All Eyes on Brittlestars

Once thought eyeless, some brittlestars may essentially be one big eye.

A brittlestar's lanky arms reach out like five thin snakes from the animal's headless central disk. A new study shows that some species' elegant, eye-catching arms serve as one big light-catching eye.

Echinoderms such as starfishes, sand dollars, and sea urchins build their skeletons by secreting crystals of calcium carbonate. Some brittlestars such as Ophiocoma wendtii have taken crystal construction further.

Brittlestar, photo taken in Palau (Palau)
Photo:
Eugene Weber, California Academy of Sciences.
Brittlestar, Amphiopoda occidentalis. Photo taken at Salt Point State Park (Sonoma County, California, US). Photo: Gerald and Buff Corsi, California Academy of Sciences.
Brittlestar, Ophioplocus esmarki. Photo taken at Salt Point State Park (Sonoma County, California, US). Photo: Gerald and Buff Corsi, California Academy of Sciences.

The outer surface of O. wendtii is studded with beadlike, microscopic lenses that together may function like a compound eye. While the animal probably doesn't see images, each lens collects light from a single direction and focuses it onto underlying nerve bundles, allowing it to detect a predator's shadow from nearly anywhere on its body.

The lenses focus light at least ten times as well as anything ever constructed in a laboratory. And since the structures act as single crystals, the light passes through without distortion.

Taking notes from brittlestar build should one day help scientists synthesize better micro-optic structures.