The Squaliformes are an

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of d ...

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...

s that includes about 126 species in seven families. Members of the order have two

dorsal fin 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 c ...

s, which usually possess spines, they usually have a sharp head, no

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as s ...

nictitating membrane The nictitating membrane (from Latin '' nictare'', to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision. All An ...

, and five to seven

gill slit Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In co ...

s. In most other respects, however, they are quite variable in form and size. Most species of the squaliform order live in saltwater or brackish water. They are found worldwide, from northern to tropical waters, and from shallow coastal seas to the

open ocean The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...

. All members of the family Eptomeridae and Dalatiidae and ''Zameus squamulosus'' possess photophores, luminous organs, and exhibit intrinsic bioluminescence. Bioluminescence evolved once in Squaliformes, approximately 111–153 million years ago, and helped the Squaliformes radiate and adapt to the deep sea. The common ancestor of Dalatiidae, Etmopteridae, Somniosidae, and Oxynotidae possessed a luminous organ and used bioluminescence for camouflage by counterillumination. Counterillumination is an active form of camouflage in which an organism emits light to match the intensity of downwelling light to hide from predators below. Currently, bioluminescence provides different functions for Squaliformes based on the family. Dalatiidae and ''Zameus squamulosus'' possess simple photophores and use bioluminescence for ventral counter-illumination. Etmopteridae possess more complex photophores and utilize bioluminescence for ventral counter illumination as well as species recognition.

Classification

Family

Centrophoridae The Centrophoridae are a family of squaliform sharks. The family contains just two genera and about 15 species. They are sometimes called gulper sharks, but this is also the name of a specific species in the family, '' Centrophorus granulosus' ...

Bleeker, 1859 (gulper sharks) * Genus ''

Centrophorus ''Centrophorus'' is a genus of squaliform sharks. These deep-water sharks, found in temperate and tropical oceans throughout the world, are characterized by grey or brown bodies, large green eyes, and spines on both dorsal fins. These spines g ...

'' * Genus ''

Deania ''Deania'' is a genus of long-snouted, deepwater dogfish sharks in the family Centrophoridae. Species * '' Deania calcea'' R. T. Lowe, 1839 (birdbeak dogfish) * '' Deania hystricosa'' Garman, 1906 (rough longnose dogfish) * '' Deania profundoru ...

'' Family Dalatiidae ( J. E. Gray, 1851) (kitefin sharks) * Genus '' Euprotomicroides'' * Genus '' Heteroscymnoides'' * Genus '' Mollisquama'' * Genus '' Dalatias'' * Genus '' Isistius'' * Genus '' Euprotomicrus'' * Genus '' Squaliolus'' Family Echinorhinidae

Theodore Gill Theodore Nicholas Gill (March 21, 1837 – September 25, 1914) was an American ichthyologist, mammalogist, malacologist and librarian. Career Born and educated in New York City under private tutors, Gill early showed interest in natural histor ...

, 1862 (bramble sharks) * Genus '' Echinorhinus'' Family Etmopteridae Fowler, 1934 (lantern sharks) * Genus ''

Aculeola The hooktooth dogfish, ''Aculeola nigra'', is a small, little-known dogfish, the only member of the genus ''Aculeola''. The type specimen is held at the National Natural History Museum, Santiago, Chile. Description The hooktooth dogfish has a ...

'' * Genus ''

Centroscyllium ''Centroscyllium'' is a genus of big-eyed, deepwater dogfishes with no anal fin, a grey or black-brown body, and dorsal spines, with the second one being much larger than the first. Seven extant species are described. Species * '' Centroscyll ...

'' * Genus ''

Etmopterus ''Etmopterus'' is a genus of lantern sharks in the squaliform family Etmopteridae. They are found in deep sea ecosystems of the Atlantic, Indian and Pacific Oceans. Ecology A number of species in this genus function as host to the specialized ...

'' * Genus ''

Trigonognathus The viper dogfish or viper shark (''Trigonognathus kabeyai'') is a rare species of dogfish shark in the family Etmopteridae, and the only extant member of its genus. It has been found in the Pacific Ocean off southern Japan, the Bonin Islands, P ...

'' Family

Oxynotidae ''Oxynotus'' is a genus of sharks in the order Squaliformes, commonly known as the rough sharks. It is the only extant genus in the family Oxynotidae. They live in deep waters in the Atlantic and western Pacific Oceans. Rough sharks are small ...

Gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...

, 1872 (rough sharks) * Genus ''

Oxynotus ''Oxynotus'' is a genus of sharks in the order Squaliformes, commonly known as the rough sharks. It is the only extant genus in the family Oxynotidae. They live in deep waters in the Atlantic and western Pacific Oceans. Rough sharks are small ...

'' Family Somniosidae D. S. Jordan, 1888 (sleeper sharks) * Genus ''

Centroscymnus ''Centroscymnus'' is a genus of squaliform sleeper sharks in the family Somniosidae. Species * ''Centroscymnus coelolepis The Portuguese dogfish (''Centroscymnus coelolepis'') or Portuguese shark, is a species of sleeper shark of the fa ...

'' * Genus ''

Centroselachus The longnose velvet dogfish (''Centroselachus crepidater'') is a sleeper shark of the family Somniosidae, found circumglobally in southern hemisphere subtropical seas, at depths of between 230 and 1,500 m. It reaches a length of 130 cm. ...

'' * Genus '' Scymnodalatias'' * Genus '' Scymnodon'' * Genus ''

Somniosus ''Somniosus'' is a widely distributed genus of deepwater dogfish sharks in the family Somniosidae. Several members of the genus are believed to attain lengths up to , thus ranking among the largest of sharks. Species * '' Somniosus antarcticus'' ...

'' * Genus '' Zameus'' Family Squalidae Blainville, 1816 (dogfish sharks) * Genus '' Cirrhigaleus'' * Genus '' Squalus''

Classification

References

Further reading