http://news.nationalgeographic.com/news ... leton.html
To sum up the basics of the study, they found that a crab sheds its exoskeleton by swallowing air to inflate its body (thus cracking the old exo). But during the few days that it is soft, it seems to use that high internal air pressure to also create a temporary rigid "air skeleton" to help protect itself until the new exo hardens.
The study was done on blackback (land) crabs and blue (marine) crabs, but it's interesting to think that it may be common to many more species of crabs too."You can imagine a similar thing when you inflate a balloon: As you add more air it becomes a little more rigid," she explained.
"In the case of this crab the inflation of the gut is increasing the pressure of body fluids throughout the whole animal, making it more rigid. That provides something for muscles to contract against."