Flatfish are a group of ray-finned fish belonging to the suborder Pleuronectoidei of the order Carangiformes. 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") lies on the substrate. This loss of symmetry, a unique adaptation in vertebrates, stems from one eye "migrating" towards the other during the juvenile's metamorphosis. Due to interspecific variation, some species tend to face their left side upward, some their right side, and others face either side upward; members of Pleuronectidae lie on their left side, with eyes on the right, Paralichthyidae lie on their right side, with eyes on the left, while the "primitive" genus Psettodes may develop into "right-facing" or "left-facing" individuals.
They are one of the most diverse groups of demersal fish. Their cryptic coloration and habits, a form of camouflage, conceals them from potential predators. Many species are of interest for fisheries.
Common names
There are a multitude of common names for flatfish, as they are a widespread group of fish and important food fish across the world. The following are common flatfish names in English:
Brill Dab Flounder Halibut Megrim Plaice Sanddab Sole Tonguefish Turbot
As these are merely common names, they do not conform with the "natural" relationships that are recovered through scientific studies of morphology or genetics. As examples, the three species consistently called "halibut" are themselves part of the right-eye flounder family, while the spiny turbots 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.
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 this centre of diversity. 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 hadal zones of the deep sea; a reported observation of a flatfish from the Bathyscaphe Trieste's dive into the Mariana Trench (at a depth of almost ) has been questioned by ichthyologists, 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 is Symphurus thermophilus, a tonguefish which congregates around "ponds" of sulphur at hydrothermal vents on the seafloor; no other flatfish is known from hydrothermal vent ecosystems.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.
Conversely, many species will enter brackish or fresh water, and the families Achiridae, Soleidae, andCynoglossidae include species that are entirely restricted to freshwater.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
Description
The most obvious characteristic of the flatfish is their 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 turbots 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 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 chromatophores, 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 aposematic 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 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 halibuts, with the Atlantic halibut measuring up to long, and the Pacific halibut weighing up to .
Many species such as flounders and spiny turbots eat smaller fish, and have well-developed teeth. These species sometimes hunt 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), and feed on benthic invertebrates. They show a more extreme asymmetry, and may lack teeth on one side of the jaw.
Development
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 ichthyoplankton. Bilaterally symmetric fish such as goldfish maintain balance using a system within their inner ears which involves the otolith, but larval and metamorphizing flatfish require visible light (such as sunlight) to properly orient themselves.
The length of the planktonic stage varies between different types of flatfishes, but through the influence of thyroid hormones, they eventually 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.
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.
Scientists have been proposing since the 1910s that flatfishes evolved from more "typical" percoid 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 otoliths 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. doi:10.13140/2.1.1725.5043 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 was 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. doi:10.1038/nature07108 The more recently described fossil genera Quasinectes and Anorevus have been proposed to show similar morphologies and have also been classified as "stem-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."