There are probably no other animals on earth that people fear as much as
sharks. Movies and books have made it seem that the world's oceans are
full of hungry sharks waiting to attack every swimmer who enters the
water. Most people also think of sharks as large, gray, torpedo-shaped
animals. Neither view is completely true. Sharks can
be short, fat, skinny, brown, and even spotted, with unusually shaped
heads and tails. Sharks range in size from the 15-centimeter (6 inch)
cigar shark to the whale shark, which may reach lengths of 18 meters (60 feet).
However, about half of the 400+ different species of sharks are actually less than 1 meter (3 feet) long. Humans are not a normal part of a shark's diet. On occasion, sharks do mistake a person for a seal or sea turtle. Once it realizes its mistake, however, the shark will usually release the person. These incidents are also very rare; more people die from bee stings every year than from shark attacks.
No Bones About It Sharks and their relatives- skates, sawfish, and rays- are elasmobranchs, fishes with skeletons made of cartilage, not bone. Cartilage is a tough flexible material, which can be found in human ears and noses. Sharks differ from bony fish in other ways. Sharks have five to seven clearly visible gill slits while bony fish have a hard covering over their gill slits. Instead of smooth scales, sharks have rough skin with tiny tooth-like structures called dermal denticles. Also, sharks lack the gas-filled swim bladder found in bony fish; they rely on their large, oily liver to stay afloat. Sand tiger sharks are the only species that gulp air; this trapped air acts as a swim bladder. Many sharks must swim constantly to keep fresh oxygenated water flowing over their gills, allowing them to breathe. Others use spiracles to pump oxygen over their gills when not swimming.