3214 INTRODUCTION Predator–prey interactions result in strong selection for efficient escape responses in prey, and in some crustaceans a tail-flip response provides this vital function. Tail-flipping consists of fast backward propulsion resulting from the contraction of the abdominal musculature in response to a predator attack or other external stimuli. The anaerobic fibers that power tail-flipping in some species of crustaceans are often among the largest cells in the animal kingdom. Mammalian muscle cells typically range from approximately 10 to much less than 100 μm in diameter (Russell et al., 2000). This fundamental design constraint yields a high fiber-surface-area to volume ratio (SA:V) and short intracellular diffusion distances. Exceeding this maximum size might therefore compromise aerobic metabolism, which relies on oxygen flux from the blood to the mitochondria and ATP-equivalent diffusion within the cytoplasm (Mainwood and Rakusan, 1982; Meyer, 1988; Hubley et al., 1997; Boyle et al., 2003; Johnson et al., 2004; Kinsey et al., 2005; Hardy et al., 2006). However, some crustacean anaerobic fibers can be hundreds of microns in diameter (Hoyle et al., 1973; Kinsey and Ellington, 1996; Boyle et al., 2003). Furthermore, muscle growth in crustaceans largely occurs hypertrophically – that is, muscle mass increases owing to an increase in fiber diameter and length, whereas fiber number remains nearly constant (Bittner and Traut, 1978; Boyle et al., 2003). As some species of crustaceans undergo an extreme increase in body mass during development, there is a parallel increase in muscle fiber size. Thus, crustacean muscle fibers from juveniles fall within the fiber size range typical of many animals, but, as the animals grow, the fibers can greatly exceed the usual threshold on cellular dimensions, while still preserving function (Boyle et al., 2003; Johnson et al., 2004). During tail-flipping, energy requirements exceed aerobic capacity and contraction is anaerobic and therefore is not constrained by diffusion of O 2 to mitochondria or ATP from mitochondria (England and Baldwin, 1983). Following bursts of tail-flipping, recovery must occur before the animal can generate another round of high-force contractions. Metabolism during an anaerobic burst–escape response in crustacean muscle follows the same pattern as in vertebrates. Contraction is initially powered by the hydrolysis of the phosphagen arginine phosphate (AP), which is the crustacean analog of phosphocreatine (PCr) in vertebrates. Once AP pools are nearly depleted, ATP for additional contractions is supplied by anaerobic glycogenolysis, which is reflected by the accumulation of lactate and depletion of glycogen (England and Baldwin, 1983; Booth and McMahon, 1985; Head and Baldwin, 1986; Milligan et al., 1989; Morris and Adamczewska, 2002). These glycogenolytically powered contractions are slower and less forceful than those powered by phosphagen hydrolysis (England and Baldwin, 1983; Head and Baldwin, 1986; Baldwin et al., 1999; Boyle et al., 2003). In contrast to burst contraction, metabolic recovery following contraction in crustaceans does not always follow the vertebrate paradigm. Vertebrates rely exclusively on aerobic metabolism to power resynthesis of creatine phosphate, and lactate does not accumulate following contraction (Kushmerick, 1983; Meyer, 1988; Curtin et al., 1997), whereas it is widely known that crustaceans The Journal of Experimental Biology 211, 3214-3225 Published by The Company of Biologists 2008 doi:10.1242/jeb.020677 The influence of oxygen and high-energy phosphate diffusion on metabolic scaling in three species of tail-flipping crustaceans Ana Gabriela Jimenez 1, *, Bruce R. Locke 2 and Stephen T. Kinsey 1 1 Department of Biology and Marine Biology, University of North Carolina Wilmington, 601 South College Road, Wilmington, NC 28403-5915, USA and 2 Department of Chemical and Biomedical Engineering, Florida State University, FAMU-FSU College of Engineering, Tallahassee, FL 32310-6046, USA *Author for correspondence (e-mail: agj6818@uncw.edu) Accepted 21 July 2008 SUMMARY 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 O 2) 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 μm. 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 O 2 (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 O 2 was low, which might explain some of the divergence in the scaling exponents of muscle aerobic capacity and muscle aerobic flux. Key words: oxygen consumption, arginine phosphate, citrate synthase activity, aerobic metabolism, anaerobic metabolism, metabolic scaling, diffusion. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY