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 ...

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 or nictitating membrane, 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. 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''. T ...

Bleeker, 1859 (gulper sharks) * Genus '' Centrophorus'' * 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 profundor ...

'' Family Dalatiidae ( J. E. Gray, 1851) (kitefin sharks) * Genus ''

Euprotomicroides The taillight shark (''Euprotomicroides zantedeschia'') is a little-known species of shark in the family Dalatiidae and the only member of its genus.Stehmann, M.F.W., Van Oijen, M. & Kamminga, P. (2016): Re-description of the rare taillight ...

'' * Genus '' Heteroscymnoides'' * Genus ''

Mollisquama ''Mollisquama'' is a genus of pocket sharks in the family Dalatiidae. There are two known species each only known from a single specimen; one found off the coast of Chile and the other found in the Gulf of Mexico. Species There are current ...

'' * Genus '' Dalatias'' * Genus ''

Isistius ''Isistius'' is a genus of dogfish sharks in the family Dalatiidae. They are commonly known as cookiecutter sharks. Members of the genus are known for their unusual behaviour and dentition. Species *''Isistius brasiliensis'' Quoy & Gaimard, ...

'' * Genus ''

Euprotomicrus The pygmy shark (''Euprotomicrus bispinatus''), the second-smallest of all the shark species after the dwarf lanternshark, is a squaliform shark of the family Dalatiidae, the only member of the genus ''Euprotomicrus''. Their lengths are up to ...

'' * Genus ''

Squaliolus ''Squaliolus'' is a genus of deep-sea squaliform sharks in the family Dalatiidae. Species * ''Squaliolus aliae'' Teng, 1959 ( smalleye pygmy shark) * ''Squaliolus laticaudus'' H. M. Smith & Radcliffe, 1912 (spined pygmy shark The spi ...

'' Family

Echinorhinidae ''Echinorhinus'' is the only extant genus in the family Echinorhinidae. Taxonomy Echinorhinidae are traditionally classified in the order Squaliformes, together with kitefin and gulper sharks.Compagno, 2005. "Sharks of the World". However, a ...

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 his ...

, 1862 (bramble sharks) * Genus ''

Echinorhinus ''Echinorhinus'' is the only extant genus in the family Echinorhinidae. Taxonomy Echinorhinidae are traditionally classified in the order Squaliformes, together with kitefin and gulper sharks.Compagno, 2005. "Sharks of the World". However, a ...

'' Family Etmopteridae Fowler, 1934 (lantern sharks) * Genus '' Aculeola'' * 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'' * Genus '' Trigonognathus'' 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 smal ...

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 ...

'' Family

Somniosidae The Somniosidae are a family of sharks in the order Squaliformes, commonly known as sleeper sharks. The common name ''"sleeper shark"'' comes from their slow swimming, low activity level, and perceived non-aggressive nature. Distribution and hab ...

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 family ...

'' * 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 ''Scymnodon'' is a genus of squaliform sharks in the family Somniosidae. Species There are currently four recognized species in this genus: * ''Scymnodon ichiharai'' Ka. Yano & S. Tanaka (II), 1984 (Japanese velvet dogfish) White, W.T., Vaz, ...

'' * 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 ''Cirrhigaleus'' is a genus of sharks in the Squalidae (dogfish) family, which is part of the Squaliformes order. Species * '' Cirrhigaleus asper'' Merrett, 1973 (roughskin spurdog) * '' Cirrhigaleus australis'' W. T. White, Last A last ...

'' * Genus ''

Squalus ''Squalus'' is a genus of dogfish sharks in the family Squalidae. Commonly known as spurdogs, these sharks are characterized by smooth dorsal fin spines, teeth in upper and lower jaws similar in size, caudal peduncle with lateral keels; upper ...

Classification

References

Further reading