A flatfish is a member of the order Pleuronectiformes of ray-finned demersal
fishes, 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 sides upward, some face their right sides upward, and others
face either side upward. 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 700 species are in the 11 families.
The largest families are Bothidae, Cynoglossidae, Paralichthyidae,
Pleuronectidae, and Soleidae, with more than 100 species each. 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. The Pleuronectidae may
be split further still, as some authorities elevate Paralichthodinae,
Poecilopsettinae, and Rhombosoleinae to families instead of subfamilies.
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. New species are described with some
regularity and undescribed species likely remain.
= Hybrids= Hybrids are well known in flatfishes.
The Pleuronectidae, of marine fishes, have the largest number of reported
hybrids. Two of the most famous intergeneric hybrids are between the
European plaice and European flounder in the Baltic Sea, and between the English
sole and starry flounder 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. Distribution
Flatfishes are found in oceans worldwide, ranging from the Arctic,
through the tropics, to Antarctica. Most species are found in depths between 0
and 500 m, but a few have been recorded from depths in excess of 1,500 m. 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 has been questioned by fish experts, and recent
authorities do not recognize it as valid. 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. Many species will enter brackish or fresh water, and a
smaller number of soles and tonguefish are entirely restricted to fresh water.
Characteristics 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, and in others, they are usually on the left. 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, 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 a
chameleon. The side of the body without the eyes, which faces the seabed, is
usually colourless or very pale. In general, flatfishes rely on their
camouflage for avoiding predators, but some have conspicuous eyespots and
several small tropical species are poisonous. Juveniles of Soleichthys
maculosus mimic toxic flatworms of the genus Pseudobiceros in both colours and
swimming mode. Conversely, a few octopus species have been reported to mimic
flatfishes in colours, shape and swimming mode.
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.
Flatfishes range in size from Tarphops oligolepis, measuring about 4.5 cm in
length, and weighing 2 g, to the Atlantic halibut, at 2.5 m and 316 kg.
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. 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 In 2008, a 50-million-year-old fossil,
Amphistium, was identified as an early relative of the flatfish and
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. 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."
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.
Fisheries As food
Flatfish is considered a Whitefish because of the high concentration of
oils within its liver. Its lean flesh makes for a unique flavor that differs
between each individual species. Methods of cooking include grilling, pan-frying,
baking and deep-frying. Timeline of genera
See also Sinistral and dextral
References Further references
Sepkoski, Jack. "A compendium of fossil marine animal genera". Bulletins of
American Paleontology 364: p.560. Retrieved 2011-05-17. CS1 maint: Extra
text Gibson, Robin N Flatfishes: biology and
exploitation. Wiley. Munroe, Thomas A "Distributions and
biogeography." Flatfishes: Biology and Exploitation: 42-67.
External links Information on Canadian fisheries of
plaice