Flatfish

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Flatfish
Temporal range: Paleocene–Recent [1]
Flounder camo md.jpg
A camouflaged flatfish
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Clade: Percomorpha
Order: Pleuronectiformes
Type species
Pleuronectes platessa
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)

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.

Contents

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. [4]

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. [8] New species are described with some regularity and undescribed species likely remain. [6]

Hybrids

Hybrids are well known in flatfishes. The Pleuronectidae have the largest number of reported hybrids of marine fishes. [9] Two of the most famous intergeneric hybrids are between the European plaice (Pleuronectes platessa) and European flounder (Platichthys flesus) in the Baltic Sea, [10] 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. [9]

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. [11] 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. [12] 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. [13] 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. [14] [15] [16]

Characteristics

Flatfish are asymmetric, with both eyes lying on the same side of the head Pseudopleuronectes americanus.jpg
Flatfish are asymmetric, with both eyes lying on the same side of the head
European flounder, like other flatfish, experience an eye migration during their lifetime. Platichthys flesus - en.pdf
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 aposematic traits such as conspicuous eyespots (e.g., Microchirus ocellatus ) and several small tropical species (at least Aseraggodes , Pardachirus and Zebrias ) are poisonous. [6] [17] [18] Juveniles of Soleichthys maculosus mimic toxic flatworms of the genus Pseudobiceros in both colours and swimming mode. [19] [20] Conversely, a few octopus species have been reported to mimic flatfishes in colours, shape and swimming mode. [21]

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

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.

Origin and evolution

Scientists have been proposing since the 1910s that flatfishes evolved from percoid ancestors. [22] 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, [23] [24] and certain molecular studies conclude that the primitive family of Psettodidae evolved their flat bodies and asymmetrical head independently of other flatfish groups. [25] [26] Many scientists, however, argue that pleuronectiformes are monophyletic. [27]

The fossil record indicates 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). [28]

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. [29]

Fossil of Amphistium. Amphistium.JPG
Fossil of Amphistium .

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. [30] [31] The more recently described fossil genera Quasinectes [23] and Anorevus [24] 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." [31]

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. [32]

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. [30] It has been suggested that the more primitive forms were eventually outcompeted. [31]

As food

Flatfish is considered a Whitefish [33] 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

QuaternaryNeogenePaleogeneHolocenePleist.Plio.MioceneOligoceneEocenePaleoceneSymphurusParophrysIsopsettaEopsettaChibapsettaPegusaLyopsettaLimandaGlyptocephalusClidodermaAtheresthesPleuronichthysParalichthysMonochirusCitharichthysEvesthesMicrostomusMicrochirusAchiurusPlatichthysParaplagusiaDicologoglossaLepidorhombusHippoglossoidesBuglossidiumSoleaMonoleneBothusArnoglossusPsettodesCitharusScophthalmusTurahbuglossusJoleaudichthysImhoffiusEobuglossusEobothusAmphistiumQuaternaryNeogenePaleogeneHolocenePleist.Plio.MioceneOligoceneEocenePaleoceneFlatfish

See also

Related Research Articles

<span class="mw-page-title-main">Flounder</span> Group of flatfish species

Flounders are a group of flatfish species. They are demersal fish, found at the bottom of oceans around the world; some species will also enter estuaries.

<span class="mw-page-title-main">American sole</span> Family of fishes

The American soles are a family (Achiridae) of flatfish occurring in both freshwater and marine environments of the Americas. The family includes about 35 species in seven genera. These are closely related to the soles (Soleidae), and have been classified as a subfamily of it, but achirids have a number of distinct characteristics.

Sole may refer to:

<span class="mw-page-title-main">Soleidae</span> Family of fishes

The true soles are a family, Soleidae, of flatfishes. It includes saltwater and brackish water species in the East Atlantic, Indian Ocean, and West and Central Pacific Ocean. Freshwater species are found in Africa, southern Asia, New Guinea, and Australia.

<span class="mw-page-title-main">Pleuronectidae</span> Family of fishes

Pleuronectidae, also known as righteye flounders, are a family of flounders. They are called "righteye flounders" because most species lie on the sea bottom on their left sides, with both eyes on their right sides. The Paralichthyidae are the opposite, with their eyes on the left side. A small number of species in Pleuronectidae can also have their eyes on the left side, notably the members of the genus Platichthys.

<span class="mw-page-title-main">European plaice</span> Species of fish

The European plaice, commonly referred to as simply plaice, is a species of marine flatfish in the genus Pleuronectes of the family Pleuronectidae.

