A Lens Into Nature’s Gifts

Researchers pushing the limits of optical design are learning from a simple sea creature crusted with tiny lenses, each one smaller and more perfect than any human engineer could devise.

The brittlestar, as the organism is called, sees with its bones.

Built into the starfish’s tough, calcite skeleton are arrays of microscopic crystals that focus light 10 times more precisely than any manufactured micro-optics, Joanna Aizenberg and her colleagues at Lucent Technologies and the Los Angeles County Museum of Natural History recently determined.

“This is a highly original discovery,” said molecular biologist Daniel Morse, who directs the marine biotechnology program at UC Santa Barbara. “It’s significant because it demonstrates that living organisms control nanostructures ... with a precision beyond the reach of present-day engineering.”

Each brittlestar lens is a thousandth of an inch across, composed of a calcite crystal that grows into a flawless eyepiece naturally corrected for any distortions, double images or other optical aberrations, Aizenberg said.

The bead-like lenses cover skeletal plates on the brittlestar’s five supple arms and its disk-shaped body.

Linked by networks of nerve fibers, the thousands of micro-lenses together appear to form a kind of single compound eye that covers the creature’s entire body in all-seeing armor.

“From the point of view of the art of design, the lenses are incredible,” said Aizenberg, an expert in biomaterials.

She led the international team of biologists, physicists and chemists who analyzed the brittlestar eyes. “The actual optical performance of these lenses is far beyond current technology.”

Indeed, for microengineers trying to craft infinitesimal lenses for faster optical computers, sensors and switches, the brittlestar eye is a living blueprint. It could lead to better crafted and more efficient telecommunication systems and optical networks.

“Once again we find that nature foreshadowed our technical developments,” said physicist Roy Sambles at the University of Exeter in England, who reviewed the research published in the journal Nature.

The finding is a telling example of the trial and error of basic research, in which the open-ended curiosity of naturalists can have unexpected dividends.

So simple and subtle is the brittlestar eye that it took researchers 30 years to fully comprehend the evidence it presented.

The work began when a young researcher named Gordon Hendler dived into the warm waters off the coast of Panama during a field expedition in the 1970s.

When Hendler first plucked specimens of the five-legged creatures from a reef, he thought he had discovered a new kind of brittlestar among the thousands of species known to exist. The new sort, scooped up from the deeper pools or at night, had arms striped in bands of pale gray and dark brown. It stood out from the dark brownish-red brittlestars that marine biologists had long found in well-lit shallow waters.

He soon realized, however, that he was looking at a single species, Ophiocoma wendtii , with photosensitive tissues able to change its protective coloring in response to light.

“It took me a while to convince myself I was looking at an animal that was capable of changing colors in response to light, something that had never been reported in brittlestars before,” said Hendler, who today is a curator at the Los Angeles County Museum of Natural History.

Scientists knew that many marine invertebrates, such as sea anemones and mollusks, can sense variations in light with their skin. But the brittlestar seemed able to do more. It apparently could pick out the darker shadows that signaled a hole to hide in amid a reef’s sun-dappled surface. It could detect its own prey.

Perhaps, Hendler and his colleagues thought, in some undiscovered way this star could actually see.

When they studied the brittle-star under an electron microscope, the researchers could see rounded transparent structures covering its arms and body “like little drops of water that collect on the surface of a newly waxed car,” Hendler said.

“You could see right through them into the skeleton,” he said.

Whenever the brittlestar is exposed to light, pigmented cells expand to slide out and cap the lenses like sunglasses. In darkness or shadow, the cells contract, slipping back into holes around the lenses. Hendler and his colleagues determined that networks of nerve fibers ran through the skeletal plates under the lenses and that the cells appeared to respond to different intensities of light. That accounted for the color changes. But it was not clear how well the lenses worked or whether they were part of a vision system.

That’s as far as Hendler could take the research on his own.

There the matter would have rested, as a series of notes published in the academic journals that only zoologists and marine ecologists read, but for a series of letters Hendler wrote in 1988 to a colleague at Caltech.

That led to a conversation in 1990 with a visiting scientist from Israel, which in turn led to a possible dissertation topic for Aizenberg. She decided to study calci-sponges instead.

The nagging question of the brittlestar stayed with her for another eight years until, at her Lucent laboratory, it worked its way to the top of the list of organisms she had been studying.

Aizenberg and her colleagues used optical lithography, a technique used to etch intricate circuits on computer chips, to better understand how the microscopic lenses worked.

They took a brittlestar lens, placed it on photosensitive material and then shined a light through the crystal, just as a child might focus the light of the sun through a magnifying glass.

As they investigated its optical properties, they were astonished by what they found. The crystal collected light and focused it with no distortion on a point that corresponded precisely to nerve bundles under the brittlestar’s skin, Aizenberg said.

Moreover, it did so far better than possible in the most sophisticated optical engineering laboratories.

“It was really a great surprise,” she said. “In a relatively simple organism, nature has suggested to us a very clever design solution to a very complex problem in optics materials science.

“It is an inspiration,” she said. “There is a clever natural solution to almost every question we have in industry these days.”

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Simple organism, sophisticated optics

Brittlestars exhibit a wide range of responses to light. The Ophiocoma wendtii, however, may be unique. With thousands of microscopic lenses better than any engineer could design and neural receptors throughout its structure, the brittlestar may have a photoreceptor system with a compound-eye capability, much like a fly.

Brittlestar top view

One of the most abundant brittlestars on coral reefs of Florida and the Caribbean, it can be found from the Bahamas to Brazil, in depths of 1 foot to 89 feet.

Disk: May grow to 1.4 inches in diameter and has a varying pattern on the topside. The underside has jaws for feeding.

Arms: Five irregularly cylindrical arms branch out from the disk and may reach 6.9 inches. Brittlestars use their arms to snake along the ocean floor. If an arm breaks, the brittlestar begins regrowing the arm where the break occurred.

Dorsal plate: The top plates of the cylindrical arm have hundreds of lenses. Others dot the two side plates.

The dorsal plate

The black dots are uneven pigmented cells that spread across the lenses during daylight and regulate light passing through each lens. During the day, the brittlestar takes on a darker appearance. At right, a brittlestar at night.

Lens cross-section

Light: Passes through the lens in this direction.

Lens: Composed of tiny calcite crystals that allow light to pass through without refraction. They have nearly perfect optical properties.

Calculated lens: Shows profile of a manufactured lens with the same optical properties as the brittlestar crystal. Lower lens: Light is concentrated at this point, the operational part of the lens.

Nerve bundle: These bundles appear to pick up the light signal. Located below the lens, these nerves run through the skeletal structure of each arm.

Lens width, 35 microns

Average lens width 50 microns

Average diameter of human hair follicle, 60 microns

Distance from lens to nerve bundle, 5 microns

Sources: Dr. Gordon Hendler, Natural History Museum of Los Angeles County; Joanna Aizenberg, Bell Laboratories/ Lucent Technologies