The Squaliformes are an order of sharks 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 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 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 Bleeker, 1859 (gulper sharks) Genus Centrophorus Genus Deania 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, 1862 (bramble sharks) Genus Echinorhinus Family Etmopteridae Fowler, 1934 (lantern sharks) Genus Aculeola Genus Centroscyllium Genus Etmopterus Genus Trigonognathus Family Oxynotidae Gill, 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 References Further reading Extant Late Jurassic first appearances Taxa named by Edwin Stephen Goodrich Cartilaginous fish orders