Meeting Abstract
P2.160 Saturday, Jan. 5 Stride Frequency and Body Size in Running Ghost Crabs WHITTEMORE, SB*; MORRIS, K; MEDLER, S; SUNY Fredonia; University at Buffalo; SUNY Fredonia scott.medler@fredonia.edu
Body size has a major impact on the skeletal muscles that power locomotion. Smaller animals operate with higher frequencies of limb and body movements, and the muscles driving these movements have correspondingly faster contractile properties. In mammals, interspecific comparisons show that smaller species possess myosin heavy chain isoforms with faster shortening velocities than their orthologs in larger species. Ghost crabs exhibit similar shifts in stride frequency as they grow from small crabs into larger animals. Does the slowing of contractile kinetics reflect a fundamental shift in muscle organization, similar to that observed among mammalian species? Or, do changes in relative body proportions that accompany increases in scale drive the slowing of stride frequency? We studied ghost crab running performance in animals representing a 50-fold range in body mass, and related this performance to changes in body dimensions and relative mass. As we have found previously, stride frequency systematically declines in larger crabs, with frequency being proportional to mass-0.15. Another consequence of changes in scale is that the relative load of the crabs increases as they grow larger. This stems from the fact that mass increases more rapidly than the cross section area of muscles as crabs grow larger. We tested the hypothesis that increases in relative load cause a slowing of stride frequency by attaching weights to crabs and measuring their stride frequencies. Crabs carrying an extra 15% of their body mass showed no slowing of stride frequency, indicating that the relative load carried by the crabs during running does not limit their performance. Although body dimensions have an important impact running performance, we predict that size-related changes in stride frequency are affected by reorganization of the muscles at the cellular and molecular levels.
