Simple animals stretch our physical understanding of cilia, sticky cells and tissue rheology

In animals, epithelial tissues mainly provide a barrier function, but these tissues are also subjected to extreme strains during daily activities such as locomotion. The mechanics of tissues and their adaptive response in these dynamic force landscapes is an area of active research. We complement this research by carrying out a multi-modal study of the locomotion in a simple yet highly dynamic marine animal, Trichoplax adhaerens, that lacks both muscles and neurons. We report the discovery of abrupt, bulk epithelial tissue fractures and healing induced by the organism’s own motility that cause dramatic shape changes and physiological asexual division. By developing a suite of quantitative experimental and numerical techniques, we demonstrate a force-driven ‘ductile-to-brittle’ transition in these tissues.

We will also demonstrate how monociliated epithelial cells work collectively to give rise to ‘ciliary flocking’. Via direct visualization of large ciliary arrays, we report the discovery of sub-second ciliary reorientations under a rotational torque that is mediated by collective tissue mechanics and the adhesion of cilia to the underlying substrate. We develop a framework to explain how perturbations propagate information in this array as linear speed traveling waves in response to mechanical stimulus. We show how ciliary flocking dynamics and emergent active-elastic instabilities enable collective agility in this non-neuromuscular animal.