Brittle Star Found Covered With Optically Advanced "Eyes"

By John Roach
for National Geographic News
August 22, 2001

In a clever twist of nature, the sea has eyes in its stars.

Scientists have discovered a species of brittle star whose outer skeleton is covered with crystalline lenses that appear to work collectively as an all-seeing eye.

The visual system of lenses in the species Ophiocoma wendtii is the first of its kind observed in nature and is superior to any manufactured lenses, said Joanna Aizenberg, a researcher at Bell Laboratories in Murray Hill, New Jersey.

"These lenses have exceptional optical performance," said Aizenberg, who is co-author of a report on the discovery published in the August 23 issue of Nature. "They are compensated for physical effects that bother us when we fabricate lenses in the laboratory"—effects known as birefringence and spherical aberration.

Brittle stars are sea creatures that have long, thin arms emanating from a small disk-shaped body. They belong to the phylum of echinoderms, which includes sea urchins, sea cucumbers, and sea stars.

O. wendtii is about the size of an outstretched human hand and lives in coral reefs from Bermuda to Brazil.

Light Sensitive

Gordon Hendler, a marine biologist at the Natural History Museum of Los Angeles County in Los Angeles, California, and co-author of the study, discovered that O. wendtii is sensitive to light and can change color.

"They are dark reddish in color during the daytime and go through a striking change in color at night, turning a blackish brown and gray," he said.

After several years of studying the species, Hendler realized that the change in color is controlled by chromatophores, or cells that contain pigment. The chromatophores are clustered around clear "windows" in the bones on a brittle star's arms.

"I started thinking then that those windows might be involved in the reception of light by brittle stars," he said. "The chromatophores could control the amount of light let in."

Hendler presented his hypothesis to Aizenberg and her colleagues Steve Weiner and Lia Addadi at the Weizmann Institute of Science in Rehovot, Israel, who began studying the phenomenon. One of the group's main areas of investigation is the exceptional properties of biologically formed minerals, in aprticular calcitic skeletal tissues.

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