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

¹ Boston University Marine Program, Marine Biological Laboratory, Woods Hole. Massachusetts 02543; and Scarborough College, University of Toronto, West Hill, Ontario M1C 1A4, Canada

1. 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 three larval stages.

2. Unlike the adult lobster, where the claws and closer muscles are asymmetrical, the claws and closer muscles of the larval stages are symmetrical.

3. In the first and second larval stages, the closer muscle is composed of over 50% intermediate sarcomere fibers, 30-40% short sarcomere fibers and less than 10% long sarcomere fibers.

4. By the late third stage the long sarcomere fibers have increased to a maximum of 40% with a corresponding decrease in number of intermediate fibers.

5. Thus, 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.

6. The data are consistent with the hypothesis that intermediate fibers are transformed into long sarcomere fibers but not into short sarcomere fibers.

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