Spin-and-Tumble Particles (STPs): Local Alignment and Collective Mixing in Starfish Embryo Swarms

Active matter systems—from bacteria to animals—exhibit striking collective behaviors that arise from local interaction rules and symmetry breaking at the single-particle level. Yet, experimental realizations of active particles with intrinsic chiral dynamics remain limited. We report the discovery that starfish embryos serve as a model chiral active system, exhibiting a spin-and-tumble dynamics within confinement. Each embryo undergoes persistent rotations in a direction that is stochastically flipping. In dense ensembles, local alignment of rotational direction emerges through information transfer. When confined to circular geometries, the system develops spontaneously rotating clusters and complex mixing dynamics. This system represents the inception of Spin-and-Tumble Particles (STPs), expanding the non-equilibrium phase space of active matter and providing an experimental platform to explore how microscopic stochastic chirality drives large-scale emergent phenomena.