The Squaliformes are an

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

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 fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...

s, 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 ...

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. Most ...

, 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 c ...

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. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the surfa ...

. All members of the family Etmoperidae and Dalatiidae and ''Zameus squamulosus'' possess

photophore A photophore is a specialized anatomical structure found in a variety of organisms that emits light through the process of boluminescence. This light may be produced endogenously by the organism itself (symbiotic) or generated through a mut ...

s, 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 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 gi ...

'' * 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 The Dalatiidae are the family of kitefin sharks of the order Squaliformes (the term "kitefin shark" also refers specifically to the species '' Dalatias licha''). Members of this family are small, under long, and are found worldwide. They have c ...

( 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 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

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 ''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 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'' * Genus '' Centroselachus'' * Genus ''

Scymnodalatias ''Scymnodalatias'' is a genus of squaliform sharks in the 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 swimmi ...

'' * Genus ''

Scymnodon ''Scymnodon'' is a genus of squaliform sharks in the family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms t ...

'' * Genus ''

Somniosus ''Somniosus'' is a widely distributed genus of deepwater Squaliformes, dogfish sharks in the family (biology), family Somniosidae. Several members of the genus are believed to attain lengths up to , thus ranking among the largest of sharks. Spec ...

'' * Genus '' Zameus'' Family

Squalidae Squalidae, more commonly known as dogfish, dog sharks, or spiny dogfish, are one of several families of sharks categorized under Squaliformes, making it the second largest order of sharks, numbering 119 species across 7 families. Having earned ...

Blainville, 1816 (dogfish sharks) * Genus ''

Cirrhigaleus ''Cirrhigaleus'' is a genus of sharks in the Squalidae (dogfish) family (biology), family, which is part of the Squaliformes order. Species * ''Cirrhigaleus asper'' Nigel Merrett, Merrett, 1973 (roughskin spurdog) * ''Cirrhigaleus australis'' ...

'' * 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