Muscle and Muscle Fiber Type Transformation In Clawed Crustaceans

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Abstract
SYNOPSIS. The first pair of thoracic limbs in many crustaceans is elaborated into claws in which the principal muscle is the closer. Changes in the fiber composition of the closer muscle during claw development, regeneration and reversal are reviewed here and the hypothesis is advanced that such changes are nerve-dependent. In adult lobsters, Homarus amencanus, the paired claws and closer muscles are bilaterally asymmetric, consisting of a minor or cutter claw with predominantly fast fibers and a small ventral band of slow and a major or crusher claw with 100% slow fibers. Yet in the larval and early juvenile stages the paired claws and closer muscles are symmetric consisting of a central band of fast fibers sandwiched by slow. Differentiation into a cutter or crusher muscle during subsequent juvenile development is by appropriate fiber type transformation. Experimental manipulation of the claws or the environment in early juvenile stages when the claws are equipotent revealed that the determination of claw and closer muscle asymmetry is dependent on the convergence of neural input from the paired claws: the point of convergence most likely being the CNS. Bilaterally symmetrical input results in the development of paired cutter claws while bilaterally asymmetric input gives rise to dimorphic, cutter and crusher claws. In the northern crayfish, Orconectes rusticus, where the paired claws are bilaterally similar, the closer muscle transforms its central band of fast fibers to slow, both during primary development and regeneration. Whether these fiber type transformations are nerve-dependent is unknown. In adult snapping shrimps, Alpheus sp., the paired claws and closer muscles are asymmetric: the minor or pincer claw has a central band of fast fibers flanked by slow while the major or snapper claw has 100% slow fibers. Claw reversal occurs with removal of the snapper resulting in the transformation of the existing pincer to a snapper and the regeneration of a new pincer at the old snapper site. Transformation of the closer muscle from pincer to snapper type is by degeneration of the fast fiber band and hypertrophy of the slow fibers. Claw transformation can be either prevented if the pincer nerve is sectioned at the time of snapper removal or promoted if the snapper nerve is sectioned: both results implicating a neural basis for muscle transformation.

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