Metachronal coordination in the larvae of the coral Acropora millepora*

Habitat selection is an important factor in the survival of reef corals. As the only motile stage of the reproductive cycle, coral larvae must choose a suitable location in which to settle and reach maturity. Larvae modify their swimming behaviour in response to cues such as light and chemicals, although the sensing and control mechanisms of this are poorly understood. Here, we present the first detailed biophysical study of motility in the multiciliated larvae of the common reef coral A. millepora. Using high-speed, high-resolution imaging, particle image velocimetry, and electron microscopy, we resolve individual cilium beat dynamics, their coordination patterns, and the resultant flow fields. We deduce that the cilia perform a diaplectic metachronal wave (MCW), whose orientation and periodicity are evident in the flows. While many recent models study the emergence and stability of MCWs based on the properties of individual cilia in idealized geometries, there have been few studies on real experimental systems, and almost no recent experiments on diaplectic MCWs. We compare and contrast our measured cilia and wave parameters with existing theoretical predictions. Our novel system with an unusual form of MCW exposes new challenges for fluid-dynamical and behavioural modelling.