Bilateral asymmetry of the paired claws of the lobster Homarus americanus is determined during the fourth and fifth juvenile stages by differential reflex activity; the side with the greater activity becomes the crusher while the contralateral side becomes the cutter. Juvenile lobsters reared during this critical period with a substratum that could not be grasped or with reduced input from predominantly internal mechanoreceptors (proprioceptors) (achieved by cutting the dactyl and its chordotonal organ or by tenotomizing the claw opener or closer muscles) failed to develop a crusher claw and hence remained bilaterally symmetrical: they developed paired cutter claws. Therefore, the proprioceptive component of the reflex activity is implicated in bringing about the initial lateralization of the claw ganglion into a crusher and a cutter side.

Moreover, lobsters with a single claw reared without a substratum developed a crusher on the intact side only if the intact claw was exercised. In the unexercised condition, differences in reflex activity between the side with a claw and the side without one were insufficient for the development of a crusher claw on the intact side. A minimal amount of reflex activity is necessary for the development of a crusher. Lobsters reared with this minimal amount of activity in both claws developed asymmetrical claws rather than paired crusher claws. This means that initial lateralization of the claw ganglion into a crusher side, on a random basis, inhibited the opposite side from also becoming a crusher. This would explain why we failed to produce lobsters with paired crusher claws and why they were seldom found in the wild.

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