<span class="mw-page-title-main">Paralichthyidae</span> Family of fishes

Large-tooth flounders or sand flounders are a family, Paralichthyidae, of flounders. The family contains 14 genera with a total of about 110 species. They lie on the sea bed on their right side; both eyes are always on the left side of the head, while the Pleuronectidae usually have their eyes on the right side of the head.

<span class="mw-page-title-main">Tonguefish</span> Family of fishes

Tonguefishes are flatfish in the family Cynoglossidae. They are distinguished by the presence of a long hook on the snout overhanging the mouth, and the absence of pectoral fins. Their eyes are both on the left side of their bodies, which also lack a pelvic fin. This family has three genera with a total of more than 140 species. The largest reaches a length of 66 cm (26 in), though most species only reach half that size or less. They are found in tropical and subtropical oceans, mainly in shallow waters and estuaries, though some species are found in deep sea floors, and even a few in rivers.

<span class="mw-page-title-main">Sole (fish)</span> Fish name belonging to several families

Sole is a fish belonging to several families. Generally speaking, they are members of the family Soleidae, but, outside Europe, the name sole is also applied to various other similar flatfish, especially other members of the sole suborder Soleoidei as well as members of the flounder family. In European cookery, there are several species which may be considered true soles, but the common or Dover sole Solea solea, often simply called the sole, is the most esteemed and most widely available.

<span class="mw-page-title-main">Yellowtail flounder</span> Species of fish

The yellowtail flounder, also known as the rusty dab, is a species of flatfish in the family Pleuronectidae. Reaching 56 cm (22 in) in length, it has reddish brown upperparts, pale underparts and yellow fins. Both its eyes are on the right (upper) side of its body. Found in the western North Atlantic, it has been fished commercially by North American fisheries for food. A victim of overfishing, the yellowtail flounder is categorized as "Vulnerable" by the International Union for Conservation of Nature.

<span class="mw-page-title-main">European flounder</span> Species of fish

The European flounder is a flatfish of European coastal waters from the White Sea in the north to the Mediterranean and the Black Sea in the south. It has been introduced into the United States and Canada accidentally through transport in ballast water. It is caught and used for human consumption.

<i>Achirus lineatus</i> Species of fish

The lined sole is a ray-finned flatfish found in the Western Atlantic. Its common length is 17 cm. Often considered a trash fish in commercial trawling, it is of little or no economic value.

<span class="mw-page-title-main">Samaridae</span> Family of fishes

Samaridae is a family of crested flounders, small flatfishes native to the Indo-Pacific. The family contains four genera with a total of 29 species.

<i>Amphistium</i> Extinct genus of fishes

Amphistium paradoxum, the only species classified under the genus Amphistium and the family Amphistiidae, is a fossil fish which has been identified as a Paleogene relative of the flatfish, and as a transitional fossil. 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.

Heteronectes chaneti is a fossil fish which has been identified as a primitive flatfish, dating to the early Eocene of France.

<i>Symphurus thermophilus</i> Species of fish

Symphurus thermophilus is a species of tonguefish notable for being the only flatfish known to be an obligate inhabitant of hydrothermal vents. It is known to inhabit several widely dispersed locations in the western Pacific Ocean and occurs in great numbers. These flatfish are distinguished by the prominent dark crossbands on their brown eyed side, black abdominal cavity membrane lining known as the peritoneum, and white blind side. They are tolerant of harsh conditions and are often found in close association with elemental sulfur, including molten sulfur pools that exceed 180 °C in temperature. As they are not significantly different in appearance and feeding habits from other tonguefishes, they are thought to be relatively recent colonizers of vent ecosystems.

<i>Symphurus</i> Genus of fishes

Symphurus is a genus of fish in the family Cynoglossidae found in the Atlantic, Indian and Pacific Ocean. Most species mainly occur in relatively shallow water, including estuaries. Some species are also found in deeper water, including S. thermophilus that lives at hydrothermal vents. These species are distinguished by merged dorsal, caudal and anal fins, the absence of a lateral line and pectoral fins, and the presence of only one pelvic fin. They are sinistral flatfishes, meaning that as adults, their crania are asymmetrical, with both eyes on the left side. The largest species grows to about 32 cm (1 ft) long.

<i>Pleuronichthys</i> Genus of fishes

Pleuronichthys is a genus of fish in the family Pleuronectidae found in the Pacific Ocean.

<span class="mw-page-title-main">Pacific sand sole</span> Species of fish

The Pacific sand sole, also known as simply sand sole, is a species of flatfish found in the north-eastern Pacific waters where it lives on sandy bottoms. It is the only species in the genus Psettichthys, and ranges from the Bering Sea to northern California.

<span class="mw-page-title-main">Plaice</span> Common name for a group of flatfish

Plaice is a common name for a group of flatfish that comprises four species: the European, American, Alaskan and scale-eye plaice.

References

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