Abstract
Typical brittle stars have five radially symmetrical arms, which coordinate to move their body in a certain direction. However, some species of them show individual difference in the number of arms: commonly five or six, rarely four or seven. We find this trait unique since intact legged animals each own a fixed number of limbs in general. How does a single species manage such different numbers of motile organs to realize adaptive locomotion? Our study aims to describe four-to seven-armed locomotion to explore a common rule across different arm numbers in brittle stars. Gathering several quantitative indices obtained from Ophiactis brachyaspis, we figured out an average locomotion where a front position emerges at one of the second neighboring arms to a mechanically stimulated arm, while side arms adjacent to the front synchronously work as left and right rowers, regardless of the total number of arms. The idea would provide a general scheme of how ‘left and right’ emerges in a radially symmetrical body.