Award for Outstanding Doctoral Thesis Research in Biological Physics (2020): Vortex arrays and chaotic mixing by swimming invertebrate larvae*

Many marine invertebrates have larval stages covered in linear arrays of beating cilia, which propel the animal while simultaneously entraining planktonic prey. These ciliary bands are strongly conserved, and they are responsible for the unusual morphologies of many invertebrates. However, few studies have investigated their underlying hydrodynamics. Using starfish larvae as a model system, we study the fluid dynamics of ciliary bands, and discover that they create a beautiful pattern of slowly-evolving vortices around swimming invertebrate larvae. Closer inspection of the bands reveals unusual ciliary "tangles" analogous to topological defects that break-up and re-form as the animals adjust their swimming strokes. 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 representing each behavior. We find that this low-dimensional behavior effectively functions as a stirring protocol, creating intricate fluid dynamical patterns reminiscent of mixing patterns found in chaotic dynamical systems. We use three-dimensional Stokesian dynamics simulations to place these findings in the context of the broader morphospace of invertebrate ciliary bands. We find a quantitative interplay between larval form and hydrodynamic function that generalizes to other invertebrates, providing an example of how physical forces shaped anatomical adaptation by early animals.