ORDER-SQUALIFORMES

The Squaliformes, or common name Dogfish sharks are an order of sharks that includes about 130 species in seven families.

Most sharks in this order have two dorsal fins, which may or may not have spines. They can have a sharp head, no anal fin or nictitating membrane, but do have spiracles. The nostrils are not connected to the mouth. The caudal fin has a vertebral column that is elevated with a somewhat long dorsal lobe. There are 5 gill slits all in front of the pectoral fin origins. Squaliformes range in size from dwarf to huge.

All known species are ovoviviparous, and range from 1 to over 20 pups per litter. Some of these sharks have the oldest known age at maturity, and the longest known gestation period. Some of these sharks are solitary, and others form shoals or even schools. In addition, some families have sharks that possess photophores capable of bioluminescence. Some functionality of this is counter illumination, but other functions exist among the families too. Bioluminescence evolved once in Squaliformes, approximately 111-153 million years ago, and helped the Squaliformes adapt to the deep sea. Other sharks among families are considered to be a facultative parasite.

Most species of the Squaliformes order live in a salt-water or brackish water. They are found worldwide, from northern to tropical waters, and from shallow coastal seas to the open ocean. The greatest diversity is among the deepwater sharks. In addition, this order contains the only sharks at high latitudes and even close to poles.