Octopuses move with uncoordinated arms

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Octopuses have no rhythm.

Their legs can shimmy in any direction, without any clear pattern and regardless of which way their head is pointed. Other animals don’t move this way, suggesting that the octopus has a motion command center in its nervous system that is unlike any other, researchers report in the May 4 Current Biology. The finding may lead to more nimble robots, the scientists say.

Videos of Octopus vulgaris crawling across the bottom of a tank and between cinder blocks reveal that an arm of an octopus contracts, sticks to a surface and then extends, propelling the animal forward. The videos also show that each arm pushes the octopus in only one direction, so the direction of movement depends only on which arms are recruited for pushing and not on how they push. Such agility may have been an adaptation to the octopus’ ecological niche amid the rocks and crevices of the seafloor.

“Octopuses change crawling direction, without changing their body direction, in the blink of an eye,” says Michael Kuba of the Max Planck Institute for Brain Research in Frankfurt. “Their movements are unpredictable, something that may save their lives.”

Like other animals, octopuses have control systems in the nervous system that execute movements by commanding muscles or body regions to move. The system must direct the correct muscles to move in the correct order. Scientists assumed that the control system computes the commands according to strategies developed only to fit the body. But a new concept suggests that the control system can evolve together with the body so both can work efficiently in their environment.

The concept, called embodied organization, originates from the field of robotics. But the octopus is perhaps the best example of this concept, says study coauthor Guy Levy of the Hebrew University of Jerusalem.

Octopuses have bilateral symmetry in their body, meaning the left side is a mirror image of the right side. Animals with bilateral symmetry always have a preferred direction of locomotion relative to the orientation of the body. “For example, we walk forward and some crabs walk sideways,” Levy says. Octopuses, however, don’t have to have a preferred direction. That’s because octopus arms are also radially symmetrical around the body — the arms are all around it. Animals with radial symmetry, such as starfish, can move in any direction relative to their body orientation.

Because octopus arms are extremely flexible, they can move in many ways. The animal can’t rely on traditional control commands to move each arm. If it did, its command center would need to do complex computations, making the control extremely difficult and perhaps even impossible. Using a control system that instead decides only which arms to use for pushing the body frees the system from having to also decide in which direction the arm will push. Adapting such a simple solution for a complex problem illustrates embodied organization, Levy says. 

The next step is to identify the nervous system circuitry involved in octopus locomotion. Studying how motion and embodied organization work in octopuses and other animals can help engineers design robots that are better able to adapt and respond to their environments.

Kuba says the new study’s findings and other preliminary work have already been applied to build a prototype bot called, unsurprisingly, OCTOPUS.