Fig. 1. Arginine kinase (AK) mediates ATP-equivalent flux in crustacean muscle. Diffusive flux of arginine phosphate (AP) occurs over short distances in small white fibers of juvenile crabs (A) but, as the fibers grow, diffusive flux must occur across hundreds of microns (B). Consequently, there is expected to be a reduction in rates of aerobic processes, such as aerobic recovery following exercise, as fiber size increases during development. In large fibers of adult crustaceans anaerobic glycogenolysis occurs following contraction, presumably to speed up phases of the recovery process.