Correlated physiological and ultrastructural studies on specialized muscles. IIIb. Fine structure of the power‐stroke muscle of the swimming leg of Portunus sanguinolentus

The fine structures of muscle fibers of the white, light pink and deep pink portions of the power‐stroke muscle of the swimming leg of Portunus sanguinolentus are described. Although the parts are very different functionally, there are only slight differences in the sarcomere lengths of component fibers. The sub‐structures of thick filaments, Z lines, double M lines and sarco plasmic reticulum are also similar. The principal differences between them lies in different numbers of mitochondria which are very numerous in the deep pink fibers, occupying up to half the fiber volume, fairly numerous in light pink fibers, but sparse in white fibers. The mitochondria are responsible for the color differences. Neuromuscular junctions in deep pink fibers lie deeply embedded, close to the contractile material. A slow relaxation rate of pink fibers appears to be related to the presence of the mitochondria.