These observations indicate that the conditions that trigger differentiation must operate in a random manner when the paired claws are intact but in a nonrandom manner when one of the claws is lost. One possible explanation is that differential use of the claws determines their asymmetry. Perhaps the claw that is used more becomes the crusher. This would explain why, when one of the claws is missing during the fourth or fifth stage, the intact claw always becomes a crusher. With two intact claws, initial use of one claw might prompt the animal to use it more than the other throughout the juvenile fourth and fifth stages, causing it to become a crusher.
To test this hypothesis, researchers raised lobsters in the juvenile fourth and fifth stages of development in a laboratory environment in which the lobsters could manipulate oyster chips. Under these conditions, the lobsters developed asymmetric claws, half with crusher claws on the left, and half with crusher claws on the right. In contrast, when juvenile lobsters were reared in a smooth tank without the oyster chips, the majority developed two cutter claws. This unusual configuration of symmetrical cutter claws did not change when the lobsters were subsequently placed in a manipulatable environment or when they lost and regenerated one or both claws.
21. The passage is primarily concerned with
drawing an analogy between asymmetry in lobsters and handedness in humans
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