Shark

A shark is a type of fish that lives in oceans all over the world. Sharks belong to a group of fish with skeletons made of cartilage instead of bone. Cartilage is the same bendy material that shapes your ears and the tip of your nose. Rays and skates belong to this same group. There are more than 500 species of sharks, and they come in many shapes and sizes.

The biggest shark is the whale shark, which can grow up to 40 feet long. That is longer than a school bus. The smallest is the dwarf lanternshark, which fits in a human hand. Most sharks fall somewhere in between. A great white shark, one of the most famous kinds, usually grows to about 15 feet.

Sharks have some unusual body parts. Their skin is covered in tiny tooth-shaped scales called denticles, which feel rough like sandpaper. Their teeth grow in rows, and when one falls out, another moves forward to take its place. A single shark may go through 30,000 teeth in its lifetime. Sharks can also sense tiny electric signals given off by other animals. Small jelly-filled pores on their snouts pick up the weak electricity made by a fish's beating heart or twitching muscles. This helps sharks find prey even in dark or cloudy water.

Sharks are powerful hunters, but most are not dangerous to people. Only about a dozen species have ever been involved in attacks on humans. Sharks attack fewer than 100 people worldwide in a typical year, and most of those people survive. Humans are far more dangerous to sharks. People kill about 100 million sharks every year, mostly for their fins, which are used in soup. Many shark species are now endangered.

Sharks matter a lot to the health of the ocean. As top predators, they keep other animal populations in balance. When shark numbers drop, the whole food web below them can fall apart.

Sharks are very old. Fossils show that sharks were swimming in the seas more than 400 million years ago, before dinosaurs existed and even before trees grew on land. They survived several mass extinctions that wiped out most other animals. One ancient shark called megalodon lived until about 3.6 million years ago. Megalodon grew to about 60 feet long, and its teeth were the size of a grown-up's hand.

Despite thousands of years of stories about shark attacks, most sharks are shy. They usually swim away when divers get close. The scary shark of movies and nightmares is mostly an invention of humans.