February 18, 2002

magine trying to stand on the top of Mount Washington in a wind storm, carrying an awkward burden three times your weight.

According to Bill Chapple, hermit crabs face a similar situation every day, toting their borrowed shells across the ocean floor while fighting strong currents.

Chapple, a professor of physiology and neurobiology with joint appointments in bioengineering and marine sciences, is working to understand how hermit crabs keep their balance and move.

An arthropod like the hermit crab is a good subject, he says, "because many arthropods have nervous systems which, compared with human beings', have relatively few elements." Crabs have around 100,000 neurons, the cells in the brain that control behavior, whereas humans have billions.

Still, Chapple hopes, similarities among animal nervous systems will make his discoveries applicable to humans. "All animals seem to use the same molecular and cellular mechanisms to be able to do their business," he says. "There are a tremendous number of common features."

The dissimilarities can be informative as well. Crabs and humans keep their balance under different conditions, Chapple says, and this is reflected in their nervous systems. The differences shed light on why particular nervous systems are organized as they are.

Chapple applies a wide variety of experimental techniques to the hermit crab, drawing on computer programming, electrical engineering, and several other engineering fields. At one time, he even obtained specimens by scuba diving for them himself.

He begins his investigation of the crab nervous system by probing individual cells with electrodes. By examining the electrical signals sent by the crab's cells, Chapple can determine what triggers a crab's muscle movements, and which pathways the signals traverse.

Using dyes and electron microscopes, he has found two types of nerve receptors, one that feels objects brushing against the crab's skin, and another that feels objects pressing against the skin by detecting stresses in the skin. These receptors send signals to neurons, which send the signals that tell muscles to move.

So when a crab feels its shell shift, its muscles react to maintain its balance.

Chapple has built computer models of the crab nervous system, and has discovered aspects of the system that are difficult to model and understand.

One such aspect is positive feedback. A crab's nerve receptors not only detect outside stimuli, but also send signals to the neurons when its own muscles tense. The muscles should contract again when they receive this signal. They do not, however, suggesting that the crab must be able to distinguish external from internal stimuli.

A similar phenomenon accounts for the squeal of a sound system when its microphone is held too close to its speakers. Chapple is trying to understand how the crab deals with the feedback signals. "There's got to be some way that the animal can sort that out from the external information," he says.

An engineer designing a robot, he says, would avoid positive feedback because it overpowers useful signals.

Crabs are also difficult to model because, like other animals, their nervous systems perform many tasks at once, making them like computers with many processors. "Animals are parallel processors to an unbelievable degree," Chapple says.

Understanding how the hermit crab handles feedback and simultaneous tasks, says Chapple, will contribute to making better robots, as well as giving insight into human nervous systems. Although they are simpler than humans, he says, "hermit crabs are a lot more sophisticated than we think."