Development of bilateral asymmetry in sensory innervation to lobster claws

Sensory innervation to the paired claws of the lobster. Homarus americanus, was examined during their differentiation from a bilaterally symmetric state to an asymmetric state of a slender cutter and a stout crusher claw. This was done by estimating the total number and size distribution of axons in the second nerve root which provides most (approximately 90%) of the innervation to the claws and has few, bilaterally distributed motor axons. The paired claws which are undifferentiated and resemble each other in the 1st larval stage correspondingly have nerve roots that are bilaterally symmetric. In early juvenile (4th and 5th) stages when claw type is determined, as well as in subsequent (6th, 7th, 8th, 16th) juvenile stages when the claws gradually differentiate into cutter and crusher types, the paired homologous roots are also similar. It is only in adults that asymmetry in sensory innervation is seen with more axons in the crusher root than in its cutter counterpart. The difference in number of axons between the dimorphic claws is related to differences in surface area between the claws. Thus, bilateral asymmetry in sensory innervation is acquired by the continual but differential addition of axons to the paired claws.