Microscopic non-reciprocity drives emergent dynamic phase of living crystals

Active matter can exhibit non-equilibrium phases with emergent dynamics and work cycles. Theory suggests that non-reciprocal interactions in heterogeneous active matter can lead to novel time-dependent phases, but their relevance to living systems has not been demonstrated experimentally. Here we show that mixtures of swimming starfish embryos at different developmental stages spontaneously undergo transition from a dynamic phase to an ordered crystal. During the dynamic phase, the embryos exhibit chiral flocking in which the collective velocity rotates. The dynamics are persistent for hours, with repeated generation and disruption of the collective flocking.We further show that this oscillation in dynamic order is coupled with the oscillation in hexagonal order, hinting toward emergent non-reciprocal interaction between dynamics and structure. Finally, we demonstrate that the transition from dynamic to static phase is driven by the loss of microscopic non-reciprocity.