Crab: The sideways walker that must change its armour

The crab is one of the commonest little animals on the shore: it wears hard "armour", walks sideways and brandishes a pair of small pincers. It looks clumsy, yet it is full of clever design. Let's meet it.

Its body

A crab's body is wrapped in a hard exoskeleton, like a suit of armour, protecting it from bites. But there is a problem: armour does not grow with the body. So a crab must moult regularly — shedding the old shell, revealing the soft new body inside, and slowly growing a bigger shell. Right after moulting a crab is very vulnerable, so it hides until the new shell hardens.

It has a pair of powerful claws for grabbing food, defence and contests over territory. It has eight legs (two more have evolved into claws), and the leg joints swing best from side to side — which is why most crabs walk sideways.

Where it lives and what it eats

Crabs eat almost anything: algae, shellfish, small fish, debris. That is why they are called the clean-up crew of the shore, helping clear leftovers from the seabed and water's edge. They live in the sea and on the coast, and some live in fresh water or on land.

One fun fact

The largest crab in the world is the Japanese spider crab, whose claw span can exceed three metres. Imagine a crab wider than an adult's outstretched arms — hard to believe, isn't it?

A note for grown-ups

You could copy a crab's sideways walk with a child, then talk about why it changes its shell. Helping a child see that hard protection can also bring limits — and that crabs solve this by moulting — makes for a lovely little reflection.