MadSci Network: Zoology
Query:

Re: Why does a crab move only sideways?

Date: Sun Jul 16 20:39:21 2000
Posted By: Allison J. Gong, Graduate student, Biology
Area of science: Zoology
ID: 963336216.Zo
Message:

Hello Zaki,

I did a lot of thinking about your question, then went to my books to see if 
I could find an answer for you.  I did not find anything that directly 
answers your question, but did come across some more general information 
about locomotion in crabs.  I hope this helps you.

The group of crustaceans that includes crabs and shrimps typically has three 
body sections - a head, thorax, and abdomen.  Each of these sections, or 
tagma, has appendages that are associated with different functions.  The 
head appendages are used for feeding, the thoracic appendages are used for 
walking, and the abdominal appendages are used for swimming.  A shrimp's 
body is linear and elongate, with the head followed by the thorax and 
abdomen in a more or less straight line.  Shrimps have large abdomens (if 
you've ever eaten a shrimp, you've eaten the tail, or abdomen) and often 
are better swimmers than walkers.

Crabs, on the other hand, have a much reduced abdomen that is tucked 
underneath the large thorax.  This is probably an adaptation for walking, as 
it shifts the animal's center of gravity forward to a point underneath the 
thoracic (walking) appendages.  Crabs can walk slowly in a forward 
direction, but move much more quickly when walking sideways.  When walking 
sideways, the leading legs pull the body by flexing, while the trailing legs 
push the body by extending.

I did come up with some hypotheses about why sideways walking may be 
beneficial to crabs.  First of all, a crab's eyes are in the front of the 
body and are on flexible stalks, so by walking sideways, a crab may be able 
to see both ahead and behind itself.  Secondly, because crabs' bodies are so 
low and squat, it may be more biomechanically efficient for the legs to move 
side-to-side rather than front-to-back.

Can you think of any other reasons why crabs walk sideways?

Allison J. Gong
Mad Scientist



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