DEVELOPMENT OF ASYMMETRY IN THE NEUROMUSCULAR SYSTEM OF LOBSTER CLAWS

Abstract

The paired claws of the lobster Homarus americanus which are symmetrical in form and function in the larval and early juvenile stages gradually transform into a slender, fast-acting cutter claw and a stout, slow-acting crusher claw during later juvenile and adult stages. Correspondingly, changes occur in the neuromuscular system of the claws. The paired claw-closer muscles are initially symmetrical in their fiber composition and consist of a band of fast fibers sandwiched on either side by slow fibers. During development, one of the muscles transforms into a cutter with a majority of fast fibers and a small ventral band of slow fibers and the other muscle transforms into a crusher with only slow fibers. The firing pattern of the juvenile fast closer excitor motoneuron consisting of high frequency, long duration bursts, is essentially retained in the adult crusher but changed in the adult cutter to low frequency, short duration bursts. In the paired juvenile closer muscles almost all fibers receive mixed innervation from both fast and slow axons whereas in the adult cutter muscle innervation by the fast axon predominates while in the crusher both are equitably distributed. The development of asymmetry in the closer muscle is regulated by impulse-mediated muscle tension though how the neural asymmetry arises is unknown.