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Invisible "Skeleton" Supports Shell-less Crab, Study Finds

Brian Handwerk
for National Geographic News

April 19, 2006

Surviving without a skeleton can be a dicey challenge.

But crabs and other animals that periodically shed their hard shells, or exoskeletons, face just such a predicament.

New research suggests that at least one crab species adapts to this dangerous period by creating an internal "skeleton" made of gas from its guts.

Blackback crabs live almost entirely on land, returning to the water only to spawn.

Like other crustaceans, insects, and arachnids, the crabs must occasionally get rid of their exoskeletons to grow larger (related photo: molting mayfly).

"They essentially secrete what looks like a whole [shell] under the old one," said Jennifer Taylor, a biology doctoral student at the University of North Carolina in Chapel Hill.

"They pump themselves up and inflate their gut, and that increased pressure will cause the old outer skeleton to crack, so that the crab can back out of it."

Once out of its shell, the crab's covering is soft. That should mean that the animal must lie helpless and vulnerable during the several days it takes a new exoskeleton to harden.

But Taylor has found that the crab's high internal air pressure also produces a temporary rigidity—an air "skeleton" that allows the animal to move while still soft.

Taylor co-authored a new study on the crab, which appears in tomorrow's issue of the journal Nature.

"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."

Liquid Bones?

In 2003 Taylor and co-author William M. Kier, professor of biology at UNC, described related research in the journal Science on blue crabs, which live entirely in water.

Those crabs were found to use a similar skeletal support mechanism based on internal water pressure.

For the land-based blackbacks, Taylor said, "One fact that may be important is that, by filling their gut with air rather than water, it makes the newly molted animal a lot lighter.

"On land, where they have to overcome gravity, that may be more significant."

Taylor notes that some insects swallow air to inflate their bodies when shedding their shells, but it's unknown whether they also use the air for skeletal support.

Study co-author Kier said, "At this point, we don't know how widespread pneumohydrostatic skeletons are.

"But they indeed may have been crucial to the process by which marine animals escaped from the sea millions of years ago and came to live on land."