Low-dimensional behavior and chaotic mixing by swimming starfish larvae*

Physical constraints from environmental forces strongly constrained the evolution of behavior in early animals. We investigate this phenomenon in starfish larvae, a model system for cilia-based locomotion in basal organisms. We find that starfish larvae exhibit a unique behavior in which the animal surrounds its body with a time-varying number of vortices as it adjusts its feeding rate and swimming speed. We show that the animal’s entire repertoire of behaviors is readily decomposed into a finite set of time-varying parameters, which appear within an analytical solution for the motion of fluids at low Reynolds numbers. We observe that this low-dimensional behavior creates fluid dynamical patterns reminiscent of mixing patterns found in chaotic dynamical systems, and that the appearance of these patterns changes in response to local food density and other stimuli. Our work suggests that the simple nervous systems developed by early animals allowed them to adapt surprisingly complex behaviors, even under strongly-constrained conditions such as viscous environments.