Sharks are fish that live in all the world’s oceans. A few species also live in rivers. They belong to a group of predatory fish: Most eat fish and other marine animals.
When sharks swim at the surface of the water, they can be recognised by their triangular dorsal fin that sticks out of the water. Sharks were already swimming through the sea 400 million years ago, so they are one of the oldest animal species in the world.
The dwarf shark is the smallest shark at 25 centimetres long, the whale shark the longest at 14 metres. The whale shark is also the heaviest shark: at up to twelve tonnes, it weighs as much as ten small cars. In total, there are about 500 species of sharks.
Sharks have a special set of teeth: behind the first row of teeth, more rows grow. If teeth then fall out in a fight with other animals, the next teeth move up. In this way, a shark «uses» up to 30,000 teeth in its lifetime.
The shark’s skin is not made of normal scales, but of the same material as the teeth. These scales are called «skin teeth». From the head to the tail fin, this skin feels very smooth, but rough the other way round.
Sharks are still poorly studied, so little is known about them. However, one special feature is known: Sharks have to keep moving constantly so that they do not sink to the bottom of the sea. This is because, unlike other fish, they do not have a swim bladder that is filled with air.
Most shark species feed on fish and other larger marine animals. But some of the largest shark species feed on plankton, which are small animals or plants that float in the water. Around the world, about five people are killed by sharks every year.