What are the physical characteristics of sharks?

There are thousands of kinds of marine animals, and sharks may be the fiercest animals in the ocean. They are fierce and cruel by nature, and attack when they meet the target, but most sharks will not harm humans.

Sharks are flat and long, wide at the front and narrow at the back, streamlined, with dorsal fins and a sharp knife-like tail standing up and down. It swims fast in the water, like lightning. Shark skin is tough with patterns and spots. There are many differences between sharks and ordinary fish: ordinary fish have movable gill covers, while sharks use mouths on their skin instead; Its spine is cartilage; Its mouth is surprisingly big, and its teeth don't grow on its chin like ordinary fish. Its teeth are just "dentate protrusions" in the mouth, and they can be replaced constantly, so its teeth are extremely sharp and can bite any living thing into pieces.

There are more than 250 kinds of sharks. They mainly eat fish, but they also eat dolphins, seals and penguins. Tiger sharks and great white sharks are the most dangerous. Tiger sharks live in tropical waters, some of which weigh 270 kilograms. They can swallow whole sea lions and sometimes attack humans. The most terrible thing is the great white shark, which can reach 12 meters in length. Eating people is very common for it, and even a huge walrus can be swallowed by it.