The Squaliformes are an order of

shark Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the ...

s that includes about 126 species in seven families. Members of the order have two dorsal fins, which usually possess spines, they usually have a sharp head, no

anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...

or nictitating membrane, and five to seven gill slits. 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 Etmoperidae 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. Many squaliforms have a spine in front of each of the two dorsal fins, likely a trait of the common ancestor of this clade. The clade likely originated in the post-Jurassic shallow waters of the northern Tethyal margin.

Classification

Family Centrophoridae Bleeker, 1859 (gulper sharks) * Genus '' Centrophorus'' * Genus ''

Deania ''Deania'' is a genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classifi ...

'' 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 s ...

'' * 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 The Gulf of Mexico () i ...

'' * 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 & Gaimar ...

'' * 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 Genus (; : genera ) is a taxonomic ran ...

'' * Genus ''

Squaliolus ''Squaliolus'' is a genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV cla ...

'' Family

Etmopteridae The Etmopteridae are a family of sharks in the order Squaliformes, commonly known as lantern sharks. Their name comes from the presence of light-producing photophores on their bodies. The members of this family are small, under long, and are ...

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 * '' Centroscylli ...

'' * Genus ''

Etmopterus ''Etmopterus'' is a genus of Etmopteridae, lantern sharks in the Squaliformes, squaliform family (biology), family Etmopteridae. They are found in deep sea ecosystems of the Atlantic Ocean, Atlantic, Indian Ocean, Indian and Pacific Oceans. Ecol ...

'' * Genus '' Trigonognathus'' Family Oxynotidae

Gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, 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 r ...

, 1872 (rough sharks) * Genus '' Oxynotus'' Family Somniosidae D. S. Jordan, 1888 (sleeper sharks) * Genus '' Centroscymnus'' * Genus '' Centroselachus'' * Genus '' Scymnodalatias'' * Genus '' Scymnodon'' * Genus '' Somniosus'' * Genus '' Zameus'' Family Squalidae Blainville, 1816 (dogfish sharks) * Genus '' Cirrhigaleus'' * Genus ''

Squalus ''Squalus'' is a genus of dogfish sharks in the family (biology), family Squalidae. Commonly known as spurdogs, these sharks are characterized by smooth dorsal fin spines, teeth in upper and lower fish jaw, jaws similar in size, caudal peduncle ...

Classification

References

Further reading