Check the next lobster you eat. One claw will be slender and lightly built, while the other will be massive with "teeth" along the edges. Now check some other lobsters. Odds are that there will be no consistency as to whether the massive, "crusher" claw is on the left or right.

To two University of Toronto biologists, this posed a mystery.

They knew that juvenile lobsters have identical claws, both of the slender, or "cutter," variety. They knew that as lobsters mature, either the left or the right claw will develop into the crusher. The choice seemed to be random, and they began looking for reasons.

Lab experiments showed that one factor is exercise. Lobsters raised in bare tanks with nothing to grab never developed a crusher. Both claws remained cutters. If there was even one pebble in the tank, however, the animal had something to pick up, and one claw became a crusher.

To show that the exercised claw was the one to develop, young lobsters in bare tanks were stroked repeatedly on one claw -- always the same claw -- with a brush until they grabbed the bristles. In nearly all cases the stroked claw, whether right or left, became the crusher.

Oddly, they found that stroking both claws equally caused neither to become a crusher. Most of the tested animals matured with two cutter claws.

This posed a new mystery. Exercise could not be the only factor. If so, the Toronto biologists reasoned, lobsters in the wild ought to develop two crushers, which is almost unheard of.

The researchers conclude that exercise does not simply promote crusher development; it also suppresses crusher development in the opposite claw. Exercise both claws, and each suppresses the other. For the effect to travel from one claw to the other, it must travel as a nerve signal through the lobster's primitive central nervous system.

Study of that simple nervous system may shed light on such human parallels as the development of left- or right-handedness and the localization of certain mental functions on one side of the brain or the other. Speech, for example, is normally controlled by the left side of the brain.

The scientists, C.K. Govind and Joanne Pearce, have published their findings in Science magazine.