Starfish coordinate hundreds of feet to hop about – and they do it without a brain. A new understanding of how they manage this could inspire underwater exploration robots that work on the same principles.
The marine invertebrates, also known as sea stars, lift their bodies off the ground with their tiny tubular feet to move across underwater surfaces like rocks and sand. “[The feet are] almost like mini-organisms, all sort of attached to the same body – and you’ve got hundreds of them,” says at the University of California, Irvine.
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These feet have muscular membranes that allow them to fill up with water or squeeze it out, much like a hydraulic spring. While they aren’t very coordinated when the animals crawl, they can become highly synchronised to generate a faster, “bouncing gait”, during which sea stars push upwards and forwards in leaping bounds, says McHenry.
Curious about how such coordination could occur in an animal without a central nervous system, McHenry and his colleagues filmed the movement of five adult chocolate chip sea stars (Protoreaster nodosus) in an aquarium. They also observed the animals’ motion while they were attached to experimental weights or foam floaters, which changed their submerged weight.
The researchers found that the heavier the load, the more the feet compensated by synchronising their hydraulic cycles. With each bounce, they generated a stronger “power stroke” that further fuelled the following bounce, says team member at the University of Southern California in Los Angeles.
The mechanism is a bit like a group of people working together to move a stuck car. “If another person just stops lifting, you’ll feel it immediately – even if they don’t say anything – and you’ll try to pick up the load,” says Kanso.
A customised computer model supported the team’s hypothesis about the mechanics of sea star movement. Based on these principles, McHenry also built a rectangular robot with 10 feet. Like the sea stars, the robot used more feet – known in robotics as actuators – in response to higher loads, enabling it to bounce across a table.
“You get this sort of resonance out of the system, because of the bouncing in the beginning, which finally forces the actuators to move in synchrony,” says McHenry. “Once they’re moving together in time, they’re generating force together, and then that really locks in the bounce. So it’s a collective, and it’s allowing local decision-making to occur, but the whole system profits from that.”
Using actuators on a need-by-need basis would have benefits in robotics, says at the University of Wollongong in Australia. It would allow for efficient energy use and less wear-and-tear on actuators and devices, ultimately making them longer-lasting, he says.
Such bio-inspired engineering might facilitate more compact designs and rapid responses for robots in challenging environmental conditions, such as sea exploration, says at Vanderbilt University in Tennessee.
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