How would you walk if you had five arms and no brains? If you're a brittle star, the answer turns out to be quite well (for an echinoderm)—although it's a little complicated.
The blunt-spined brittle star (Ophiocoma echinata) looks like a claymation creature from an alien horror movie as it moves its disk-like body along the sea floor with unexpected agility. It was this creepy crawl that captivated graduate student Henry Astley, who couldn't quite figure out how the animal was coordinating its movements. "It was too confusing," he said in a prepared statement. "There's no obvious front. There are five arms that are moving, and I'm trying to keep track of all five while the disk was moving."
Animals with two matching sides (bilateral symmetry, as in humans, as well as most other animals) have a leg or fin up in terms of coordinating movement, compared with organisms that are radially symmetrical, such as the brittle star or jellyfish. Unlike brittle stars, jellyfish solve the problem of lacking a "front" by moving along a central axis (think up and down), rather than picking one direction among their equal sides.
Each of a brittle star's legs can sense light, chemical and tactile stimuli, so some scientists have proposed that each limb might be moving based on its own local senses rather than being controlled centrally and coordinated with the other limbs for a unified crawl (a robot brittle star has already been built based on this model). But that didn't seem like a very practical way to run a whole organism.
So Astley, an evolutionary biology graduate student at Brown University, studied 13 blunt-spined brittle stars as they moved across sand on the bottom of an inflatable pool for several trials each. How do you motivate an echinoderm to move when you want it to? Wide open spaces. "They hate being exposed," Astley said. "So we put them in the middle of this sandy area, and they'd move."
After analyzing the movement of each leg from videotape, Astley found that these animals were moving like bilateral animals, but were randomly switching their "front" whenever they changed direction.
"Even though their bodies are radially symmetrical, they can define a front and basically behave as if they're bilaterally symmetrical," Astley said. This allows them to be flexible in choosing direction while also taking advantage of streamlined two-way symmetry.
A brittle star's nervous system is wrapped around in a ring shape around their body disk and has no discernible front or back. So that means they never really have to turn. Want to go another direction? Just pick a new front arm and go with it. For us to turn, "we need to not only change the direction of movement, but we have to rotate our bodies," he said. "With these guys, it's like, 'now that's the front. I don't have to rotate my body disk.'"
We might find them slow, but for echinoderms—a group that includes sea urchins and sand dollars—brittle stars are pretty quick. Astley found that it took them about two seconds to coordinate movement with all of their arms.
"For an animal that doesn't have a central brain, they're pretty remarkable," he said.