MadSci Network: Zoology |
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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