Frustration and flocking in chiral active fluids of sea urchin embryos

Broken symmetries are a hallmark of living systems, shaping how active matter organizes and moves. We use sea urchin embryos as a model active fluid to investigate how shape anisotropy, polarity, and chirality influence self-organization and pattern formation. These broken symmetries, together with time-reversal symmetry breaking, lead to density-dependent flocking dynamics. Tracking embryo trajectories reveals the simultaneous emergence of frustration and long-range velocity correlations at high densities. Surprisingly, despite the left-handed chirality of individual embryos, collective motion exhibits both clockwise and counterclockwise vortices. Combining experiment, simulation, and theory, we identify the physical mechanisms underlying these behaviors, revealing how local asymmetries give rise to global order in active biological matter and suggesting design principles for bio-inspired metamaterials.