Five-limbed brittlestars move like people

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Despite being brainless, research has shown that brittlestars actually move in a very coordinated way, and move bilaterally, like people.

Amazingly, the way in which brittlestars move doesn't appear to have been investigated before Henry Astley, an evolutionary biologist from Brown University, decided to take a further look after noticing for himself the way in which brittlestars coordinated their arms.

 "It was too confusing," said the fourth-year graduate student in the Department of Ecology and Evolutionary Biology. "There's no obvious front. There are five arms that are all moving, and I'm trying to keep track of all five while the (central body) disc was moving."

Many animals, humans included, are bilaterally symmetrical, meaning they can be divided into matching halves. 

Brittlestars are pentaradially symmetrical, meaning there are five different ways to divide them into matching halves. Bilaterally symmetrical organisms designate a "head" to chart direction, which then commands other body parts to follow, whereas animals with a radial symmetry have no such central directional control.

"What brittlestars have done is throw a wrench into the works," Astley said. "Even though their bodies are radially symmetrical, they can define a front and basically behave as if they're bilaterally symmetrical and reap the advantages of bilateral symmetry."

"For an animal that doesn't have a central brain, they're pretty remarkable," said Astley, the sole author of the paper.

To move, brittlestars usually designate one arm as the front, depending on which direction it seeks to go. An arm on either side of the central arm then begins a rowing motion, much like a sea turtle, Astley said. The entire sequence of movement takes about two seconds.

Instead of physically turning, the brittlestar chooses a new centre arm instead, and heads off in a new direction.

"If we as animals need to turn, we need to not only change the direction of movement, but we have to rotate our bodies," Astley explained. "With these guys, it's like, 'Now, that's the front. I don't have to rotate my body disk.'"

The brittlestar also uses another curious method of locomotion about 25% of the time, which would seem like moving backwards to a bilateral.

The brittlestar keeps the same front, but now uses the non-forward-rowing limbs to move it. So, why doesn’t the brittlestar just define a new front and move forward? "There's clearly something that determines that," Astley said. "It could be the relative stimulus strength on the arms."

The research has been published in the Journal of Experimental Biology.

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