Meeting Abstract
Theories explaining animal locomotion abound. None of these theories, however, have been applied to underwater legged locomotion. To adjust and adapt those theories to underwater legged locomotion, we consider sea stars, which locomote using numerous podia that are operated hydraulically. In at least four species of sea stars a coordinated action of podia is used to develop a relatively rapid gait, which is characterized by a marked oscillatory raising and lowering of the center of mass. We have extensive video observations of two of those species, Protoreaster nodosus and Asterias forbesi. Curiously, any one podium is used only once every three steps. In both species, bounce frequencies decrease and maximum velocities increase exponentially with increasing mass; both trends are similar to those seen in terrestrial walkers but sometimes with different coefficients and exponents. In both species this mode of locomotion is similar to terrestrial walking in that the sea star maintains contact with the ground at all times, but is more similar to terrestrial running in that potential and kinetic energy occur in phase with each other. However, for a given mass, Protoreaster move at a slower maximum speed and lower frequency of bounce than do Asterias. Even the maximum speeds achieved by these sea stars are quite slow, on the order of mm s-1. Applying existing theories of walking to underwater circumstances requires some modifications to account for forces that are important underwater such as drag, acceleration reaction and buoyancy. Even modified, these theories struggle to account for these very slow speeds. Although the speeds are slow, they may be matters of life and death in predator prey races with snails and other sea stars, possibly in cannibalistic interactions.
