Squaliformes

Common card · No. 275472

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 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...

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