Meeting Abstract
We have confirmed that at least five species of sea stars have a special oscillatory gait that they use to increase their speed. Although this gait is driven by coordinated movement of tens of tube feet, or podia, rather than a few well-defined legs, these gaits have an identifiable step frequency and stride length. The product of step frequency and stride length is speed, and changes in either of these variables could be responsible for speed increases. In human studies, subjects can be asked to aim for walks with constant stride frequency, constant stride length, constant speed or preferred stride length and frequency across a range of speeds. In sea stars we cannot survey such a range of conditions, but we can get a range of behaviors by studying voluntary gaits within individuals, within a species and across species (other possibilities might include different temperatures and motivational states). We filmed many individuals of three species of sea star (Protoreaster nodosus, Asterias forbesi, Luidia clathrata), for whom periodic vertical locomotory oscillations are correlated with increases in speed. We found that most of the variation in speed was due to changes in stride length. Stride frequency varied less and sometimes even declined with increasing speed. These correlates of speed suggest that this locomotory system behaves like a forced damped harmonic oscillator. The frequency may be related to sea star mass and the stride length may be related to the length of the podia, which are hydraulically variable in length. By observing among a size series within a species and by surveying among species, a range of mechanical conditions varying in mass and density and allometric morphology can be surveyed to uncover underlying mechanisms and constraints.
