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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