Fast calcium-dependent reorientation of motile cilia basal bodies in the simple metazoan, Trichoplax

Ciliary-based animal locomotion relies on the spatially coordinated beating of motile cilia to displace the body in liquid environments or across solid surfaces. The orientation of ciliary beating largely depends on the rotational polarity of the basal body, which in most animals is fixed and controlled by cues linked to the main body axes. The small marine animal Trichoplax exploits the beating of motile cilia in its lower epithelium to crawl on substrates. However, Trichoplax lacks defined body axes and exhibits rapid changes in body shape and direction of movement, thus raising the question of what controls the orientation and reorientation of ciliary beating. We show here that the basal bodies of the cilia in the lower epithelium of Trichoplax are oriented along the direction of the animal movements, and that they change their orientation throughout the lower epithelium in a few seconds’ time when the animal modifies its shape or changes direction following external mechanical stimuli. We also show that Ca ²⁺ is required for fast basal body reorientation. Such rapid ciliary basal body reorientation has never been observed in metazoans before. Thus, our results identify a previously undescribed mechanism underlying directional motility in a metazoan and shed light on its subcellular determinants, bridging the scale between intra-cellular ciliary machinery and animal movement.