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 a saltwater or brackish waters. 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.
Gulper sharks are usually deepwater fish. While some, such as the gulper sharkCentrophorus granulosus, are found worldwide and fished commercially, others are uncommon and little-known. Their usual prey is other fish; some are known to feed on squid, octopus, and shrimp. Some species live on the bottom, while others are pelagic. They are ovoviviparous, with the female retaining the egg-cases in her body until they hatch. They are small to medium sharks, ranging from to in adult body length.
Kitefin sharks are small, under long, and are found worldwide. They have cigar-shaped bodies with narrow heads and rounded snouts. Several species have specialized bioluminescent organs. The term kitefin shark is also used as the common name for the type species of the family, Dalatias licha.
Bramble sharks are usually benthic fish found in tropical and temperate waters worldwide, while the prickly shark is found in the deep waters of the Pacific Ocean. Their usual prey is small fish, cephalopods, and crustaceans. They are ovoviviparous, with the female retaining the egg cases in her body until they hatch. They are relatively large sharks, ranging from in adult body length.color|white|Etmopteridaeconvert|90|cm|in|abbr=oncolor|white|Oxynotidaeconvert|49| to|150|cm|ftcolor|white|Somniosidaecolor|white|Squalidae
