[paul.fr]

Why do crabs move sideways? Do they have one set of 'legs'/claws longer than the other? Is their body off centre? Or what.

[neilep]

This is a great question and I am astonished that crab lovers community have not pounced on this !

I can only speculate that they do so because due to circumstances , evolution has dictated that they evolve so.
That's about as general an answer you can get I reckon without actually really answering the question !!..LOL

[Karen W.]

I found this on ( Crustacea "The British Marine life study society Information page ) at the following link!

http://www.glaucus.org.uk/crustace.htm#Why%20Do%20Crabs%20Walk


Why Do Crabs Walk Sideways?
Because that's the way their legs bend. Muscles work in pairs. A muscle can only retract, or pull; to lengthen again it must relax and be pulled back by another 'antagonistic' muscle. The muscle blocks in crabs are attached to the inner surface of the exoskeleton, including the ten tubular legs, including the legs with claws, or chelae, as well as other appendages. Crabs do not have ball-and-socket joints but the legs pivot at numerous peg-in-socket joints that are sealed by flexible chitin, and can move in one plane (similar to our knee). Each joint moves in a different plane, and so together they allow the crab to move in all directions like our shoulder and hip joints. However, many crabs have joints in their legs moving in a restricted number of planes so that they can only move sideways. The sharp ends of each leg grip on to surfaces and can grip on to tiny irregularities (watch a Hermit Crab climbing up an almost smooth rock). In some crabs, the rear legs are shaped like paddles for limited swimming.
Many crabs like the Shore Crab  need to scamper sideways to avoid the legs getting tangled up with each other.
exoskeleton = external skeleton (see the above article for details of moulting, ecdysis).

Water is a very dense medium, about 830 times as dense as air, and has a viscosity about 60 times that of air. This means that marine life does not require the supporting skeletons of land dwellers; the large spider crabs will collapse out of the water. Water is more difficult to move through, and marine creatures have evolved shapes to minimise the resistance. This can be seen in the flattened bodies of many common crabs.
 

by Andy Horton and Jane Lilley 1997.

At least 67 species of true Brachyuran crab inhabit the seas surrounding the British Isles. They vary in size from the Giant Box Crab, Paramola cuvieri, which is a rare deep water species, to the inquiline Pea Crab, Pinnotheres