Sharks are fish. There are about 350 species of sharks. The smallest sharks are only 12. 5 cm long. The largest, the whale shark, is the world's largest fish. It grows to about 1Sm long. Sharks are different from other fish. For example, other fish have skeletons made of hard bones. Sharks have skeletons made of cartilage. In addition, most fish have a special organ called a swim bladder to stop them from sinking but sharks do not. If a shark does not swim constantly, it sinks. Shark babies grow inside eggs. The baby sharks are called pups. Most sharks eat fish. Some also eat seals and other sharks. Many people are afraid of sharks be cause of their reputation as' man-eaters'. However, there are fewer than one hundred shark attacks on humans in the world each year. Humans are the sharks' worst enemies. Fishermen kill them because of their valuable dorsal fins. Often, they catch a shark, cut off its fin and throw it back into the sea. If a shark has no fin, it dies because it cannot swim. Scientists believe that humans kill more than one million sharks every year. Sharks are ______.