Tube feet dynamics control sea star locomotion.

Even if for most of us sea stars seem motionless, they actually can move to catch their prey or climb the rocks. Indeed, their oral surface is covered by many small and active tubular projections, known as tube feet or podia, connected to their water vascular system. Extension and retraction of the tube feet make possible the highly organized stepping movement by which sea stars can move. While the morphology and internal architecture of sea star tube feet have been studied extensively, the locomotion mechanism and tube feet dynamics are still not fully understood.
To address this challenge, we developed an optical method based on frustrated total internal reflection (FTIR) to visualize and quantify in real time the number of tube feet adhering to the substrate. By using a wide range of sea star sizes, we showed that crawling speed is not related to sticking contact area, a metric which is linearly proportional to the mass. However, we found that the crawling speed is inversely proportional to tube foot adhesion time, which is itself dependent on the mass, suggesting a mechanical adaptation of the crawling speed to the mass. To confirm these observations, we equipped sea stars with a 3D-printed harness loaded with a weight corresponding to 25% or 50% of their initial mass. Our findings showed that the artificial increase in mass leads to a significant increase in the adhesion time of the tube feet, confirming the existence of an adaptation mechanism of the crawling speed to the mass through the modulation of the tube foot adhesion time. We then perturbed their locomotion by studying how the sea star tube feet dynamics can adapt to an inverted locomotion mode by placing them upside down, as observed in their natural environment.