Aflatfish is a member of theray-finneddemersal fishsuperorderPleuronectoidei, 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, thethreadfins, do not resemble the flatfish but are their closest relatives.
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 theCarangiformes, which also includesjacks andbillfish. Specifically, flatfish are most closely related to thethreadfins, which are now also placed in the suborder Pleuronectoidei. Together, the group is most closely related to thearcherfish andbeachsalmons withinToxotoidei. Due to this, they are now treated as a suborder of the Carangiformes.[5][6]
Over 800 described species are placed into 16 families.[7] 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.[8] The largest families areSoleidae,Bothidae andCynoglossidae with more than 150 species each. There also exist two monotypic families (Paralichthodidae andOncopteridae). Some families are the results of relatively recent splits. For example, theAchiridae were classified as a subfamily of Soleidae in the past, and theSamaridae were considered a subfamily of the Pleuronectidae.[9][10] The familiesParalichthodidae,Poecilopsettidae, andRhombosoleidae were also traditionally treated as subfamilies of Pleuronectidae, but are now recognised as families in their own right.[10][3][4] TheParalichthyidae has long been indicated to be paraphyletic, with the formal description ofCyclopsettidae in 2019 resulting in the split of this family as well.[7]
Hybrids are well known in flatfishes. The Pleuronectidae have the largest number of reported hybrids of marine fishes.[12] Two of the most famousintergeneric hybrids are between theEuropean plaice (Pleuronectes platessa) andEuropean flounder (Platichthys flesus) in theBaltic Sea,[13] and between theEnglish sole (Parophrys vetulus) andstarry flounder (Platichthys stellatus) inPuget 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.[12]
Flatfishes are found in oceans worldwide, ranging from theArctic, through the tropics, toAntarctica. Species diversity is centered in the Indo-West Pacific and declines following both latitudinal and longitudinal gradients away from the Indo-West Pacific.[14] Most species are found in depths between 0 and 500 m (1,600 ft), but a few have been recorded from depths in excess of 1,500 m (4,900 ft). None have been confirmed from theabyssal orhadal zones. An observation of a flatfish from theBathyscaphe Trieste at the bottom of theMariana Trench at a depth of almost 11 km (36,000 ft) has been questioned by fish experts, and recent authorities do not recognize it as valid.[15] Among the deepwater species,Symphurus thermophilus lives congregating around "ponds" ofsulphur athydrothermal vents on the seafloor. No other flatfish is known from hydrothermal vents.[16] Many species will enterbrackish or fresh water, and a smaller number of soles (familiesAchiridae andSoleidae) and tonguefish (Cynoglossidae) are entirely restricted to fresh water.[17][18][19]
Flatfish are asymmetric, with both eyes lying on the same side of the headEuropean flounder, like other flatfish, experience an eye migration during their lifetime.
The most obvious characteristic of the flatfish is itsasymmetry, 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 primitivespiny turbots include equal numbers of right- and left-sided individuals, and are generally less asymmetrical than the other families.[1] Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on theseabed (benthos), and the extension of the dorsal fin onto the head.
Polynemus, a threadfin, belongs to the same group as flatfish, but looks completely different
The most basal members of the group, thethreadfins, do not closely resemble the flatfishes.
The surface of the fish facing away from the sea floor is pigmented, often serving tocamouflage 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 somecephalopods. The side of the body without the eyes, facing the seabed, is usually colourless or very pale.[1]
Theflounders 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. Thesoles, by contrast, are almost exclusively bottom-dwellers, and feed on invertebrates. They show a more extreme asymmetry, and may lack teeth on one side of the jaw.[1]
Flatfishes range in size fromTarphops oligolepis, measuring about 4.5 cm (1.8 in) in length, and weighing 2 g (0.071 oz), to theAtlantic halibut, at 2.5 m (8.2 ft) and 316 kg (697 lb).[1]
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 aswim bladder, and do not dwell on the bottom, instead dispersing from their hatching grounds asplankton.[1]
The length of the planktonic stage varies between different types of flatfishes, but eventually they begin to metamorphose into the adult form. One of the eyes migrates across the top of the head and onto the other side of the body, leaving the fish blind on one side. The larva also loses its swim bladder and spines, and sinks to the bottom, laying its blind side on the underlying surface.
Scientists have been proposing since the 1910s that flatfishes evolved frompercoid ancestors.[25] 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 theEocene,[26][27] and certain molecular studies conclude that the primitive family ofPsettodidae evolved their flat bodies and asymmetrical head independently of other flatfish groups.[28][29] Many scientists, however, argue that pleuronectiformes aremonophyletic.[30]
The fossil record indicates that flatfishes might have been present before theEocene, based on fossilotoliths resembling those of modern pleuronectiforms dating back to theThanetian andYpresian stages (57-53 million years ago).[31]
Flatfishes have been cited as dramatic examples of evolutionary adaptation.Richard Dawkins, inThe Blind Watchmaker, explains the flatfishes' evolutionary history thus:
...bony fish as a rule have a marked tendency to be flattened in a vertical direction.... It was natural, therefore, that when the ancestors of [flatfish] took to the sea bottom, they should have lain on oneside.... 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.[32]
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 ofsaltation 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 generaAmphistium andHeteronectes, 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.[33][34] The more recently described fossil generaQuasinectes[26] andAnorevus[27] 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 vianatural selection—not suddenly, as researchers once had little choice but to believe."[34]
To explain the survival advantage of a partially migrated eye, it has been proposed that primitive flatfishes likeAmphistium 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.[35]
While known basal genera likeAmphistium andHeteronectes 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 theEocene and contemporaneous with them.[33] It has been suggested that the more primitive forms were eventually outcompeted.[34]
TheEuropean plaice is the principal commercial flatfish in Europe.
American soles are found in both freshwater and marine environments of the Americas.
Halibut are the largest of the flatfishes, and provide lucrative fisheries.
Theturbot is a large, left-eyed flatfish found in sandy shallow coastal waters around Europe.
Flatfish is considered aWhitefish[36] 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.
^Nelson, Joseph S.; Grande, Terry C.; Wilson, Mark V. H. (2016-03-28).Fishes of the world. John Wiley & Sons.ISBN9781118342336.OCLC958002567.
^abcRandall, J. E. (2007).Reef and Shore Fishes of the Hawaiian Islands.ISBN1-929054-03-3
^abCooper, J.A.; and Chapleau, F. (1998).Monophyly and intrarelationships of the family Pleuronectidae (Pleuronectiformes), with a revised classification. Fish. Bull. 96 (4): 686–726.
^abGarrett, 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
^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.
^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.
^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
^Regan C.T. (1910). "The origin and evolution of the Teleostean fishes of the order Heterosomata".Annals and Magazine of Natural History6(35): p. 484-496.doi.org/10.1080/00222931008692879
^Campbell M.A., Chen W-J. & López J.A. (2013). "Are flatfishes (Pleuronectiformes) monophyletic?".Molecular Phylogenetics and Evolution69(3): p. 664-673.doi.org/10.1016/j.ympev.2013.07.011
^Campbell M.A., López J.A., Satoh T.P., Chen W-J. & Miya M. (2014). "Mitochondrial genomic investigation of flatfish monophyly".Gene551(2): p. 176-182.doi.org/10.1016/j.gene.2014.08.053
^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 Genetics56: p. 1069-1072.doi.org/10.1038/s41588-024-01784-w
^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
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