Fast mechanosensitive and Ca2+-dependent reorientation of motile cilia basal bodies in the placozoan Trichoplax

Locomotion driven by ciliary beating is widespread among Animals and represents one of the most ancient movement strategies.¹ The direction of ciliary beating is largely set by the orientation of the ciliary basal bodies (BBs),^{2,3,4,5,6,7,8,9} itself determined by the ciliary rootlet.⁹ In most Animals, BB orientation is set along global body axes.¹⁰ The placozoan Trichoplax challenges this view. This small flat marine animal lacks a defined body plan^11,12,13,14 yet efficiently crawls on solid substrates and continuously changes shape thanks to the thousands of cilia on its lower epithelium that can swiftly change beating direction.^{15,16,17,18,19,20} This raises a key question: how is ciliary beating orientation coordinated and reoriented across scales in the absence of a fixed body plan? Here, we immunostain rootlets to map BB orientation across the entire Trichoplax lower epithelium. Combining this approach with animal tracking reveals that BBs are globally aligned with the instantaneous direction of animal movement and that large-scale spatial variation of BB orientation underlies body-shape changes. Strikingly, BBs can also reorient collectively in a few seconds in response to external mechanical stimuli, revealing a coupling between mechanical cues and ciliary beating direction that enables rapid negative mechanotactic responses. Treatment with pharmacological inhibitors shows that this phenomenon depends on the activity of voltage-gated calcium channels. Together, our results uncover a rapid and coordinated mechanism of BB reorientation that links subcellular organization to whole-animal behavior, providing a framework to understand how ciliary locomotion emerges in the absence of fixed body axes.