Hydrodynamic self-assembly of living chiral crystals*

Living matter exhibits complex non-equilibrium behavior. Here, we report on the formation of chiral crystals of starfish embryos which undergo autonomous order-disorder transitions. During the gastrula stage, individual swimming embryos frequently enter a bound state at the air-water interface. At sufficiently high embryo densities, the near field interactions between rotating embryos lead to the formation of stable 2D crystals with hexagonal order. By measuring the flow field around surface bound embryos, we show their negative buoyancy gives rise to Stokeselet flows that generate a hydrodynamic attraction and consequently lead to the formation of crystals. Our system and hydrodynamic model offer a framework to further investigate the role of chiral interactions in active matter systems.