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

TL;DR

This study reveals how starfish larvae's ciliary vortex patterns facilitate a tradeoff between feeding and swimming, highlighting the hydrodynamic mechanisms underlying larval behavior and morphology.

Contribution

It uncovers the hydrodynamic basis of feeding-swimming tradeoffs in starfish larvae through experimental observation and modeling of ciliary vortices and tangles.

Findings

Vortex arrays surround swimming starfish larvae.

Ciliary tangles act as topological defects affecting flow.

Distinct flow patterns correspond to feeding and swimming modes.

Abstract

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.