Brittle stars and people have something in common: They move in fundamentally similar ways. Though not bilaterally symmetrical like humans and many other animals, brittle stars have come up with a mechanism to choose any of its five limbs to direct its movement on the seabed. It’s as if each arm can be the creature's front, capable of locomotion and charting direction. Results appear in the Journal of Experimental Biology.
It appears that the brittle star, the humble, five-limbed dragnet of the seabed, moves very similarly to us.
In a series of first-time experiments, 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. 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.
The findings are published in the Journal of Experimental Biology.
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 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.
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 watched brittle stars move about, he couldn’t figure out how the individual arms were coordinating. “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.”