A marine shark, belonging to one of at least 61 species in the family Squalidae, that has two dorsal fins. Dogfish give birth to live young, and vary in size from about 90 cm (Atlantic velvet belly, Etmopterus spinax) to about 6.4 m (Greenland shark, Somniosus microcephalus). Dogfishes often travel in large schools. They occur in all oceans.