If you dive into a pool and open your eyes, the underwater realm shimmers and blurs. Likewise, most fish plopped onto the deck of a ship take in a similarly warped world view. Animals' eyes are typically optimized for either air or water, but now scientists have discovered that one species of chiton, a type of algae-eating mollusk, may use simple, rocklike lenses to see equally well by land or by sea.
Chitons are also called "coat of mail shells" because they sport eight separate back plates, giving them the appearance of an armored knight. Hundreds of tiny eyes dot the surface of these plates, but scientists know very little about how the animals actually see. When biologist Daniel Speiser began studying chiton vision at Duke University in Durham, North Carolina, even the material that formed the animals' eyes was a mystery.
So Speiser and his colleagues sliced off the eyes' lenses and performed x-ray analysis to determine their chemical composition and structure. They found that the lenses were made of aragonite, a form of calcium carbonate sometimes used in jewelry and the same mineral that makes up chiton shells. Although other animals, such as trilobites, have sported eyes of rock, chitons can lay claim to the first aragonite lenses ever discovered.
Speiser and his team next tested how well these eyes worked. They placed a small black disc into the animals' field of view or dimmed the background from white to a uniform shade of gray that more diffusely blocked the same amount of light as the disc. The chitons clamped their shells tight when shown a 3-centimeter-diameter circle, roughly equivalent to a human seeing an object 20 times larger than the moon. They were unperturbed, however, when the researchers instead dimmed the light. The findings, reported online today in Current Biology, suggest that chitons can distinguish between approaching predators and the general dimming of light that might be caused by a passing cloud.
The scientists were surprised to find that chiton eyes worked equally well in air and in water, since light travels at different speeds through the two fluids. The researchers suspect that chitons capitalize on aragonite's unusual ability to transmit light at different speeds. The mineral bends the incoming rays in two directions and creates a double image. Chiton lenses may curve such that one image falls on the light-sensitive cells of the eye when the animal is in air, and the other image falls on these same cells when the animal is under water, Speiser says. The adaptation makes sense, as chitons live in intertidal zones and spend time above and below the water line. "I think this is a very clever and simple solution to the problem of making eyes that work in these two different settings," he says.
Todd Oakley, a biologist at the University of California, Santa Barbara, is intrigued by the possible discovery of a biological lens with dual focus capabilities. "This conclusion could be put to further tests by actually measuring the different focal lengths of the lenses," he says. Speiser says he hopes the chitons' unusual aragonite eyes may even inspire optical engineers designing bifocal lenses for glasses or cameras.