Five breaks synchrony while six keeps synchrony: individual difference in the coordinated pattern of five- and six-armed brittle stars

AbstractPhysiological experiments and mathematical models have supported that neuronal activity is crucial for coordinating rhythmic movements of animals. On the other hand, robotics studies have suggested the importance of physical properties made by body structure. However, it remains unclear how morphology affects movement coordination in animals independent of neuronal activity. To begin to understand this issue, this study reports a rhythmic movement in the green brittle star. We found this animal moved five radially symmetric parts in a well-ordered unsynchronized pattern. We explained the coordinated pattern without considering neuronal activity, by building a phenomenological model where internal fluid flows between the body parts. Changing the number of body parts from five to six simulated a synchronized pattern, which is also demonstrated by a rare individual with six symmetric parts. This model suggests the different number in morphology changes the symmetry of the fluid flow, leading to the different synchronization patterns.