To most people, the most significant difference between hard- and soft-shelled crabs is how easy they are to eat. But new research indicates that the distinction is in fact much more striking than that. According to a report published today in the journal Science, blue crabs actually alternate between two types of skeletons as they transition from hard shells to soft ones and back again.

Crustaceans such as crabs periodically molt, shedding their rigid exoskeletons before forming a new one about a week later. Until the new shell hardens, the outer layer (known as the cuticle) is too soft and flexible to transmit muscle contraction. But crustaceans can still move or flee directly following a molt, so Jennifer R. A. Taylor and William M. Kier of the University of North Carolina, Chapel Hill, tested blue crabs to determine how the creatures accomplish this motion. The scientists measured the internal pressure inside crabs immediately following a molt and found that it was significantly higher then than it was before the animal shed its skeleton. This hydrostatic pressure, the authors report, can be controlled to spur legs and claws to action. "This is an exciting concept for us because it's not something that we thought animals could do," Kier notes. "Crabs certainly are more vulnerable without the tough body armor they grow to protect themselves, but they are not at all helpless. It turns out that they can run around, swim and exert considerable force."

Although many animals, particularly worms and polyps, rely on hydrostatic skeletons, in which muscle contractions are transmitted through an incompressible fluid, the blue crab is the first animal shown to use two skeletal types. Once a crab's new shell is hardened it no longer uses hydrostatic support and instead returns to contracting muscles attached to its rigid skeleton in order to move. The scientists suggest that other arthropods may undergo similar skeleton changes. "We haven't looked for this switching back and forth of the skeleton in insects yet, but we will," Keir comments. "There is a good chance that we will find it."