DEVELOPMENT OF THE DIMORPHIC CLAW CLOSER MUSCLES OF THE LOBSTERHOMARUS AMERICANUS.IV. CHANGES IN FUNCTIONAL MORPHOLOGY DURING GROWTH

Abstract

1. The changes in functional morphology of the claws were examined during growth of the lobster by comparing claw structure (mechanical advantage) with muscle fiber properties in each of the closer muscles.2. In the first two juvenile stages (stages 4-5) the claws and the closer muscles are essentially identical to one another. During late stage 5 or in stage 6, claw asymmetry becomes apparent in terms of the complement of muscle fiber types found in the closer muscles. During stage 6, claw morphology becomes asymmetric.3. The fibers of the closer muscles in both claws at all stages were organized into distinct groups. In adult claws each group contained an homogeneous population of fast or slow muscle fibers.4. In the adult cutter claw closer muscle, dorsal groups contained fast muscle while ventral groups had only slow muscle. On a dry weight basis, the cutter claw closer muscle consisted of 81 % fast muscle and 19% slow muscle. The closer muscle of the crusher claw contains all slow muscle.5. In postlarval animals (stages 4-6), the dorsal groups of the closer muscles in both claws contained a mixed population of fibers. Commencing at late stage 5, these groups in the presumptive cutter claw began to lose intermediate and slow fibers. At stage 6 the homologous groups in the presumptive crusher claw began to lose their fast fibers. The ventral groups in both claws never contained fast fibers.6. At stages 4-5 the mechanical advantage (M.A.) of the paired claws was the same. During maturation the M.A. of the crusher claw increased until in the adult it averaged 2.8 times that of the cutter claw, illustrating the correlation of claw structure to fiber type in its closer muscle.7. Thus, discrete changes occur in the claws during growth of the lobster so that each possesses many specialized features, in regard to muscle fiber properties and claw mechanics, to provide the particular function of speed for the cutter claw and of strength for the crusher claw.