  Many marine invertebrates have larval stages covered in linear arrays of
beating cilia, which propel the animal while simultaneously entraining
planktonic prey. These bands are strongly conserved across taxa spanning four
major superphyla, and they are responsible for the unusual morphologies of many
invertebrate larvae. However, few studies have investigated their underlying
hydrodynamics. 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 modeling
demonstrate that these vortices create a physical tradeoff between feeding and
swimming in heterogenous environments, which manifests as distinct flow
patterns or "eigen-strokes" representing each behavior---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.

Fluid Dynamics;Biological Physics;Quantitative Methods