Brainless brittle stars move (sort of) like us

BROWN (US) — Even without a brain, the thick-spined brittle star moves in fundamentally the same way we do.

Though not bilaterally symmetrical like people and many other animals, brittle stars have come up with a mechanism to choose one of five limbs to direct movement on the seabed.

It’s as if each arm can be the creature’s front, capable of locomotion and charting direction, reports Brown University evolutionary biologist Henry Ashley, in the Journal of Experimental Biology.

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Yet when the brittle star wants to change direction, it designates a new front, meaning that it chooses a new center arm and two other limbs to move. Brittle stars have come up with a mechanism to choose any of its five limbs to be central control, each capable of determining direction or pitching in to help it move.

Many animals, including humans, are bilaterally symmetrical—they can be divided into matching halves by drawing a line down the center. In contrast, brittle stars are pentaradially symmetrical: There are five different ways to carve them into matching halves.

Whereas bilateral symmetrical organisms have perfected locomotion by designating a “head” that charts direction and then commands other body parts to follow suit, radial symmetrical animals have no such central directional control.

“What brittle stars have done is throw a wrench into the works,” Astley says. “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.”

Astley decided to study brittle stars after noticing that their appendages acted much like a snake’s body, capable of coiling and unfurling from about any angle. Yet when he watched brittle stars move about, he couldn’t figure out how the individual arms were coordinating.

“It was too confusing,” says Astley, a 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.”

He decided to take a closer look. On a trip to Belize in January 2009 led by professor and department chair Mark Bertness, Astley plopped thick-spined brittle stars (Ophiocoma echinata) into an inflatable pool and filmed them. The animals were willing subjects.

They hate being exposed, Astley says, so we put them in the middle of this sandy area and they’d move.

To move, brittle stars 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 says. The entire sequence of movement takes about two seconds. “They’re pretty slow in general.”

To turn, the brittle star chooses a new center arm and the accompanying rowing arms to move it along. “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 explains. “With these guys, it’s like, ‘Now, that’s the front. I don’t have to rotate my body disk.'”

Oddly, the brittle star also chooses another type of locomotion—that to bilaterals would appear to be moving backward—about a quarter of the time, Astley documented. In this motion, the animal keeps the same front, but now designates the non-forward-rowing motion limbs to move it.

The question, then, is why doesn’t the brittle star define a new front and simply move forward?

“There’s clearly something that determines that,” Astley says. “It could be the relative stimulus strength on the arms.”

The research was funded by the Bushnell Foundation.

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