Sharks are among the oldest living animals on earth today. There are approximately 200 to 250 species of sharks and they live in all the oceans of the world. Despite the shark's reputation for viciousness, there are really only about 30 species of shark that are known to attack humans. Large sharks, such as the great white, tiger, and bull sharks, that include human-sized prey in their diet, are the most dangerous. Other sharks that may attack humans include: hammerhead, gray reef, lemon, dusky, blue, spinner, sand tiger, nurse, and Ganges River sharks.
Sharks have skeletons composed of cartilage rather than of bone. They may be reddish brown, bronze, metallic blue, gray or nearly black and are often patterned with spots, bands, marbling or protuberances. Sharks have five to seven gill slits. Many sharks must keep moving in order to breathe. This is not true for all sharks though. The nurse shark, for example, can lie still and fan their gills to bring oxygen rich water across the slits. Because sharks lack a swim bladder, all sharks must swim in order to avoid sinking toward the ocean floor. Finally, sharks are know for their speed and maneuverability in the water. Most species can swim at speeds of 20 to 30 miles per hour, and the speed of the mako shark has been recorded at more than 60 miles per hour.
