Sea star locomotion mechanism: Tube foot dynamics and mass-driven adaptation

Although they have no proper brain and are not very mobile, sea stars have the remarkable ability to move around actively, whether hunting or climbing rocks. These movements are enabled by numerous tubular adhesive organs, referred to as tube feet or podia, which cover their oral surface and are connected to a water vascular system. The intricate stepping motion of sea stars is achieved through the extension, flexion, and retraction of these tube feet. Despite extensive studies on the internal morphology and ultrastructure of sea star tube feet, their dynamics and the precise locomotion mechanism remain incompletely understood. To address this challenge, we developed an optical method based on frustrated total internal reflection (TIRF) to dynamically visualize and quantify the adhesion of tube feet to the substrate in real time. Our investigation, conducted of individuals of Asterias rubens spanning a range of sizes, revealed that crawling speed is not directly correlated with the sticking contact area — a metric linearly proportional to mass. Instead, our findings showed an inverse relationship between crawling speed and tube foot adhesion time, a factor influenced by mass. These results suggest a mechanical adaptation of crawling speed to mass through the modulation of tube foot adhesion time. We found that less than 10% of tube feet adhere to the surface during sea star locomotion. To validate these findings, some individuals were equipped with a 3D-printed harness loaded with a weight equivalent to 25% or 50% of their initial mass. The artificial increase in mass resulted in a significant increase in tube foot adhesion time, confirming the existence of an adaptation mechanism wherein crawling speed adjusts to mass through the modulation of tube foot adhesion time. Further insights were gained by perturbing the locomotion of A. rubens and studying how sea star tube feet dynamics adapt to inverted locomotion modes, such as being placed upside down.