Creased ciliary flocks shape unfolding dynamics via information bottlenecks in an aneural animal

Multicellular organisms utilize thin sheet folding to achieve functional three-dimensional forms. During embryonic development, stereotypical epithelial folds emerge from active cellular and molecular processes including cell shape change and differential cell growth. Active thin sheet folding promises to be a powerful design technique in the fields of active solids, soft robotics, and synthetic biology. However, the general principles of active thin sheet folding remain poorly understood. Here we discover a non-canonical cilia-driven thin sheet folding behavior exhibited by basal animal Trichoplax adhaerens. Through volumetric imaging, we found that, despite having no nervous system, T. adhaerens has the remarkable ability to resolve complex body folding states in a non-stereotypical fashion using a carpet of collectively flocking cilia. Cilia-resolved imaging revealed that folds create crease defects in the animal's ciliary carpet, which act as information bottlenecks to break collective behavior of cilia. In turn, these bottlenecks enable the disjointed locomotion required for fold removal. These findings point to a two-way coupling mechanism, wherein ciliary activity shapes the animal's folding state and vice versa. Our work demonstrates the broad configuration space of non-stereotypical active folding and highlights the power of distributed activity to drive folding and unfolding of a thin multicellular sheet. We anticipate our study to be a starting point for the establishment of a new class of distributed active origami wherein fold lines themselves are dynamic and motile, with implications in engineering of self-folding materials. Additionally, our work reveals a new facet of the Placozoan behavioral repertoire, which extends our understanding of mechanical intelligence in the absence of a nervous system.