The spiny dogfish , spurdog , mud shark , or piked dogfish [2] Squalus acanthias is one of the best known species of the Squalidae dogfish family of sharks , which is part of the Squaliformes order. It is found mostly in shallow waters and further offshore in most parts of the world, especially in temperate waters. Spiny dogfish in the northern Pacific Ocean have recently been reevaluated and found to constitute a separate species, now known as " Pacific spiny dogfish ", Squalus suckleyi. The spiny dogfish has dorsal fins, no anal fin, and dark green spots along its back. The caudal fin has asymmetrical lobes, forming a heterocercal tail.