Skip to story
ELI5 Yahaaa

Legs that bend the wrong way

Why Does a Crab Walk Sideways?

A crab is not being awkward. Its legs simply do not bend in the direction that would let it walk forwards.

Your leg Swings forwards andbackA crab's leg Hinges only side toside
Sideways is not a choice. It is the only smooth direction.

  1. Step 1 of 5

    The joints only hinge one way.

    1 The leadinglegs pull2 The trailinglegs push3 The whole crabslides sideways

    Your knee swings forward and back, so walking forwards is easy. A crab's leg joints hinge left and right instead, so swinging a leg forward means fighting the joint the whole way.

  2. Step 2 of 5

    Sideways is the fast direction.

    Forwards Slow, and every jointfights itSideways Eight legs pullingtogether

    Because eight legs all pull the same way at once, a sideways scuttle is quick and steady. A ghost crab can cross sand faster than most people can run.

  3. Step 3 of 5

    Its skeleton is on the outside.

    You Bones inside, softoutsideA crab Hard outside, softinside

    You have bones inside and soft skin outside. A crab is the other way round: a hard shell outside, with all the soft parts and the muscles tucked in underneath it.

  4. Step 4 of 5

    A hard shell cannot grow.

    1 The old shellsplits at theback2 It backs out,soft and floppy3 Puffs up withwater andhardens larger

    Armour has one problem: it will not stretch. So a crab has to climb out of its whole shell — legs, eyestalks and all — swell up with water while the new one is still soft, and then wait for it to harden.

  5. Step 5 of 5

    Claws are for more than pinching.

    One claw for crushingOne for cutting andpickingWaved about to signalEight walking legsbehind

    The two front legs became tools. Crabs use them to dig, to carry, to crack shells, and to wave at each other — the fiddler crab's oversized claw is mostly for signalling.

    A crab that loses a claw can grow it back over the next few moults, starting small and getting closer to full size each time.

The short version

A crab walks sideways because its leg joints only bend that way — and it wears its skeleton on the outside, so it has to shed it to grow.

Try it yourself

Stand with your feet apart and try to walk sideways quickly, then forwards with your knees locked. The second one is what a crab is avoiding.

Topics

Next tiny story