The leg flexor muscle of Carcinus. II. Distribution of muscle fiber types

Three types of muscle fiber were recognized in the leg flexor mus- cle of Carcinus maenas on the basis of histochemical staining €or the oxida- tive enzyme NADHD and analysis of fiber cross-sectional area. The distribution of these fiber types within the muscle is described. The oxidative capacity and cross-sectional area of the fiber was correlated with the fiber type determined physiologically. Key words crab leg muscle, NADHD histochemistry, fiber types, oxidative capacity Investigation of the contraction time of crus- tacean muscle fibers has revealed a number of muscle fiber types. Two extremes can be eas- ily recognized: "slow" and "fast." Intermedi- ate types can also be identified (Atwood, '76), although they are better considered as pre- senting a continuum from one extreme to the other. A variety of histological methods have also been used to identify muscle fiber types. These include histochemical staining for adenosine triphosphatase (ATPase) and oxidative en- zymes, and measurements of sarcomere length. Traditionally, sarcomere length has been used as an indicator of fiber type (Atwood, '72, '76). Recently sarcomere length and muscle fiber type have come to be regarded as syn- onomous, with short sarcomere fibers being equated to fast fibers and long sarcomere fi- bers being equated to slow fibers (Lang et al., '80; Ogonowski et al., '80). Such extrapolation may not be valid in all cases. More recently, Lang et al. ('80) have used ATPase activity as well as sarcomere length to classify fibers. Some confusion seems to have arisen with attempts to correlate these morphological and histochemical measures with physiological results. This is well shown by the perplexing conclusion that "the oxida- tive capacity of the muscle fibers is not di- rectly correlated with muscle fiber type (based on adenosine triphosphatase activity and sar- comere length)" (Lang et al., '80). Biologically, it would appear to be more meaningful to re- late metabolic status to intrinsic physiological function. Close examination of the photomicro- graphs of Lang et al.'s ('80) histochemical sec- tions (the reproduction being admittedly poor) suggests that they could readily support such a correlation between physiological fiber type, oxidative capacity, and ATPase activity. That