  In tissues as diverse as amphibian skin and the human airway, the cilia that
propel fluid are grouped in sparsely distributed multiciliated cells (MCCs). We
investigate fluid transport in this "mosaic" architecture, with emphasis on the
trade-offs that may have been responsible for its evolutionary selection. Live
imaging of MCCs in embryos of the frog $Xenopus~laevis$ shows that cilia
bundles behave as active vortices that produce a flow field accurately
represented by a local force applied to the fluid. A coarse-grained model that
self-consistently couples bundles to the ambient flow reveals that hydrodynamic
interactions between MCCs limit their rate of work so that when the system size
is large compared to a single MCC, they best shear the tissue at low area
coverage, a result that mirrors findings for other sparse distributions such as
cell receptors and leaf stomata.
