Why do crabs walk sideways

A.Because that's the only way they can. Crab leg joints have very little movement from side to side, but more in an up and down direction, similar to if you place your hand palm down on a table and move your fingers. You can move them much more up and down than you can from side to side.

The reason that a crab's upper limbs have a limited range of movements is that they're restricted by a hard exoskeleton. A crab's exoskeleton is essential for protecting it from hungry predators.

However strange a crabs' walk may look to us, the combination of restricted leg movement with a strong exoskeleton is obviously a successful one for them to have evolved in this way.

Q. How many legs do crabs have

A. All crabs have ten legs, two of which have developed into very large and powerful claws, used for grasping their food and fighting each other. Crabs often loose their claws during these fights.

Q. Do crab shells grow

A. Crabs belong to a group of animals known as the Crustacea. Their soft inner body is protected by the hard outside shell, an exoskeleton, which does not grow. As a crab grows and gets too big for its exoskeleton an inner, softer shell also develops.

When the time is right the crab sheds its exoskeleton, a process known as 'moulting'. During moulting the crab is very vulnerable to attack as the new softer shell takes time to harden and normally tries to stay hidden from predators.

Q. Do crab's lost claws ever grow back

A. During the moult, the crab can grow back a lost claw, but it will be smaller than the one that was broken off.

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by Lisa Cardy