The Squaliformes are an order 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 con ...

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 gi ...

'' * Genus '' Deania'' 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 ci ...

(

J. E. Gray John Edward Gray, Fellow of the Royal Society, FRS (12 February 1800 – 7 March 1875) was a British zoology, zoologist. He was the elder brother of zoologist George Robert Gray and son of the pharmacologist and botanist Samuel Frederick 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 shar ...

'' * 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 currently ...

'' * Genus ''

Dalatias ''Dalatias'' is a genus of kitefin sharks that have lived since the Middle Eocene. It was thought to be a monotypic taxon with the type species, '' D. licha'', considered as the only species until 2022, when Malyshkina and her colleagues describ ...

'' * 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, 1 ...

'' * 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 a ...

'' * Genus ''

Squaliolus ''Squaliolus'' is a genus of deep-sea squaliform sharks in the family Dalatiidae. Species * ''Squaliolus aliae'' Teng Teng may refer to: *Teng (surname) (滕), a Chinese surname *Teng (state), an ancient Chinese state *Teng (mythology), a flyi ...

'' 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 histo ...

, 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 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 photophore A photophore is a glandular organ that appears as luminous spots on vari ...

Fowler, 1934 (lantern sharks) * Genus '' Aculeola'' * Genus '' Centroscyllium'' * Genus '' Etmopterus'' * Genus '' Trigonognathus'' Family Oxynotidae

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 are ...

, 1872 (rough sharks) * Genus '' Oxynotus'' 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 ha ...

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

Centroscymnus ''Centroscymnus'' is a genus of squaliform sleeper sharks in the family Somniosidae. Species * '' Centroscymnus coelolepis'' Barbosa du Bocage & Brito Capello, 1864 (Portuguese dogfish) * ''Centroscymnus owstonii'' Garman Garman is a surname ...

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

Scymnodalatias ''Scymnodalatias'' is a genus of squaliform sharks in the family Somniosidae. Species * ''Scymnodalatias albicauda'' Taniuchi & Garrick, 1986 (whitetail dogfish) * †'' Scymnodalatias cigalafulgosii'' (Adnet, 2006) * ''Scymnodalatias garrick ...

'' * 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'' * Genus ''

Zameus The velvet dogfish (''Zameus squamulosus''), the only member of the genus ''Zameus'', is a small sleeper shark of the family Somniosidae, found around the world between latitudes 64°N and 48°S, from the surface to 2,000 m. Description Its ...

'' 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 la ...

'' * 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