Correlated physiological and ultrastructural studies on specialized muscles. IIIa. Neuromuscular physiology of the power‐stroke muscle of the swimming leg of Portunus sanguinolentus

The power stroke for swimming in Portunid crabs is provided by the basi‐ischiopodite levator muscle of the fifth leg. This was studied in Hawaiian specimens of Portunus sanguinolentus and found to consist of three distinct portions comprising differently‐colored muscle fibers: white, light pink and deep pink. The innervation, membrane properties and the behavioral role of these fibers in swimming have been examined. Each portion is separately innervated. Fibers of the white portion have propagated action potentials producing rapid twitch contractions. They are used only in fast swimming or in escape responses. Membrane responsiveness of the deep pink fibers ranges from complete electrical inexcitability to moderate graded electrogenesis, whilst their contractions are slow and are probably used to maintain limb posture or in slow swimming. Fibers of the light pink portion range from ones capable of propagated action potentials and rapid twitches to others which produce graded responses only and slower contractions. The light pink portion provides the main thrust for swimming: during slow swimming only two axons are active but as the speed increases further units are recruited so that during escape responses all four axons innervating this portion are active.