The secret behaviour of sharks

These mysterious fish are one of the least understood creatures of the ocean.

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Sharks have been swimming in the seas for more than 450 million years. That's even longer than trees have existed - but how much do we know about them?

There are more than 500 shark species and they are all very different. The biggest - whale sharks - are about the size of a bus, while the smallest - dwarf lantern sharks - are no longer than a pencil. The well-known great white shark snacks on seals and sea lions, while hammerheads love dining out on stingrays. Bonnetheads confuse scientists by munching seagrass; and some deep-sea species even have glow-in-the-dark skin.

It's hard to study these creatures because they often live in the open ocean, are constantly on the move and travel huge distances. For many sharks, we don't know how they sleep, how they give birth, what their social life is like or how they survive different diseases. Let's dive in and investigate some of the biggest mysteries that keep ocean scientists awake at night.

The Megalodon