The influence of oxygen and high-energy phosphate diffusion on metabolic scaling in three species of tail-flipping crustaceans

We examined the influence of intracellular diffusion of O-2 and high-energy phosphate (HEP) molecules on the scaling with body mass of the post-exercise whole-animal rate of O-2 consumption ((V) over dot(O2)) and muscle arginine phosphate (AP) resynthesis rate, as well as muscle citrate synthase (CS) activity, in three groups of tail-flipping crustaceans. Two size classes in each of three taxa (Palaemonetes pugio, Penaeus spp. and Panulirus argus) were examined that together encompassed a 27,000-fold range in mean body mass. In all species, muscle fiber size increased with body mass and ranged in diameter from 70 +/- 1.5 to 210 +/- 8.8 mu m. Thus, intracellular diffusive path lengths for O-2 and HEP molecules were greater in larger animals. The body mass scaling exponent, b, for post-tail flipping (V) over dot(O2) (b=-0.21) was not similar to that for the initial rate of AP resynthesis (b=-0.12), which in turn was different from that of CS activity (b=0.09). We developed a mathematical reaction-diffusion model that allowed an examination of the influence of O-2 and HEP diffusion on the observed rate of aerobic flux in muscle. These analyses revealed that diffusion limitation was minimal under most conditions, suggesting that diffusion might act on the evolution of fiber design but usually does not directly limit aerobic flux. However, both within and between species, fibers were more diffusion limited as they grew larger, particularly when hemolymph P-O2 was low, which might explain some of the divergence in the scaling exponents of muscle aerobic capacity and muscle aerobic flux.