Meeting Abstract

25.3  Saturday, Jan. 4 14:00  The bounce in a seastar’s step: classifying gaits in underwater legged locomotion ELLERS, O.; JOHNSON, A.S.*; GUTTENPLAN, K.; MOTOKAWA, T.; Bowdoin College; Bowdoin College; Bowdoin College; Tokyo Institute of Technology ajohnson@bowdoin.edu

We describe a novel mode of underwater legged locomotion in which a seastar, Protoreaster nodosus, uses the coordinated action of podia to develop a bouncing gait when moving relatively quickly. This mode of locomotion can be provoked by poking, especially after a righting reaction; so perhaps this behavior is part of an escape response. Fastest average speeds are about 2 mm s^-1 and the periodicities of the bounces range from 5 to 12 seconds per step with larger seastars having longer periods. This mode of locomotion is similar to terrestrial walking in that the seastar maintains contact with the ground at all times, but it is dissimilar to classic inverted pendular walking in several ways. In the classic inverted pendulum, potential energy gained on each rise of the center of mass is converted to kinetic energy as the center of mass falls, with kinetic and potential energy being of similar magnitudes. But in this seastar, kinetic energy is three orders of magnitude less than potential energy. Potential energy is, however, converted to kinetic energy with each bounce but most of the kinetic energy is lost with each step, making each step more of a thud than a bounce. Greater average horizontal speed is correlated with greater fluctuations in vertical speed, and horizontal speed develops as bounces form during an accelerative phase at the start of movement. Once the bounces have developed, the temporal relationship of vertical and horizontal velocity most resembles that of terrestrial running, despite the lack of an aerial phase. This novel mode of locomotion explores some limits and characteristics of underwater legged locomotion.