Science Questions

Why do crabs walk sideways?

Mon, 20th Apr 2015

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James Bentley asked:

Why do crabs walk sideways when most other species walk forwards? Is their an advantage of sorts?


We put James' question to Naked Scientist Ginny Smith...Eriocheir sinensis, Chinese mitten crab.

Ginny - Well, the simple answer is because itís 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. Weíve 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 itís okay for them to walk sideways.

They spend a lot of their life buried under the sand and theyíve 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 donít really need to walk that fast - they're scavengers. They donít chase prey very much. So actually, they donít 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.


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Neat idea, but there's a simpler explanation. Crabs can swim forwards and backwards, so no need to walk in that direction. Minimum joint flexibility gives maximum strength. alancalverd, Thu, 2nd Oct 2014

There is a problem with this theory. It doesn't explain why it doesn't apply to all crabs since some crabs don't walk sideways. Creating an experiment would have to entail something which would tell us what the crab is thinking because if it's for efficiency there's no physical test which could test that it's why the crab does it. I hope that makes some sense?

I looked this up on Wikipedia which said that some crabs walk sideways because of the articulation of the legs which makes a sidelong gait more efficient but that some crabs walk forwards or backwards, including raninids, Libinia emarginata and Mictyris platycheles.

See PmbPhy, Thu, 9th Oct 2014

i would suggest that the ones that dont walk sideways dont bottom feed or scavenge in the same way as the ones that do jamesbentley, Thu, 15th Jan 2015

As alancalverd pointed out, less flexibility means greater strength, so your question might be answered by what they mainly eat - how they are using their appendages the break open or crush things, or if they are just manipulating softer tissue prey. Sideways motion seems less speedy, but if you yourself are encased in heavy armor, it might not matter. cheryl j, Thu, 15th Jan 2015

I have eaten a few crabs, but haven't really observed a lot of live ones. 

There may be some flexibility at the shoulder joint, but all of the other leg joints just have simple flexion/extension in a single direction.  I suppose somewhat like your knee joint.  It is likely they have evolved without the musculature and joints to have rapid forward movement.

Say, with humans, we can move to the side, but much prefer walking, running, jogging in line with our knee joints and muscles.

The question might be what evolutionary niche was filled with strong legs moving to the side.  The OP's suggestion of more sensing the ground is an interesting one. 

Another thought, obviously evolution put some effort into protection (including a thick shell).  When staring down a predator, one can turn around and run, or take evasive action.  If they can't outrun the predator, then evasive action would be much better, and a quick side-step may be the best for evasive actions. CliffordK, Thu, 15th Jan 2015

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