Crabs switch skeleton types

Working with blue crabs, biologists at the University of North Carolina at Chapel Hill have discovered what may turn out to be a previously unrecognized, fundamental and widespread support mechanism in crabs, lobsters, insects and other arthropods that periodically shed their hard external skeletons.

Doctoral student Jennifer R.A. Taylor and William M. Kier, professor of biology, have found that rather than being flaccid and mostly immobile after molting, crabs switch to what’s called hydrostatic support. That very different “skeleton” allows the creatures to move around efficiently, hide and even defend themselves during the week or so it takes for their soft, newly grown shells to stiffen completely.

A report on the discovery appears as the cover story in the July 11 issue of the journal Science. To their knowledge, the work is the first to show animals switching back and forth between two different skeleton forms. “This is an exciting concept for us because it’s not something that 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. It turns out that they can run around, swim and exert considerable force.”

When they need more room to grow, blue crabs swallow a lot of water to puff themselves up and then crack apart and discard their old shells. Within a day, their outer membrane, which is called the cuticle, begins to toughen and harden.

In experiments Taylor conducted under Kier’s supervision, the researchers found that internal pressures were temporarily far higher after molting than they were before. Those fluid forces can be controlled to move legs and claws almost as efficiently as when the interplay between muscles and the outer shells causes movement.

In a preliminary test to determine if hydrostatic pressure was necessary for newly molted crabs to maintain their shape and support, researchers made a small hole in a crab’s cuticle. Because it sealed the hole almost instantaneously, loss of fluid was minimal, Taylor said. After removing a larger part of an entire claw, however, the rest of the claw collapsed like a tire going flat.

Further experiments involving force and pressure sensors showed that when newly molted crabs tried to pull their claws in toward their bodies, their internal hydrostatic pressures shot up, she said. Still more measurements, taken as control experiments, showed that seven days after molting, internal pressures did not increase 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 wrote. “Most arthropods grow by molting, and thus, many may undergo this change in skeletal support. It is clear that the shedding of the exoskeleton does not incapacitate a crustacean.”

Some animals, such as worms and polyps, rely exclusively on hydrostatic support throughout their lives, Kier said. Others, such as mammals, chiefly employ a bony skeleton like humans do, but also usually require hydrostatic support to enable tongues to extend and penises to become erect. Other examples of animals’ supplementary use of such support are elephant trunks and clams’ soft, burrowing “foot” and siphons.

“This is also exciting to us since crustaceans belong to a really big group, the arthropods, which includes insects and are the most diversified non-microscopic animals on Earth,” Kier said. “We haven’t looked for this switching back and forth of the skeleton in insects yet, but we will. There is a good chance that we will find it.”

The National Science Foundation and the National Aeronautics and Space Administration (NASA) supported the UNC College of Arts and Sciences studies.

By David Williamson
UNC News Services
919-962-8596

Note: Contact Taylor and Kier at 919-962-5017 or billkier@bio.unc.edu