Flatfish
Encyclopedia
The flatfish are an order
(Pleuronectiformes) of ray-finned fish, 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 and around the head during development. Some species face their left side upward, some face their right side upward, and others face either side upward.
Many important food fish are in this order, including the flounder
s, sole
s, turbot
, plaice
, and halibut
. There are more than 400 species of this order. Some flatfish can camouflage themselves on the ocean floor.
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 always on the right side of the body (dextral or right-eyed flatfish), and in others, they are always on the left (sinistral or left-eyed flatfish). The primitive spiny turbot
s 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 sometimes with striking coloured patterns. Some flatfish are also able to change their pigmentation to match the background, in a manner similar to a chameleon
. The side of the body without the eyes, which faces the seabed, is usually colourless or very pale.
The flounder
s and spiny turbots eat smaller fish, and have well-developed teeth. They sometimes seek prey in the mid-water, away from the bottom, and show fewer extreme adaptations than other families. The sole
s, 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.
Flatfish range in size from Tarphops oligolepis, measuring about 4.5 centimetres (1.8 in) in length, and weighing 2 gram (0.0705479242102239 oz), to the Atlantic halibut
, at 2.5 metres (8.2 ft) and 316 kilograms (696.7 lb).
.
The length of the planktonic stage varies between different types of flatfish, 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.
Flatfish have been cited as dramatic examples of evolutionary adaptation
. For example, Richard Dawkins
in The Blind Watchmaker
, explains the flatfish's evolutionary history thus:
In 2008, scientists discovered that "50-million-year-old fossils have revealed an intermediate species
between primitive flatfishes (with eyes on both sides of their heads) and the modern, lopsided versions, which include sole, flounder, and halibut." The research concluded that "the change happened gradually, in a way consistent with evolution via natural selection
—not suddenly, as researchers once had little choice but to believe."
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
(Pleuronectiformes) of ray-finned fish, also called the Heterosomata, sometimes classified as a suborder of Perciformes
Perciformes
The Perciformes, also called the Percomorphi or Acanthopteri, is one of the largest orders of vertebrates, containing about 40% of all bony fish. Perciformes means perch-like. They belong to the class of ray-finned fish and comprise over 7,000 species found in almost all aquatic environments...
. In many species, both eyes lie on one side of the head, one or the other migrating through and around the head during development. Some species face their left side upward, some face their right side upward, and others face either side upward.
Many important food fish are in this order, including the flounder
Flounder
The flounder is an ocean-dwelling flatfish species that is found in coastal lagoons and estuaries of the Northern Atlantic and Pacific Oceans.-Taxonomy:There are a number of geographical and taxonomical species to which flounder belong.*Western Atlantic...
s, sole
Sole (fish)
Sole is a group of flatfish 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...
s, turbot
Turbot
The turbot is a species of flatfish in the family Scophthalmidae. It is native to marine or brackish waters of the North Atlantic, Baltic Sea and the Mediterranean Sea.-Etymology:...
, plaice
Plaice
Plaice is the common name of four species of flatfishes.Plaice or PLAICE may also refer to:* USS Plaice , a Balao-class submarine* PLAICE, an open source hardware FLASH programmer, memory emulator, and logic analyzer...
, and halibut
Halibut
Halibut is a flatfish, genus Hippoglossus, from the family of the right-eye flounders . Other flatfish are also called halibut. The name is derived from haly and butt , for its popularity on Catholic holy days...
. There are more than 400 species of this order. Some flatfish can camouflage themselves on the ocean floor.
Characteristics
Spiny turbot
The spiny turbots are a family, Psettodidae, of relatively large, primitive flatfish found in the tropical waters of the east Atlantic and Indo-Pacific. The family contains just three species, all in the same genus, Psettodes...
s 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
Seabed
The seabed is the bottom of the ocean.- Ocean structure :Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources...
(benthos
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...
), 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 flatfish are also able to change their pigmentation to match the background, in a manner similar to a chameleon
Chameleon
Chameleons are a distinctive and highly specialized clade of lizards. They are distinguished by their parrot-like zygodactylous feet, their separately mobile and stereoscopic eyes, their very long, highly modified, and rapidly extrudable tongues, their swaying gait, the possession by many of a...
. The side of the body without the eyes, which faces the seabed, is usually colourless or very pale.
