Growth of crustacean muscles and muscle fibers

Summary The superficial flexor (SF) muscle of the crayfish (Procambarus clarkii) abdomen increases in volume in direct proportion to increases in total body weight during ontogeny. This increase in SF muscle mass occurs solely (Figs. 1, 2) by an increase in the width and length of SF muscle fibers (i.e., the number of SF muscle fibers remains constant). Unlike vertebrate muscle fibers, these crustacean muscle fibers increase in length by increases in sarcomere length (Fig. 3). This increase in sarcomere length during ontogeny must occur via a continuous lengthening of actin and myosin filaments since the relative lengths of the A and I bands remain essentially unchanged as these fibers lengthen. Similar results are reported for the opener muscle of the cheliped (Figs. 5–7).We suggest that fiber number is specified for many crayfish muscle masses since for a given species of crayfish, certain muscle masses contain a set number of fibers within rather narrow limits, and the number of fibers is often significantly different in homologous muscle masses of the same species or in the same muscle mass of different species. Finally, it would seem that similar processes are operating both during embryonic growth and during regeneration in crayfish and in some other crustaceans, since fiber number is not significantly different in opener muscles from normal and regenerated limbs in crayfish and in the crabGecarcinus lateralis.We would like to thank Mr. Martis Ballinger and Mr. Mark R. Meyer for their aid and histology and photography, Mr. Michael Bouton for his assistance in several of the experiments, and Drs. Alan Templeton and Laurence Fox for their help in the statistical analysis of the data. This research was supported by NSF grant No. GB-30199 and NIH grant No. NS-08609.