Flatfish

A flatfish is a member of the ray-finned demersal fish order Pleuronectiformes, also called the Heterosomata, sometimes classified as a suborder of Perciformes. 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.

Flatfish Temporal range: Paleocene–Recent[1] PreꞒ Ꞓ O S D C P T J K Pg N
A camouflaged flatfish
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Superorder:	Acanthopterygii
Order:	Pleuronectiformes
Type species
Pleuronectes platessa Linnaeus, 1758
Families[2][3]
Suborder Psettodoidei Psettodidae (spiny turbots) Suborder Pleuronectoidei Superfamily Citharoidea Citharidae (largescale flounders) Superfamily Pleuronectoidea Scophthalmidae (turbots) Paralichthyidae (large-tooth flounders) Pleuronectidae (righteye flounders) Bothidae (lefteye flounders) Superfamily Soleoidea Paralichthodidae (measles flounders) Poecilopsettidae (bigeye flounders) Rhombosoleidae (rhombosoleids) Achiropsettidae (southern flounders) Samaridae (crested flounders) Achiridae (American soles) Soleidae (true soles) Cynoglossidae (tonguefishes)

Many important food fish are in this order, including the flounders, soles, turbot, plaice, and halibut. Some flatfish can camouflage themselves on the ocean floor.

Taxonomy

Over 800 described species are placed into 16 families.[4] Broadly, the flatfishes are divided into two suborders, Psettodoidei and Pleuronectoidei, with > 99% of the species diversity found within the Pleuronectoidei.[5] The largest families are Soleidae, Bothidae and Cynoglossidae 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.[6][7] The families Paralichthodidae, Poecilopsettidae, and Rhombosoleidae were also traditionally treated as subfamilies of Pleuronectidae, but are now recognised as families in their own right.[7][2][3] 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.[8]

The taxonomy of some groups is in need of a review, as 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.[9] New species are described with some regularity and undescribed species likely remain.[6]

Hybrids

Hybrids are well known in flatfishes. The Pleuronectidae, of marine fishes, have the largest number of reported hybrids.[10] Two of the most famous intergeneric hybrids are between the European plaice (Pleuronectes platessa) and European flounder (Platichthys flesus) in the Baltic Sea,[11] 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.[10]

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.[12] 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 the abyssal or hadal zones. An observation of a flatfish from the Bathyscaphe Trieste at the bottom of the Mariana 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.[13] Among the deepwater species, Symphurus thermophilus lives in congregating around "ponds" of sulphur at hydrothermal vents on the seafloor. No other flatfish is known from hydrothermal vents.[14] 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.[15][16][17]

Characteristics

Flatfish are asymmetrical, with both eyes lying on the same side of the head

European flounder, like other flatfish, experience an eye migration during their lifetime.

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 spiny 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 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 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.[1]

In general, flatfishes rely on their camouflage for avoiding predators, but some have conspicuous eyespots (e.g., Microchirus ocellatus) and several small tropical species (at least Aseraggodes, Pardachirus and Zebrias) are poisonous.[6][18][19] Juveniles of Soleichthys maculosus mimic toxic flatworms of the genus Pseudobiceros in both colours and swimming mode.[20][21] Conversely, a few octopus species have been reported to mimic flatfishes in colours, shape and swimming mode.[22]

The flounders 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.[1]

Flatfishes range in size from Tarphops oligolepis, measuring about 4.5 cm (1.8 in) in length, and weighing 2 g (0.071 oz), to the Atlantic halibut, at 2.5 m (8.2 ft) and 316 kg (697 lb).[1]

Species and species groups

• Brill
• Dab
• Sanddab
• Flounder
• Halibut
• Megrim
• Plaice
• Sole
• Tonguefish
• Turbot

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.[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.

Evolution

Fossil of Amphistium

In 2008, a 50-million-year-old fossil, Amphistium, was identified as an early relative of the flatfish and transitional fossil.[23] In a typical modern flatfish, the head is asymmetric, with both eyes on one side of the head. In Amphistium, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top of the head.[24] The researchers concluded, "the change happened gradually, in a way consistent with evolution via natural selection—not suddenly, as researchers once had little choice but to believe."[23]

Flatfishes have been cited as dramatic examples of evolutionary adaptation. Richard Dawkins, in The 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 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.[25]

• 
  The European 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.
• 
  The turbot is a large, left-eyed flatfish found in sandy shallow coastal waters around Europe.
• 
  Flatfish (left‐eyed flounder)

As food

Flatfish is considered a Whitefish[26] 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