The flounder
Flounder
The flounder is an ocean-dwelling flatfish species that is found in coastal lagoons and estuaries of the Northern Atlantic and Pacific Oceans.-Taxonomy:There are a number of geographical and taxonomical species to which flounder belong.*Western Atlantic...
s and spiny turbots eat smaller fish, and have well-developed teeth. They sometimes seek prey in the mid-water, away from the bottom, and show fewer extreme adaptations than other families. The sole
Sole (fish)
Sole is a group of flatfish 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...
s, 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.
Flatfish range in size from Tarphops oligolepis, measuring about 4.5 centimetres (1.8 in) in length, and weighing 2 gram (0.0705479242102239 oz), to the Atlantic halibut
Atlantic halibut
The Atlantic halibut, Hippoglossus hippoglossus, is a flatfish of the family Pleuronectidae. They are demersal fish, that live on or near sand, gravel or clay bottoms at depths of between . The halibut is among the largest teleost fish in the world. Halibut are strong swimmers and are able to...
, at 2.5 metres (8.2 ft) and 316 kilograms (696.7 lb).
Reproduction
Flatfish 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 planktonPlankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
.
The length of the planktonic stage varies between different types of flatfish, 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.
Origins
Evidence of evolution
of living things has been discovered by scientists working in a variety of fields over many years. This evidence has demonstrated and verified the occurrence of evolution and provided a wealth of information on the natural processes by which the variety and diversity of life on Earth developed...
. For example, Richard Dawkins
Richard Dawkins
Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...
in The Blind Watchmaker
The Blind Watchmaker
The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design is a 1986 book by Richard Dawkins in which he presents an explanation of, and argument for, the theory of evolution by means of natural selection. He also presents arguments to refute certain criticisms made on...
, explains the flatfish's 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.
In 2008, scientists discovered that "50-million-year-old fossils have revealed an intermediate species
Amphistium
Amphistium is a fossil fish which has been identified as an early 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...
between primitive flatfishes (with eyes on both sides of their heads) and the modern, lopsided versions, which include sole, flounder, and halibut." The research concluded that "the change happened gradually, in a way consistent with evolution via natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
—not suddenly, as researchers once had little choice but to believe."
Species
Some "species" listed here are groups of species. See individual entries for further lists.- BrillBrill (fish)The brill, Scophthalmus rhombus, is a species of flatfish in the turbot family of the order Pleuronectiformes. Brill can be found in the North Atlantic, Baltic Sea, and Mediterranean, primarily in deeper offshore waters....
- DabCommon dabThe common dab, Limanda limanda, is an edible flatfish of the family Pleuronectidae. It is a demersal fish native to shallow seas around Northern Europe, in particular the North Sea, where it lives on sandy bottoms down to depths of about...
- SanddabSanddabCitharichthys is a genus of flatfish in the large-tooth flounder family, Paralichthyidae. They have both eyes on the left side of the head....
- FlounderFlounderThe flounder is an ocean-dwelling flatfish species that is found in coastal lagoons and estuaries of the Northern Atlantic and Pacific Oceans.-Taxonomy:There are a number of geographical and taxonomical species to which flounder belong.*Western Atlantic...
- HalibutHalibutHalibut is a flatfish, genus Hippoglossus, from the family of the right-eye flounders . Other flatfish are also called halibut. The name is derived from haly and butt , for its popularity on Catholic holy days...
- MegrimMegrimThis article is about the fish, For the neurological condition, see Migraine. The megrim or whiff is a species of left-eyed Flatfish found in European seas between 100 and 700 metres below sea level. It is caught commercially in some countries.-Description:It can grow up to 60 cm in length...
- PlaicePlaicePlaice is the common name of four species of flatfishes.Plaice or PLAICE may also refer to:* USS Plaice , a Balao-class submarine* PLAICE, an open source hardware FLASH programmer, memory emulator, and logic analyzer...
- SoleSole (fish)Sole is a group of flatfish 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...
- TonguefishTonguefishTonguefishes are a family, Cynoglossidae, of flatfishes. They are distinguished by the presence of a long hook on the snout overhanging the mouth, and the absence of pectoral fins...
- TurbotTurbotThe turbot is a species of flatfish in the family Scophthalmidae. It is native to marine or brackish waters of the North Atlantic, Baltic Sea and the Mediterranean Sea.-Etymology:...