NORTH CAROLINA, USA (29 July 2003) -- When you're a blue crab and you outgrow your hard shell, what do you do for support after you shed it to grow a new one? Rather than wilt under the situation, crabs actually crank up their internal pressure to keep moving, scientists reveal in a new study published in the journal Science. Researchers at the University of North Carolina report Friday that they've found what may be a previously unrecognized and widespread alternative support mechanism used not only by crabs, but also lobsters, insects and other creatures that normally have external skeletons. Doctoral student Jennifer R.A. Taylor and biology professor William Kier found that crabs, after molting, switch to a type of hydrostatic support that allows them to maneuver - and even defend themselves - during the week or so it takes for soft, newly grown shells to harden up. Kier and Taylor think their research is the first to demonstrate how animals switch back and forth between two skeleton forms. "This is an exciting concept for us because it's not something we thought animals could do," Kier said. "Crabs certainly are more vulnerable without the tough body armor they grow to protect themselves, but they are not at all helpless, either." When blue crabs outgrow their shells, generally each spring, they swallow water to puff themselves up and crack apart their shell. A membrane, called the cuticle, still surrounds its body and begins to harden into a new shell. Taylor, working under Kier's supervision, found that water pressure inside this membrane was temporarily much higher than it was before the molting. This allows for the movement of legs and claws almost as efficiently as when the crabs have a hard shell. In one test, Taylor made a small hole in a crab's cuticle, which quickly sealed and resulted in little change in pressure or fluid loss. But after removing a large part of a claw, the rest of the claw quickly collapsed like a tire going flat. Then, the researchers attached force and pressure sensors to the crabs to show that when newly molted crabs tried to pull their claws in toward their bodies, their internal hydrostatic pressure shot up. More experiments seven days after molting showed that inside the hardened shells, pressure no longer increased when the animals pulled in their claws. | | 
"These data imply that hydrostatic support is no longer used once crabs have returned to the hard-shell condition," Taylor said. This quick-change behavior of skeletal support makes it "clear that the shedding of the exoskeleton does not incapacitate a crustacean." The research may have practical implications for the blue-crab industry. Producers cull crabs that are about to molt and sell them as softshell crabs, which bring much higher prices. But softshells, like lobsters, must be shipped live and cooked whole, which is often difficult when the crabs are so fragile. The study also reveals new information about a biological function that's vital not only to the majority of the animal kingdom that has external skeletons, but also in understanding creatures like worms and snails that rely exclusively on hydrostatic support throughout their lives. Kier said he has no evidence yet that insects also switch back and forth between skeletal types, "but we'll be looking. There is a good chance that we will find it." Humans and other mammals mainly employ an internal, bony skeleton, but still rely on hydrostatic pressure to extend tongues, allow penises to become erect, and, in elephants, to help move their trunks. SOURCE - Scripps Howard News Servic |
