Flatfish are a group ofray-finned fish belonging to thesuborderPleuronectoidei and historically the orderPleuronectiformes (thoughthis is now disputed). Their collectivecommon name is due to their habit of lying on one side of their laterally-compressed body (flattened side-to-side) upon theseafloor; 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 thesubstrate. This loss ofsymmetry, aunique adaptation invertebrates, stems from one eye "migrating" towards the other during the juvenile'smetamorphosis; due to variation, some species tend to face their left side upward, some their right side, and others face either side upward.Pleuronectidae lie on their left side, with eyes on the right.Paralichthyidae lie on their right side, with eyes on the left.Plaice lie on the left.[3]
Illustration of several common European flatfish species
There are a multitude ofcommon names for flatfish, as they are a widespread group of fish and importantfood fish across the world. The following are common flatfish names inEnglish:
As these are merely common names, they do not conform with the "natural" relationships that are recovered through scientific studies ofmorphology orgenetics. As examples, the three species consistently called "halibut" are themselves part of theright-eye flounder family, while thespiny turbots are not at all closely related to "true" turbot, but are consistently recovered in a "primitive" orbasal position at the base of flatfishphylogenetic trees.
The most obvious characteristic of the flatfish is theirasymmetry, 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 thedorsal fin onto the head.
This sequence of photos shows an individualpeacock flounder (Bothidae) changing its coloration over different substrates.
The surface of the fish facing away from the sea floor is pigmented, often serving tocamouflage the fish, but at times displaying striking patterns. Some flatfishes are also able to change their pigmentation to match the background using theirchromatophores, in a manner similar to somecephalopods. The side of the body without the eyes, facing the seabed, is usually colourless or very pale.[1]
Many species such as flounders and spiny turbotseat smaller fish, and have well-developed teeth. These species sometimes hunt in themidwater, away from the bottom, and show fewer"extreme" adaptations than other families. Thesoles, by contrast, are almost exclusively bottom-dwellers (more strictlydemersal), and feed onbenthic invertebrates. They show a more extreme asymmetry, and may lack teeth on one side of the jaw.[1]
European flounder, like other flatfish, experience an eye migration during their lifetime.
Flatfishes lay eggs that hatch intolarvae resembling typical, symmetrical, fish. These are initially elongated, but quickly develop into a more rounded form. The larvae typically have protectivespines on the head, over the gills, and in thepelvic and pectoral fins. They also possess aswim bladder, and do not dwell on the bottom, instead dispersing from their hatching grounds asichthyoplankton.[1] Bilaterally symmetric fish such asgoldfish maintain balance using a system within theirinner ears which involves theotolith, but larval and metamorphizing flatfish require visible light (such assunlight) to properly orient themselves.[19]
The length of theplanktonic stage varies between different types of flatfishes, but through the influence ofthyroid hormones,[20] they eventually begin tometamorphose 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.[21][19]
Hybrids are well known in flatfishes. The Pleuronectidae have the largest number of reported hybrids of marine fishes.[22] Two of the most famousintergeneric hybrids are between theEuropean plaice (Pleuronectes platessa) andEuropean flounder (Platichthys flesus) in theBaltic Sea,[23] 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.[22]
...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.[24]
Scientists have been proposing since the 1910s that flatfishes evolved from more "typical"percoid ancestors.[25] The fossil record indicated 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).[26] 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 ofsaltation rather than gradual evolution throughnatural selection, because a partially migrated eye was considered to have beenmaladaptive.
This started to change in 2008 with a study on the two fossil fish genera;Amphistium andHeteronectes, which dated to about 50 million years ago. These genera retainprimitive 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.[27][28] The more recently described fossil generaQuasinectes andAnorevus have been proposed to show similar morphologies and have also been classified as "stem-pleuronectiforms".[29][30] 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."[28]
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.[31] 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[examples needed] indicate that most flatfish lineages living today were present in theEocene andcontemporaneous with them.[27] It has been suggested that the more primitive forms were eventually outcompeted.[28]
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 have been recovered to be closely related to various groups, such as thethreadfins (often recovered as asister group to flatfish),archerfish, andbeachsalmons. Due to this, they are now treated as a suborder of the Carangiformes,[32][33] as represented inEschmeyer's Catalog of Fishes.[34]
Threadfins such asPolynemus have been recovered closer to the primitive spiny turbots than those are to other flatfish, or as asister group to amonophyletic flatfish group
There has been some disagreement whether flatfish as a whole are amonophyletic group. Some palaeontologists think that some percomorph groups unrelated to flatfishes were also "experimenting" with head asymmetry during theEocene,[29][30] and certain molecular studies conclude that the primitive family ofPsettodidae evolved their flat bodies and asymmetrical head independently of other flatfish groups.[41][42] The following phylogeny is from Lüet al. 2021; a whole-genome analysis usingconcatenated 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 bepolyphyletic as seen here:[43]
However, threadfins (Polynemidae) aren't universally found to be nested within the group of flatfish, as recovered by a study ofultraconserved elements from the threadfin family in Girardet al. 2022,[44] or as represented in theWorld Register of Marine Species,[45] where Pleuronectiformes is retained as a name for the flatfish group.[46] Numerous scientists continue to argue for a monophyletic group of all flatfish,[47] though the debate continues.[48]
Over 800 described species are placed into 16 families.[49] 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.[50] 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.[10][51] The familiesParalichthodidae,Poecilopsettidae, andRhombosoleidae were also traditionally treated as subfamilies of Pleuronectidae, but are now recognised as families in their own right.[51][52][53] TheParalichthyidae has long been indicated to be paraphyletic, with the formal description ofCyclopsettidae in 2019 resulting in the split of this family as well.[49] The following is themaximum likelihood phylogenetic tree from Campbellet al. 2019, which was obtained by analyzing sevenprotein-coding genes. This study erected two new families to resolve the previously non-monophyletic status of Paralichthyidae and theRhombosoleidae:[49]
Flatfish is considered awhitefish[62] 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.
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.
^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
^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
^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
^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
^abFriedman M. (2008). "The evolutionary origin of flatfish asymmetry".Nature454(7201): p. 209–212.doi:10.1038/nature07108
^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
^Nelson, Joseph S.; Grande, Terry C.; Wilson, Mark V. H. (2016-03-28).Fishes of the world. John Wiley & Sons.ISBN978-1-118-34233-6.OCLC958002567.
^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.
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