Abstract
Typical brittle stars creep on the ground with five arms. However, some species of them show individual difference in the number of arms: commonly five or six, rarely four or seven. We found this trait unique since intact legged animals each own a fixed number of limbs in general. How does a single species manage different numbers of motile organs to realize adaptive locomotion? We aim 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 number of arms. This idea would generalize how ‘left and right’ emerges in a radially symmetrical body.