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A flatfish is a member of the Ray-finned fish demersal fish 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, do not resemble the flatfish but are their closest relatives.
Many important food fish are in this order, including the , soles, turbot, plaice, and halibut. Some flatfish can camouflage themselves on the ocean floor.
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. (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 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.
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.
In general, flatfishes rely on their camouflage for avoiding predators, but some have aposematism traits 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 flatworms of the genus Pseudobiceros in both colours and swimming mode.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
The and spiny turbots eat smaller fish, and have well-developed teeth. They sometimes seek prey in the midwater, away from the bottom, and show fewer extreme adaptations than other families. The soles, by contrast, are almost exclusively bottom-dwellers, and feed on invertebrates. They show a more extreme asymmetry, and may lack teeth on one side of the jaw.
Flatfishes range in size from Tarphops oligolepis, measuring about in length, and weighing , to the Atlantic halibut, at and .
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.
The fossil record indicates 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.
Flatfishes have been cited as dramatic examples of evolutionary adaptation. Richard Dawkins, in The Blind Watchmaker, explains the flatfishes' evolutionary history thus:
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 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.Friedman M. (2008). "The evolutionary origin of flatfish asymmetry". Nature 454(7201): p. 209–212. The more recently described fossil genera QuasinectesBannikov 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. and AnorevusBannikov 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 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."
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 evidence indicate that most flatfish lineages living today were present in the Eocene and contemporaneous with them. It has been suggested that the more primitive forms were eventually outcompeted.
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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.
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. Most species are found in depths between 0 and , but a few have been recorded from depths in excess of . None have been confirmed from the abyssal or . An observation of a flatfish from the Bathyscaphe Trieste at the bottom of the Mariana Trench at a depth of almost has been questioned by fish experts, and recent authorities do not recognize it as valid.Jamieson, A.J., and Yancey, P. H. (2012). On the Validity of the Trieste Flatfish: Dispelling the Myth. The Biological Bulletin 222(3): 171-175 Among the deepwater species, Symphurus thermophilus lives congregating around "ponds" of sulphur at hydrothermal vents on the seafloor. No other flatfish is known from hydrothermal vents.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. 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.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
Characteristics
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 include equal numbers of right- and left-sided individuals, and are generally less asymmetrical than the other families. Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on the seabed (benthos), and the extension of the dorsal fin onto the head.
The most basal members of the group, the Threadfin, do not closely resemble the flatfishes.
(1998). 9780125476652, Academic Press. ISBN 9780125476652
Species and species groups
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.
Origin and evolution
Scientists have been proposing since the 1910s that flatfishes evolved from 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 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, 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 Many scientists, however, argue that pleuronectiformes are monophyletic.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
...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.
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.
Timeline of genera
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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:[[H.|Holocene]]
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from: -65.5 till: -23.03 color:paleogene text:[[Paleogene]]
from: -23.03 till: -2.588 color:neogene text:[[Neogene]]
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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]]
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color:miocene bar:NAM24 from:-11.608 till:0 text:[[Citharichthys]]
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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]]
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from: -65.5 till: -55.8 color:paleocene text:[[Paleocene]]
from: -55.8 till: -33.9 color:eocene text:[[Eocene]]
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See also
Further reading
External links
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