Echinoderms lack a centralized nervous control system, yet each extant echinoderm class has evolved unique and effective strategies for locomotion. Brittle stars (Ophiuroidea) stride swiftly over the seafloor by coordinating motions of their five muscular arms. Their arms consist of many repeating segments, requiring them to use a complex control system to coordinate motions among segments and between arms. We conducted in vivo experiments with brittle stars to analyze the functional role of the nerve ring, which connects the nerves in each arm. These experiments were designed to determine how the ophiuroid nervous system performs complex decision making and locomotory actions under decentralized control. Our results show that brittle star arms must be connected by the nerve ring for coordinated locomotion, but information can travel bidirectionally around the nerve ring so that it circumvents the severance. Evidence presented indicates that ophiuroids rely on adjacent nerve ring connections for sustained periodic movements. The number of arms connected via the nerve ring is correlated positively with the likelihood that the animal will show coordinated locomotion, indicating that integrated nerve ring tissue is critical for control. The results of the experiments should provide a basis for the advancement of complex artificial decentralized systems.

Conventional mobile robots have difficulties adapting to unpredictable environments or performing adequately after undergoing physical damages in realtime operation, unlike animals. We address this issue by focusing on brittle stars, an echinoderm related to starfish. Most brittle stars have five flexible arms, and they can coordinate among the arms (i.e., inter-arm coordination) as well as the many bodily degrees of freedom within each arm (i.e., intra-arm coordination). They can move in unpredictable environments while promptly adapting to those, and to their own physical damages (e.g., arm amputation). Our previous work focused on the inter-arm coordination by studying trimmed-arm brittle stars. Herein, we extend our previous work and propose a decentralized control mechanism that enables coupling between the inter-arm and intra-arm coordination. We demonstrate via simulations and real-world experiments with a brittle star-like robot that the behavior of brittle stars when they are intact and undergoing shortening or amputation of arms can be replicated.

A brittle star, an echinoderm with penta-radially symmetric body, can make decisions about its moving direction and move adapting to various circumstances despite lacking a central nervous system and instead possessing a rather simple distributed nervous system. In this study, we aimed to elucidate the essential control mechanism underlying the determination of moving direction in brittle stars. Based on behavioral findings on brittle stars whose nervous systems were lesioned in various ways, we propose a phenomenological mathematical model. We demonstrate via simulations that the proposed model can well reproduce the behavioral findings. Our findings not only provide insights into the mechanism for the determination of moving direction in brittle stars, but also help understand the essential mechanism underlying autonomous behaviors of animals. Moreover, they will pave the way for developing fully autonomous robots that can make decisions by themselves and move adaptively under various circumstances.

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