FIBER COMPOSITION OF ANTENNAL MUSCLES IN THE LOBSTER HOMARUS AMERICANUS AND THE CRAYFISH PROCAMBARUS CLARKII

Abstract

The fiber-type composition of the paired antagonistic remotor and promotor muscles of the second antennae in the lobster Homarus americanus and the crayfish Procambarus clarkii was investigated using enzyme histochemical and electron microscopic techniques. Muscle fibers were classified either into a narrow category of fast fibers, or a much broader category of slow fibers, based on a close correlation between their enzymatic and structural properties. Thus, fast fibers have a much higher myofibrillar ATPase activity and a lower oxidative capacity denoted by staining for NADH-diaphorase than slow fibers. Myofibrils of fast fibers also have relatively short (3-5 µm) sarcomeres and 6 thin filaments surrounding a thick filament. Slow fibers have longer sarcomeres (6―14 µm) and 10―13 thin filaments surrounding a thick filament. Within the slow category, differential staining for ATPase and NADH-diaphorase together with a wide range in sarcomere lengths denotes further subdivision. According to these criteria the paired antennal muscles in the lobster are composed of fast and slow fibers in separate bundles. Their homologs in the crayfish have fast and slow fibers in the remotor muscle, but only slow fibers in the promotor muscle. Moreover, the large fast bundle in the lobster remotor muscle, capable of contracting at high frequencies to produce a buzzing sound, has 60% sarcoplasmic reticulum and 30% myofibrils compared to the more typical 15-20% sarcoplasmic reticulum and 70-80% myofibrils in all other muscles.