Generalized locomotion of brittle stars with an arbitrary number of arms

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 found 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? We aim to describe four-, five-, six-, and seven-armed locomotion to explore a common rule across different arm numbers in brittle stars. Using Ophiactis brachyaspis, we performed behavioral experiments where an arm tip was mechanically stimulated to analyze subsequent locomotion. For assessing a directional property of arms, we introduced a quantitative index of the degree of a leftward or rightward bias in movement. In combination with other indices, 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. Supposedly, some afferent signal runs along a circular pathway while locally counting how many arms it passes, so as to position a common anterior pattern. This idea would provide a general scheme of how ‘left and right’ emerges in the behavior of a radially symmetrical body.