Vortex arrays and ciliary tangles underlie the feeding-swimming trade-off in starfish larvae

Many marine invertebrates have larval stages covered in linear arrays of beating cilia, which propel the animal while simultaneously entraining planktonic prey(1). These bands are strongly conserved across taxa spanning four major superphyla(2,3), and they are responsible for the unusual morphologies of many invertebrate larvae(4,5). However, few studies have investigated their underlying hydrodynamics(6,7). Here, we study the ciliary bands of starfish larvae, and discover a beautiful pattern of slowly evolving vortices that surrounds the swimming animals. Closer inspection of the bands reveals unusual ciliary 'tangles' analogous to topological defects that break up and re-form as the animal adjusts its swimming stroke. Quantitative experiments and modelling demonstrate that these vortices create a physical trade-off between feeding and swimming in heterogeneous environments, which manifests as distinct flow patterns or 'eigenstrokes' representing each behaviour-potentially implicating neuronal control of cilia. This quantitative interplay between larval form and hydrodynamic function may generalize to other invertebrates with ciliary bands, and illustrates the potential effects of active boundary conditions in other biological and synthetic systems.