Retarded and Mosaic Phenotype in Regenerated Claw Closer Muscles of Juvenile Lobsters

The closer muscle in the paired claws of the lobster Homarus americanus become determined into their asymmetric form of a cutter and crusher type claw during the 4th and 5th juvenile stages and differentiate their fiber composition accordingly in subsequent juvenile stages. Our aim was to study the effects of claw loss during this critical juvenile period on muscle regeneration. Hence the fiber composition of the paired closer muscles in newly regenerated claws was examined histochemically following removal of both claws either in the 4th and 5th stages or in the 4th through 7th stages. The newly regenerated muscle was retarded compared to its original counterpart in both cases. In the former case, however, the retardation was temporary as the muscle composition in later stages resembled the original. Recovery in the latter was not apparent in later stages, suggesting that retardation is more permanent. Also in both protocols the newly regenerated closer muscle occasionally displayed a mosaic distribution, with slow fibers interspersed among fast fibers in a central band that is normally homogenously fast. Therefore, loss of the paired claws during a developmentally sensitive period affects the phenotype of the regenerated muscle with the change persisting for shorter or longer periods depending on how often the claws are lost.