Nonreciprocal phases and configuration-induced transition in living mixtures

Nonequilibrium systems often exhibit complex collective behaviors driven by broken action-reaction symmetry. In this work, we investigate active mixtures of starfish embryos at distinct developmental stages, uncovering spontaneous transitions between collective flocking and phase-separated crystalline states. These nonequilibrium steady states arise from the interplay between nonreciprocal forces and dynamic system configurations. Using interaction inference techniques, we identify nonreciprocal phases that combine emergent polar order with configurational stability. While nonreciprocal interactions initiate collective motion through run-and-chase dynamics, stronger nonreciprocity leads to self-induced deformations and fractures, generating highly dynamic states. Interestingly, not all configurations are unstable — since nonreciprocal stress depends on how neighbors are arranged, some configurations can be stable without changes in microscopic interactions. This configuration-dependence enables a spontaneous flocking-crystalline transition, which is observed in both experiment and simulation. These findings advance our understanding of how nonreciprocity drives dynamic transitions in living systems, providing new avenues to engineer time-dependent collective behaviors.