See also

• Sinistral and dextral

References

1. Chapleau, Francois; Amaoka, Kunio (1998). Paxton, J.R.; Eschmeyer, W.N. (eds.). Encyclopedia of Fishes. San Diego: Academic Press. xxx. ISBN 0-12-547665-5.
2. Nelson, J. S. (2006). Fishes of the World (4 ed.). Hoboken, NJ: John Wiley & Sons. ISBN 978-0-471-25031-9.
3. J. S. Nelson; T. C. Grande; M. V. H. Wilson (2016). Fishes of the World (5th ed.). Wiley. p. 752. ISBN 978-1-118-34233-6.
4. Campbell, Matthew A.; Chanet, Bruno; Chen, Jhen‐Nien; Lee, Mao‐Ying; Chen, Wei‐Jen (2019). "Origins and relationships of the Pleuronectoidei: Molecular and morphological analysis of living and fossil taxa". Zoologica Scripta. 48 (5): 640–656. doi:10.1111/zsc.12372. ISSN 0300-3256. S2CID 202856805.
5. Nelson, Joseph S. VerfasserIn. (2016-03-28). Fishes of the world. ISBN 9781118342336. OCLC 958002567.
6. Randall, J. E. (2007). Reef and Shore Fishes of the Hawaiian Islands. ISBN 1-929054-03-3
7. 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.
8. Campbell, Matthew A.; Chanet, Bruno; Chen, Jhen‐Nien; Lee, Mao‐Ying; Chen, Wei‐Jen (2019). "Origins and relationships of the Pleuronectoidei: Molecular and morphological analysis of living and fossil taxa". Zoologica Scripta. 48 (5): 640–656. doi:10.1111/zsc.12372. ISSN 0300-3256. S2CID 202856805.
9. Hoshino, Koichi (2001-11-01). "Monophyly of the Citharidae (Pleuronectoidei: Pleuronectiformes: Teleostei) with considerations of pleuronectoid phylogeny". Ichthyological Research. 48 (4): 391–404. doi:10.1007/s10228-001-8163-0. ISSN 1341-8998. S2CID 46318428.
10. 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
11. Food and Agriculture Organization of the United Nations: Platichthys flesus (Linnaeus, 1758).. Retrieved 18 May 2014
12. Campbell, Matthew A.; Chanet, Bruno; Chen, Jhen-Nien; Lee, Mao-Ying; Chen, Wei-Jen (2019). "Origins and relationships of the Pleuronectoidei: Molecular and morphological analysis of living and fossil taxa". Zoologica Scripta. 48 (5): 640–656. doi:10.1111/zsc.12372. ISSN 1463-6409. S2CID 202856805.
13. 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
14. 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.
15. 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.
16. 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.
17. Monks, N. (2007). Freshwater flatfish, order Pleuronectiformes. Archived 2014-08-15 at the Wayback Machine Retrieved 18 May 2014
18. Elst, R. van der (1997) A Guide to the Common Sea Fishes of South Africa. ISBN 978-1868253944
19. Debelius, H. (1997). Mediterranean and Atlantic Fish Guide. ISBN 978-3925919541
20. Practical Fishkeeping (22 May 2012) Video: Tiny sole mimics a flatworm. Archived 2014-05-17 at the Wayback Machine Retrieved 17 May 2014.
21. Australian Museum (5 November 2010). This week in Fish: Flatworm mimic and shark teeth. Retrieved 17 May 2014.
22. 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
23. "Odd Fish Find Contradicts Intelligent-Design Argument". National Geographic. July 9, 2008. Retrieved 2008-07-17.
24. Matt Friedman (2008). "The evolutionary origin of flatfish asymmetry" (PDF). Nature Letters. 454 (7201): 209–212. Bibcode:2008Natur.454..209F. doi:10.1038/nature07108. PMID 18615083. S2CID 4311712.
25. Dawkins, Richard (1991). The Blind Watchmaker. London: Penguin Books. p. 92. ISBN 0-14-014481-1.
26. "Flatfish BBC".

Further references

• Sepkoski, Jack (2002). "A compendium of fossil marine animal genera". Bulletins of American Paleontology. 364: 560. Archived from the original on July 23, 2011. Retrieved 2011-05-17.
• Gibson, Robin N (Ed) (2008) Flatfishes: biology and exploitation. Wiley.
• Munroe, Thomas A (2005) "Distributions and biogeography." Flatfishes: Biology and Exploitation: 42-67.

External links

• Information on Canadian fisheries of plaice