Sharks are a group of fish characterized by a cartilaginous skeleton, five to seven-gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha and are the sister group to the rays. They range in size from the small dwarf lantern shark (Etmopterus perryi), a deep-sea species of only 17 centimeters (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 meters (39 ft) in length. Sharks are found in all seas and are common to depths of 2,000 meters (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive and be found in both seawater and freshwater.