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A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
they have independently evolved external superficial fish-like body plans adapted to their
marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...
environments, including most numerously
fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
, but also mammals such as cetaceans ( whales,
dolphin A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the b ...
s, and porpoises), and even extinct ancient
marine reptiles Marine reptiles are reptiles which have become secondarily adapted for an aquatic or semiaquatic life in a marine environment. The earliest marine reptile mesosaurus (not to be confused with mosasaurus), arose in the Permian period during th ...
such as various known species of ichthyosaurs. Most species have only one dorsal fin, but some have two or three. Wildlife biologists often use the distinctive nicks and wear patterns which develop on the dorsal fins of large cetaceans to identify individuals in the field. The bony or cartilaginous bones that support the base of the dorsal fin in fish are called ''pterygiophores''.


Functions

The main purpose of the dorsal fin is to stabilize the animal against rolling and to assist in sudden turns. Some species have further adapted their dorsal fins to other uses. The sunfish uses the dorsal fin (and the anal fin) for propulsion. In anglerfish, the anterior of the dorsal fin is modified into a biological equivalent to a fishing pole and a lure known as ''illicium'' or ''esca''. Many
catfish Catfish (or catfishes; order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the three largest species alive, ...
can lock the leading ray of the dorsal fin in an extended position to discourage predation or to wedge themselves into a crevice. Some animals have developed dorsal fins with protective functions, such as spines or venom. For example, both the spiny dogfish and the Port Jackson shark have spines in their dorsal fins which are capable of secreting venom. File:Carassius wild golden fish 2013 G1.jpg, Most fish, like this large goldfish, have one dorsal fin File:Parts of a shark.svg, Sharks typically have two dorsal fins File:Thunnus albacares.png, The yellowfin tuna also has two dorsal fins File:Haddock fins.tiff, Haddocks have three dorsal fins Billfish have prominent dorsal fins. Like tuna, mackerel and other scombroids, billfish streamline themselves by retracting their dorsal fins into a groove in their body when they swim. The shape, size, position and colour of the dorsal fin varies with the type of billfish, and can be a simple way to identify a billfish species. For example, the white marlin has a dorsal fin with a curved front edge and is covered with black spots. The huge dorsal fin, or sail, of the sailfish is kept retracted most of the time. Sailfish raise them if they want to herd a school of small fish, and also after periods of high activity, presumably to cool down.''Aquatic Life of the World''
pp. 332–333, Marshall Cavendish Corporation, 2000. .
File:Orca wal 3.jpg, Differences of dorsal fins of orcas between male and female File:White shark (cropped).jpg, The dorsal fin of a white shark contains dermal fibers that work "like riggings that stabilize a ship's mast", and stiffen dynamically as the shark swims faster to control roll and yaw.Lingham‐Soliar T (2005
"Dorsal fin in the white shark, ''Carcharodon carcharias'': A dynamic stabilizer for fast swimming"
''Journal of Morphology'', 263 (1): 1–11. tp://ftp.cicese.mx/pub/divOC/ocebiol/Erick%20O%F1ate/White%20shark%20papers/Lingham-Soliar_2005_Dorsal_fin_Dynamic_stabilizer_fast_swimming.pdf pdf/ref> File:Istiophorus platypterus .jpg, Large retractable dorsal fin of the
Indo-Pacific sailfish The Indo-Pacific sailfish (''Istiophorus platypterus'') is a sailfish native to the Indian and Pacific Oceans and is naturalized in the Atlantic where it has entered the Mediterranean Sea via the Suez Canal as a Lessepsian migrant. It is dark b ...
File:Ichthyosaurios5.jpg, Various species of Ichthyosaurs displaying different types of dorsal fins


Structure

A dorsal fin is classified as a medial, unpaired fin that is located on the midline of the backs of some aquatic vertebrates. In development of the embryo in teleost fish, the dorsal fin arises from sections of the skin that form a caudal fin fold. The larval development and formation of the skeleton that support the median fins in adults result in pterygiophores. The skeletal elements of the pterygiophore includes basals and radials. The basals are located at the base of the dorsal fin, and are closest to the body. The radials extend outward from the body to support the rest of the fin. These elements serve as attachment sites for epaxial muscles. The muscles contract and pull against the basals of the pterygiophores along one side of the body, which helps the fish move through water by providing greater stability. In these types of fish, the fins are made of 2 main components. The first component is the dermal fin rays known as lepidotrichia, and the
endoskeletal An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is an internal support structure of an animal, composed of mineralized tissue. Overview An endoskeleton is a skeleton that is on the i ...
base with associated muscles for movement is the second. File:Dorsal Fin Labeled Final.jpg, Dorsal fin of a perch showing the basals and radials of the pterygiophore that support the dorsal fin. File:Callionymus lyra dorsal fin male.jpg, Closeup of the dorsal fin of a
common dragonet The common dragonet (''Callionymus lyra'') is a species of dragonet which is widely distributed in the eastern North Atlantic where it is common near Europe from Norway and Iceland southwards. It is a demersal species that occurs over sand bottom ...


See also

* Fish fin * Shark fin soup *
Submarine sail In naval parlance, the sail (American usage) or fin (British/Commonwealth usage) (also known as a fairwater) of a submarine is the tower-like structure found on the dorsal (topside) surface of submarines. Submarine sails once housed the conning to ...
*
Vertical stabilizer A vertical stabilizer or tail fin is the static part of the vertical tail of an aircraft. The term is commonly applied to the assembly of both this fixed surface and one or more movable rudders hinged to it. Their role is to provide control, sta ...


Notes

{{diversity of fish Vertebrate anatomy es:Aleta (zoología)#Dorsales