclusively bottom-dwellers (more strictly Demersal fish), and feed on benthic invertebrates. They show a more extreme asymmetry, and may lack teeth on one side of the jaw.
The length of the stage varies between different types of flatfishes, but through the influence of thyroid hormones, they eventually begin to Metamorphosis 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.
Scientists have been proposing since the 1910s that flatfishes evolved from more "typical" Percoidea ancestors.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. doi.org/10.1080/00222931008692879 The fossil record indicated that flatfishes might have been present before the Eocene, based on fossil resembling those of modern pleuronectiforms dating back to the Thanetian and Ypresian stages (57-53 million years ago).Schwarzhans W. (1999). "A comparative morphological treatise of recent and fossil otoliths of the order Pleuronectiformes". Piscium Catalogus. Otolithi Piscium 2. 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 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 fish genera; Amphistium and Heteronectes, which dated to about 50 million years ago. These genera retain 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.Friedman M. (2008). "The evolutionary origin of flatfish asymmetry". Nature 454(7201): p. 209–212. The more recently described fossil genera Quasinectes and Anorevus have been proposed to show similar morphologies and have also been classified as "stem group-pleuronectiforms".Bannikov A.F. & Zorzin R (2019). "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.Bannikov A.F. & Zorzin R. (2020). "A new genus and species of percomorph fish ("stem pleuronectiform") from the Eocene of Bolca in northern Italy". Miscellanea Paleontologica 17: p. 5–14 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."
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.Janvier P. (2008). "Squint of the fossil flatfish". Nature 454(7201): p. 169–170 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 evidenceThis is an example. indicate that most flatfish lineages living today were present in the Eocene and Sympatry. It has been suggested that the more primitive forms were eventually outcompeted.
However, threadfins (Polynemidae) aren't universally found to be nested within the group of flatfish, as recovered by a study of ultraconserved elements from the threadfin family in Girard et al. 2022, or as represented in the World Register of Marine Species, where Pleuronectiformes is retained as a name for the flatfish group. Numerous scientists continue to argue for a monophyletic group of all flatfish,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. doi.org/10.1038/s41588-024-01784-w though the debate continues.
Over 800 described species are placed into 16 families. 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.
The 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. New species are described with some regularity and undescribed species likely remain.
Period = from:-65.5 till:15
TimeAxis = orientation:horizontal
ScaleMajor = unit:year increment:5 start:-65.5
ScaleMinor = unit:year increment:1 start:-65.5
TimeAxis = orientation:hor
AlignBars = justify
Colors =
BarData=
PlotData=
PlotData=
PlotData=
Development
Flatfishes lay eggs that hatch into Juvenile fish 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 Fish fin. They also possess a swim bladder, and do not dwell on the bottom, instead dispersing from their hatching grounds as ichthyoplankton. Bilaterally symmetric fish such as goldfish maintain balance using a system within their which involves the otolith, but larval and metamorphizing flatfish require visible light (such as sunlight) to properly orient themselves.
(2025). 9789811978586, Springer. ISBN 9789811978586
Hybrids
Hybrids are well known in flatfishes. The Pleuronectidae have the largest number of reported hybrids of marine fishes.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 Two of the most famous intergeneric hybrids are between the European plaice ( Pleuronectes platessa) and European flounder ( Platichthys flesus) in the Baltic Sea,Food and Agriculture Organization of the United Nations: Platichthys flesus (Linnaeus, 1758).. Retrieved 18 May 2014 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.
Evolution
Flatfishes have been cited as dramatic examples of evolutionary adaptation. In The Blind Watchmaker, Richard Dawkins explains the flatfishes' evolutionary history as such:
...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.
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 and billfish. Specifically, flatfish have been recovered to be closely related to various groups, such as the (often recovered as a sister group to flatfish), archerfish, and . Due to this, they are now treated as a suborder of the Carangiformes, as represented in Eschmeyer's Catalog of Fishes.
Classification
The following classification is based on Eschmeyer's Catalog of Fishes (2025):
Fossil taxa
The following fossil taxa are also placed in this suborder:
Phylogeny
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, and certain molecular studies conclude that the primitive family of Psettodidae evolved their flat bodies and asymmetrical head independently of other flatfish groups.Campbell M.A., Chen W-J. & López J.A. (2013). "Are flatfishes (Pleuronectiformes) monophyletic?". Molecular Phylogenetics and Evolution 69(3): p. 664-673. doi.org/10.1016/j.ympev.2013.07.011Campbell 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. doi.org/10.1016/j.gene.2014.08.053 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:
(2016). 9781118342336, John Wiley & Sons. ISBN 9781118342336 The largest families are Soleidae, Bothidae and Tonguefish 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.Randall, J. E. (2007). Reef and Shore Fishes of the Hawaiian Islands. 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. The families Paralichthodidae, Poecilopsettidae, and Rhombosoleidae were also traditionally treated as subfamilies of Pleuronectidae, but are now recognised as families in their own right. 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. 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:
Timeline of genera
#legends
id:CAR value:claret
id:ANK value:rgb(0.4,0.3,0.196)
id:HER value:teal
id:HAD value:green
id:OMN value:blue
id:white value:black
id:white value:white
id:cenozoic value:rgb(0.54,0.54,0.258)
id:paleogene value:rgb(0.99,0.6,0.32)
id:paleocene value:rgb(0.99,0.65,0.37)
id:eocene value:rgb(0.99,0.71,0.42)
id:oligocene value:rgb(0.99,0.75,0.48)
id:neogene value:rgb(0.999999,0.9,0.1)
id:miocene value:rgb(0.999999,0.999999,0)
id:pliocene value:rgb(0.97,0.98,0.68)
id:quaternary value:rgb(0.98,0.98,0.5)
id:pleistocene value:rgb(0.999999,0.95,0.68)
id:holocene value:rgb(0.999,0.95,0.88)
bar:eratop
bar:space
bar:periodtop
bar:space
bar:NAM1
bar:NAM2
bar:NAM3
bar:NAM4
bar:NAM5
bar:NAM6
bar:NAM7
bar:NAM8
bar:NAM9
bar:NAM10
bar:NAM11
bar:NAM12
bar:NAM13
bar:NAM14
bar:NAM15
bar:NAM16
bar:NAM17
bar:NAM18
bar:NAM19
bar:NAM20
bar:NAM21
bar:NAM22
bar:NAM23
bar:NAM24
bar:NAM25
bar:NAM26
bar:NAM27
bar:NAM28
bar:NAM29
bar:NAM30
bar:NAM31
bar:NAM32
bar:NAM33
bar:NAM34
bar:NAM35
bar:NAM36
bar:NAM37
bar:NAM38
bar:space
bar:period
bar:space
bar:era
align:center textcolor:black fontsize:M mark:(line,black) width:25
shift:(7,-4)
bar:periodtop
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:[[H.|Holocene]]
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:[[Q.|Quaternary]]
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]]
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:[[H.|Holocene]]
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:[[Q.|Quaternary]]
Relation to humans
Fishing and aquaculture
Flatfish are commonly fished using . Large species such as the halibuts are specifically targeted by fisheries, resulting in heavy fishing pressures and bycatch. Some species are , such as the tonguefish Cynoglossus semilaevis.
As food
Flatfish is considered a Whitefish 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.
Further reading
External links
|
|
