AS ADVENTUROUS diners know, soft-shell crabs are incredibly delicate creatures. In fact, they are so flimsy that it is hard to understand how they manage to move at all.
Like vertebrates, crustaceans usually move their limbs using muscles attached to a hard skeleton, albeit one on the outside of the body rather than the inside. But when a crab sheds its exoskeleton to grow a bigger shell, the muscles are left without any rigid surface to pull against. So biologists assumed they were basically immobile and helpless until their new shell hardened.
But this is obviously untrue, says William Kier from the University of North Carolina, Chapel Hill. Soft crabs have been seen crawling rapidly away from predators, and they have to make forceful movements to get out of their shells in the first place. 鈥淢oulting is a vigorous activity. They have to actually crack the shell,鈥� he points out.
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Kier suspected they use the same strategy as organisms that lack hard skeletons, such as sea anemones, and animals that can move soft tissue organs, like an elephant curling its trunk. All use hydrostatic pressure to create a stiff structure against which muscles can pull.
Graduate student Jennifer Taylor tested the idea by checking whether soft-shelled blue crabs (Callinectes sapidus) rely on fluid pressure to maintain their body shape. 鈥淚f you remove a claw, it deflates like a flat tyre,鈥� says Kier. She then found that the fluid pressure inside a claw goes up as the muscles contract, indicating that the crabs are indeed using blood and water as a hydrostatic fluid. Two days later, once the shell has hardened, the pressure does not change during muscle use (Science, vol 301, p 209).
Crabs are the first creatures known to use both techniques to move using the same muscles, though Kier suspects that some moulting insects, such as dragonflies, do the same. But there is a price to pay, says blue crab expert Richard Dillaman at the University of North Carolina at Wilmington. Increasing the pressure makes crabs more prone to injury. 鈥淚f you poke a hole in them just after a moult they squirt blood inches high.鈥