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A dorsal fin is a
fin A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fins ...
located on the back of most marine and freshwater
vertebrate Vertebrates () comprise all species of animals within the subphylum Vertebrata () (chordates with backbones). Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described. Vertebrates inclu ...
s within various
taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name ...
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 commo ...
they have independently evolved external superficial fish-like
body plan A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrate body plan is one of many: invertebrates consist of many phyla. This term, usua ...
s ideal for 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 * Ma ...
environments, including most numerously
fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. They form a sister group to the tunicates, together forming the olfactores. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony ...
, but also mammals such as
cetacean Cetaceans (from la, cetus, lit=whale, from grc, κῆτος, translit=kētos, lit = huge fish, sea monster) are aquatic mammals constituting the infraorder Cetacea (). Key characteristics are their fully aquatic lifestyle, streamlined body shape, of ...
s (
whale Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. They are an informal grouping within the infraorder Cetacea, usually excluding dolphins and porpoises. Whales, dolphins and porpoises belong to the ...
s, dolphins, and porpoises), and even extinct ancient marine reptiles 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 Molidae, 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 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 Prussian carp, have one dorsal fin File:Parts of a shark.svg, Shark, Sharks typically have two dorsal fins File:Thunnus albacares.png, The yellowfin tuna also has two dorsal fins File:Haddock fins.tiff, Haddock, 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. [ftp://ftp.cicese.mx/pub/divOC/ocebiol/Erick%20O%F1ate/White%20shark%20papers/Lingham-Soliar_2005_Dorsal_fin_Dynamic_stabilizer_fast_swimming.pdf pdf]
File:Istiophorus platypterus .jpg, Large retractable dorsal fin of the Indo-Pacific sailfish File:Ichthyosaurios5.jpg, Various species of Ichthyosaurs displaying different types of dorsal fins


Structure

A dorsal fin is classified as a Anatomical terms of location#Medial and lateral, 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 and hypaxial muscles, 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 Endoskeleton, endoskeletal 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


See also

* Fish fin * Submarine sail * Vertical stabilizer


Notes

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