Development of the dimorphic claw closer muscles of the lobster Homarus americanus: L Regional distribution of muscle fiber types in adults.

1. The closer muscles of the dimorphic claws (chelipeds) were studied for the presence and location of fast and slow muscle fibers. 2. Cutter claws were composed of about 60-70% short sarcomere (less than 4 mum) fast fibers; the remainder was longer sarcomere (greater than 6 mum) slow and intermediate (4-6 mum) fibers. 3. Crusher claws were composed of a uniform population of long sarcomere (6-13 mum) slow and intermediate (4-6 mum) fibers. 4. There was a regional distribution of fibers in the cutter claw. Ventral fibers were predominantly slow. Dorsal fibers and central medial fibers were fast. Proximal and distal fibers in the medial section were usually mixed. 5. The regional distribution of cutter fibers correlates with previous physiological studies on the distribution of the fast and slow motor axons to these muscle fibers.