The
flatfish are an
orderIn 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
PerciformesThe Perciformes, also called the Percomorphi or Acanthopteri, is the largest order of vertebrates containing about 40% of all bony fish. Perciformes means perch-like. They belong to the ray-finned fish and comprise over 7000 species found in almost all aquatic environments...
. The name means "side-swimmers" in
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
. 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
flounderFlounder is an ocean-dwelling flatfish species that is located off the Canadian and U.S. east coast of the Northern Atlantic, and the Pacific Ocean, in coastal lagoons and estuaries.-Taxonomy:...
s,
soleSole is a type of flatfish of varying families. Generally speaking, they are the 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...
s,
turbotTurbot are flatfish native to marine or brackish waters of the North Atlantic, Black Sea, Baltic Sea and the Mediterranean Sea.-Origin:...
,
plaicePlaice is the common name of four species of flatfishes:* Alaska plaice, Pleuronectes quadrituberculatus* American plaice, Hippoglossoides platessoides* European plaice, Pleuronectes platessa* Scale-eye plaice, Acanthopsetta nadeshnyi...
, and
halibutA halibut is a type of flatfish from the family of the right-eye flounders . This name is derived from haly and butt , alleged to be called so from being commonly eaten on holy-days...
.
The
flatfish are an
orderIn 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
PerciformesThe Perciformes, also called the Percomorphi or Acanthopteri, is the largest order of vertebrates containing about 40% of all bony fish. Perciformes means perch-like. They belong to the ray-finned fish and comprise over 7000 species found in almost all aquatic environments...
. The name means "side-swimmers" in
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
. 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
flounderFlounder is an ocean-dwelling flatfish species that is located off the Canadian and U.S. east coast of the Northern Atlantic, and the Pacific Ocean, in coastal lagoons and estuaries.-Taxonomy:...
s,
soleSole is a type of flatfish of varying families. Generally speaking, they are the 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...
s,
turbotTurbot are flatfish native to marine or brackish waters of the North Atlantic, Black Sea, Baltic Sea and the Mediterranean Sea.-Origin:...
,
plaicePlaice is the common name of four species of flatfishes:* Alaska plaice, Pleuronectes quadrituberculatus* American plaice, Hippoglossoides platessoides* European plaice, Pleuronectes platessa* Scale-eye plaice, Acanthopsetta nadeshnyi...
, and
halibutA halibut is a type of flatfish from the family of the right-eye flounders . This name is derived from haly and butt , alleged to be called so from being commonly eaten on holy-days...
. There are more than 400 species of this order. Some flatfish can camouflage themselves on the ocean floor.
Characteristics
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, and in others, they are always on the left. The primitive
spiny turbotThe spiny turbots are a family, Psettodidae, of primitive flatfish found in the tropical waters of the Atlantic and Indian oceans. The family contains just three species, all in the same genus, Psettodes. The common name comes from the presence of spines in the dorsal and anal fins, which may...
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
seabedThe seabed is the bottom of the ocean. At the bottom of the continental slope is the continental rise, which is caused by sediment cascading down the continental slope. The seabed has been explored by submersibles such as Alvin and, to some extent, scuba divers with special apparatuses...
(
benthosBenthos are the organisms which live on, in, or near the seabed, also known as the benthic zone. They live in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths....
), 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 similar manner to a
chameleonThe family Chamaeleonidae 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, and the...
. The side of the body without the eyes, which faces the seabed, is usually colourless or very pale.
The
flounderFlounder is an ocean-dwelling flatfish species that is located off the Canadian and U.S. east coast of the Northern Atlantic, and the Pacific Ocean, in coastal lagoons and estuaries.-Taxonomy:...
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 less extreme adaptations than other families. The
soleSole is a type of flatfish of varying families. Generally speaking, they are the 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...
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 in length, and weighing , to the
Atlantic halibutThe Atlantic halibut, Hippoglossus hippoglossus, is a flatfish of the family Pleuronectidae. It is a demersal fish that lives on sand, gravel or clay bottoms at depths of between . Its native habitat is the temperate waters of the northern Atlantic, from Labrador and Greenland to Iceland, the...
, at and .
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 consist of any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their 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
Flatfish have been cited as dramatic
examples of evolutionary adaptationThe wide range of evidence of common descent of living things strongly indicates the occurrence of evolution and provides a wealth of information on the natural processes by which the variety of life on Earth developed...
. For example,
Richard DawkinsClinton Richard Dawkins, FRS, FRSL is a British ethologist, zoologist, Neo-Darwinian evolutionary biologist and theorist and a popular science author....
in
The Blind WatchmakerThe Blind Watchmaker 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 his previous book The Selfish Gene...
, explains the flatfish's evolutionary history as:
…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.
The development of flatfish is thus considered to
recapitulateThe theory of recapitulation, also called the biogenetic law or embryological parallelism and often expressed as "ontogeny recapitulates phylogeny" is a discredited biological hypothesis...
their evolutionary history.
In 2008, scientists discovered "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 selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
—not suddenly, as researchers once had little choice but to believe."
The asymmetric geometry of flatfish has been likened to the
cubistCubism was a 20th century avant-garde art movement, pioneered by Pablo Picasso and Georges Braque, that revolutionized European painting and sculpture, and inspired related movements in music and literature. The first branch of cubism, known as "Analytic Cubism", was both radical and influential as...
paintings of
Pablo PicassoPablo Diego José Francisco de Paula Juan Nepomuceno María de los Remedios Cipriano de la Santísima Trinidad Ruiz y Picasso was a Spanish painter, draughtsman, and sculptor. Commonly known simply as Picasso, he is one of the most recognized figures in 20th-century art...
, and is often perceived as being "imperfect", "grotesque", "strange", etc. It is likely that the asymmetry contributes to their survival by helping to disguise them on the ocean floor.
Species
Some "species" listed here are groups of species. See individual entries for further lists.
- Brill
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 Sea, primarily in deeper offshore waters....
- Dab
The 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...
- Flounder
Flounder is an ocean-dwelling flatfish species that is located off the Canadian and U.S. east coast of the Northern Atlantic, and the Pacific Ocean, in coastal lagoons and estuaries.-Taxonomy:...
- Halibut
A halibut is a type of flatfish from the family of the right-eye flounders . This name is derived from haly and butt , alleged to be called so from being commonly eaten on holy-days...
- Megrim
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:...
- Plaice
Plaice is the common name of four species of flatfishes:* Alaska plaice, Pleuronectes quadrituberculatus* American plaice, Hippoglossoides platessoides* European plaice, Pleuronectes platessa* Scale-eye plaice, Acanthopsetta nadeshnyi...
- Sole
Sole is a type of flatfish of varying families. Generally speaking, they are the 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...
- Tonguefish
Tonguefishes 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...
- Turbot
Turbot are flatfish native to marine or brackish waters of the North Atlantic, Black Sea, Baltic Sea and the Mediterranean Sea.-Origin:...
External links