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.

The researchers found that each of the bones is a single calcite crystal and each window is in the shape of a double lens. To find out whether the structures worked like lenses, Aizenberg designed a lithographic experiment based on technology used to construct semiconductors.

She removed a cluster of the calcite crystals from O. wendtii and placed them on a sheet of light-sensitive material. The crystals were then exposed to light. The intensity of the light was such that an image would register on the light-sensitive material only if the crystals worked like lenses, directing and focusing the light.

"[I] found there was a dot under each lens in the array structure," she said. "This was direct evidence that these structures are optical elements that focus and direct the light inside the skeleton."

Aizenberg determined the focal distance of the brittle star's lenses by altering the distance between the calcite crystals and the photosensitive material. This distance, she found, corresponded with the depth at which nerve bundles were located beneath each lens.

The nerve bundles, which are presumed to be light-sensitive, transmit the optical information to the rest of the nervous system.

Potential Technological Benefits

Although it's yet to be proven, the whole photoreceptive system is thought to function like a compound eye, allowing brittle stars to detect predators and seek out hiding places.

"Thanks to evolution, they have beautifully designed crystal lenses that are an integral part of their calcite skeleton," said Hendler. "Those lenses appear to be acting in concert with chromatophores and photoreceptor tissues."

Hendler and Aizenberg said the study of creatures such as O. wendtii has important implications for science. How the brittle star's visual system works may be useful, for example, in developing technologies for chip design in optical networks and better lithographic techniques.

Moreover, the research suggests that the brittle star's relatives, such as sea urchins and sea stars, may use their calcite skeletons similarly for photoreception or in other ways still undiscovered.

Roy Sambles, a physicist at the University of Exeter in England, said in an accompanying article in Nature that micro-lens arrays such as the one found in O. wendtii is proof that nature is a step ahead of people: "Once again, we find that nature foreshadowed our technical developments."

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