Rapid behavioral transitions produce chaotic mixing by a planktonic microswimmer

TL;DR

This study reveals that the rapid 'blinking' behavior in invertebrate larvae, driven by ciliary band reorientation, enhances fluid mixing and feeding efficiency through hydrodynamically optimal chaotic flow patterns.

Contribution

It introduces a theoretical and experimental analysis showing that larval blinking optimizes chaotic mixing, explaining its adaptive advantage in feeding.

Findings

Larval blinking induces strong chaotic mixing of surrounding fluid.

The blinking rhythm is hydrodynamically optimal for feeding.

Chaotic flow dynamics help overcome viscous circulation constraints.

Abstract

Despite their vast morphological diversity, many invertebrates have similar larval forms characterized by ciliary bands, innervated arrays of beating cilia that facilitate swimming and feeding. Hydrodynamics suggests that these bands should tightly constrain the behavioral strategies available to the larvae; however, their apparent ubiquity suggests that these bands also confer substantial adaptive advantages. Here, we use hydrodynamic techniques to investigate "blinking," an unusual behavioral phenomenon observed in many invertebrate larvae in which ciliary bands across the body rapidly change beating direction and produce transient rearrangement of the local flow field. Using a general theoretical model combined with quantitative experiments on starfish larvae, we find that the natural rhythm of larval blinking is hydrodynamically optimal for inducing strong mixing of the local fluid environment due to transient streamline crossing, thereby maximizing the larvae's overall feeding rate. Our results are consistent with previous hypotheses that filter feeding organisms may use chaotic mixing dynamics to overcome circulation constraints in viscous environments, and it suggests physical underpinnings for complex neurally-driven behaviors in early-divergent animals.