Sharks are powerful fish. There are 400 types of sharks in the world. Most sharks live in the ocean, although some sharks live in lakes and rivers and are called freshwater sharks. Sharks differ from other fish because they do not have a skeleton made of bones. Instead they have a skeleton made of cartilage a lighter elastic material. This cartilage helps them bend and twist in the water. Many people fear sharks, but they do not often attack humans.
A shark has to spend most of its time hunting for prey. It has to be very fast. Some kinds can swim at speeds up to 40 miles per hour. A shark can come upon a seal or other prey on the surface from below and grab it. To help it detect prey, a shark has an amazing sense of smell (10,000 times better than humans). It also can hear a sound one mile away, feel vibrations made by a prey from 330 feet away and can see very well, even in the deep dark bottom of the ocean.
A shark does not chew its food. It bites off big pieces and swallows them whole. It has many rows of teeth. It is always losing some and replacing them A shark may use up more than 10,000 teeth in its life. Different sharks have different kinds of teeth. Each kind is right for the food that a shark eats. Long, spiky teeth are good for catching. Flat teeth are good for grinding. Serrated teeth (like the edge of a knife) are good for ripping. A shark usually eats alone because if a big group of sharks attacks a single prey, one shark might bite another in the confusion.
A shark has to keep moving all the time or it will drown. It moves even when asleep. Its powerful tail helps it move forward, and its fins keep them on course. As it swims through the water, the water flows through its mouth and over its gills. The gills collect oxygen for the shark to breathe. Then the water flows out through gill slits on either side of its head.
Most shark babies grow inside their mothers and are born after about 10-12 months. Babies are called pups. Lemon sharks may have as many as 17 pups at a time. Some sharks, like the cloudy cat shark, lay eggs in cases on the ocean floor. Inside the egg case, a shark embryo feeds on the yolk of the egg. It cracks out of the egg and swims away from its mother fully grown.
Whale sharks are the world's largest living fish. The great white shark is the world's largest predatory fish. The pygmy shark is one of the tiniest sharks. It is under 9 inches long. The shortfin mako is the fastest shark in the world. The three deadliest or most dangerous to humans are the white, tiger and bull sharks.
Sharks are disappearing from the world's oceans. Scientists are trying to find ways to prevent that from happening. In order to learn more about sharks and their habits, scientists safely catch them in a plastic sling, measure them, take a blood sample for DNA and attach a tag to the base of a fin. This tag can send out a sound which can be picked up by an underwater microphone. Records of sightings are kept in logbooks around the world.
In summary, there are about 400 kinds of sharks in the world. They are the world's most powerful fish. However, unlike fish, sharks don't have a skeleton made of bone, but of cartilage. The cartilage allows them to bend and twist as they swim. Most sharks live in the ocean, although a few kinds live in freshwater.