789. A Sea Creature Puts Its Best Foot Forward

A brittle star considering which foot to put forward

By Sindya N. Bhanoo, The New York Times, May 14, 2012

Brittle stars are sea creatures with five limbs and no brain. Found on the seafloor across the world, they have no obvious front, unlike humans and most other animals. Now, a new study reports that the brainless creatures are nonetheless able to move in a coordinated way, by designating one limb as the “front-facing” limb, and using two others to propel forward.

 “They are pushing forward with the front two limbs, like a turtle,” said Henry Astley, the study’s author and an evolutionary biologist at Brown University. “The back limbs aren’t highly involved.”

Mr. Astley reports his findings in The Journal of Experimental Biology.

Most animals, including humans, are bilaterally symmetrical. In other words, drawing a line down the center results in symmetrical halves. A few animals, brittle stars included, are radially symmetrical: They can be sliced in many different ways and still be symmetrical, giving them no clear “front.”

But while moving, the brittle star is able to designate a front, and act as if it is bilaterally symmetrical, Mr. Astley said. The brittle star is also able to switch its front-facing limb as needed, and this enables it to swiftly change direction, Mr. Astley said.

“If we turn when we’re walking, we have to change our direction,” he said. “They just decide another direction is front, and they’re off.”

Scientists have long thought that bilateral symmetry confers an evolutionary advantage, because it allows for directed movement when searching for food or avoiding predators. But the new study puts a twist on this notion, Mr. Astley said.

“You can get the benefits of bilateral symmetry without being bilaterally symmetrical,” he said. “You can become behaviorally bilaterally symmetrical.”