Percolation theory of intercellular communication through hydrodynamic trigger waves*

Here, akin to a chain reaction, we present the discovery of hydrodynamic trigger waves in cellular communities of the protist Spirostomum ambiguum, propagating hundreds of times faster than their swimming speed. These cells can contract their body by 50% within milliseconds, equivalent to 14g-forces. A single contraction (transmitter) generates long-ranged vortex flows that trigger neighbouring cells, in turn. We further present a microfluidic device to measure the sensitivity to hydrodynamic signals (receiver). Using percolation theory, a phase transition is revealed that requires a critical cell density to sustain communication. Our results suggest that this signalling could help organise cohabiting communities over large distances, comparable to quorum sensing. Moreover, as contractions release toxins, synchronised discharges could also facilitate the repulsion of large predators, or conversely immobilise large prey. We postulate that beyond protists numerous other freshwater and marine organisms could coordinate with variations of hydrodynamic trigger waves.