Abstract

Arrays of oscillating cilia often coordinate into a pattern known as a metachronal wave. Many theoretical and computational studies have shown that hydrodynamic coupling through the surrounding fluid is sufficient to produce such waves. Various internal biological mechanisms are also hypothesized to affect the coordination of real biological ciliary arrays. However, relatively few experimental studies have comprehensively characterised the coordination state of ciliary fields in living organisms. Here we present detailed studies of the multiciliated larvae of two species of marine invertebrates. Firstly, we study the larvae of the common reef-building coral A. millepora, quantifying the ciliary propulsion and coordination. We then study the metachronal wave exhibited by the ciliary band of the larvae of a marine annelid, Platynereis dumerilii. The ciliary band has a simple geometry, being one dimensional with periodic boundary conditions. We quantify the steady state wave behaviour and the re-establishment of coordination. To probe the mechanisms involved in the coordination, we then study the effect of physically ‘interrupting’ the wave by two different methods: using laser ablation to create gaps in the ciliary band, or placing physical barriers in the path of the wave to hydrodynamically decouple the cilia on either side of the barrier. In all, our results highlight complex features of ciliary metachronism not captured by simple hydrodynamic models.