Why do crabs walk sideways?

We put James' question to Naked Scientist Ginny Smith...

Ginny - Well, the simple answer is because it's the way their knees bend. So, if you think about our knees, they bend forwards and that allows us to take step forwards whereas crabs, their legs are on the side and their knees bend outwards, so they can only move sideways. The more interesting answer is taking into account, of course, evolution. We've evolved to walk forwards. Most animals walk forwards because you can see where you're going better. But for crabs, there must be a reason why it's okay for them to walk sideways. They spend a lot of their life buried under the sand and they've developed these kind of long, flat bodies that make it very easy for them to sort of squiggle under the sand and hide there. Having their legs on the side kind of fits in with that elongated shape. They also don't really need to walk that fast - they're scavengers. They don't chase prey very much. So actually, they don't need to be great runners and being able to hide has been more useful for them.

Chris - Does the crab nebula move sideways? Carolin - No. well, it moves outwards in every direction very, very fast.