A comparative study on striated muscle fibers of the first antenna and the claw muscle of the crab Pinnixia sp

Two types of muscle fibers, here called Type-A and Type-B muscle fibers, are found in the peduncle of the first antenna of Pinnixia sp. One type is found in the abductor claw muscle. Type-A muscle fibers are reminiscent of invertebrate fast-acting muscle: straight Z-lines, prominent M-lines, hollow tubule-shaped thick myofilaments, hexagonally arranged thin and thick myofilaments, and a thin/thick myofilament ratio of 3/1. Dyadic contacts occur at mid-sarcomere levels. Type-B antenna and abductor claw muscle fibers are reminiscent of some invertebrate slow-acting muscle fibers. They lack M-lines, dyadic contacts occur at A/I junctions, thin and thick myofilaments are non hexagonally arranged, and thin/thick myofilament ratios are 6–7/1. A distinct feature of abductor claw muscle is the presence of plasmalemmal invaginations associated with Z-lines and cisternae of sarcoplasmic reticulum. This evidence is discussed in terms of a comparison with other invertebrate muscles with known speeds of activity.