How a starfish with no brain "controls" hundreds of legs

Anew study has shown that the secret of their coordination is not in a hidden "central controller", but in local mechanical feedback: each tubular leg adjusts to the load almost autonomously.

At the centre of the work is the common starfish Asterias rubens. Each of its 'arms' has rows of tubular legs (podiums) at the bottom with a suction cup at the end; these are powered by a water-vascular system that moves fluid and helps the legs attach and detach.

To see how the legs 'work' in real time, the scientists used frustrated total internal reflection (FTIR) -based optical imaging - the surface is 'illuminated' at points of contact, and it is possible to measure exactly how much and which legs are clinging to the substrate.

The results were unexpectedly specific: the speed of the starfish is governed primarily by the time of adhesion of individual legs, not by how many legs touch the surface at the same time. The longer the legs "hold on", the slower the animal crawls.

In a series of additional experiments, starfish were loaded with more weight (including small weights/"backpacks") and forced to crawl upside down. And in both cases, the same thing was observed: coupling time increased and speed decreased. The mechanical models confirmed the conclusion: the behaviour is born from local adjustment of the legs to weight and gravity - without the need for a central control program.

The authors note that this principle of "decentralised control" could be useful for soft robotics and devices with multiple points of contact - where stability and adaptation to changing conditions are important.