DEVELOPMENT OF THE DIMORPHIC CLAW CLOSER MUSCLES OF THE LOBSTER,HOMARUS AMERICANUS:II. DISTRIBUTION OF MUSCLE FIBER TYPES IN LARVAL FORMS

Abstract

The closer muscles of the paired claws (chelipeds) of lobsters were characterized according to the distribution of short, long and intermediate sarcomere muscle fibers during the 3 larval stages. Unlike the adult lobster, where the claws and closer muscles are asymmetrical, the claws and closer muscles of the larval stages are symmetrical. In the 1st and 2nd larval stages, the closer muscle is composed of > 50% intermediate sarcomere fibers, 30-40% short sarcomere fibers and < 10% long sarcomere fibers. By the late 3rd stage the long sarcomere fibers have increased to a maximum of 40% with a corresponding decrease in number of intermediate fibers. At the end of the last larval stage, the closer muscles are symmetrical, with muscle fibers about equally distributed among short, intermediate and long sarcomere fiber types. The data are consistent with the hypothesis that intermediate fibers are transformed into long sarcomere fibers but not into short sarcomere fibers.