Great white shark (Elias Levy / CC BY)
Sharks are cartilaginous fish related to rays. They are characterized by 5 to 7 gill openings on the sides of their heads and pectoral fins that are not connected to their heads. The dwarf lantern shark (Etmopterus perryi) is the smallest species, reaching only 17 centimeters (7 inches) in length. The largest shark is the whale shark (Rhincodon typus), which reaches about 12 meters (39 feet) in length. The majority of sharks feed on other fish. Large sharks prey on marine mammals as well. Most species do not impose a threat to divers. Shark attacks are typically motivated out of curiosity or mistaken identity. In most cases, after a single sample bite, the shark retreats. Like most wild animals, sharks will usually defend themselves if provoked.