Researchers Say Brittle Stars Move Bilaterally, Like Humans

Posted on May 11, 2012

Researchers say brittle stars (Ophiocoma echinata) move bilaterally, like people. Brown University evolutionary biologist Henry Astley discovered that brittle stars, despite having no brain, move in a very coordinated fashion, choosing a central arm to chart direction and then designating other limbs to propel it along. 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.

Many animals, including humans, are bilaterally symmetrical, meaning they can be divided into matching halves by drawing a line down the center. Brittle stars are pentaradially symmetrical. There are five ways to carve them into matching halves.

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

Scientific American says the nervous system of a brittle star is "wrapped around in a ring shape around their body disk." It does not have a discernible front or back.

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 said. The entire sequence of movement takes about two seconds. To turn, the brittle star chooses a new center arm and the accompanying rowing arms to move it along.

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

Here is a video of a brittle star moving uploaded by the SeaDoc Society. Take a look:

The research was published here in the Journal of Experimental Biology.

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