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  • A rocky “see” for mollusks

Jul 2011

Combing the beach this summer, you may pick up a mollusk shell, and even if you don’t hear the ocean when you hold it to your ear, the shell could have an interesting story to share – a story about light, vision and rock.

Some mollusks actually make their eye lenses from calcium carbonate crystal – specifically, aragonite, scientists say – and these eyes of rock could have enough vision to help the sea creature spot potential predators, a recent study has shown.

The West Indian fuzzy chiton, Acanthopleura granulate, is a mollusk with eyes of rock. The eyes are very small (less than 100 μm in diameter) but can be seen in the photo as small black spots on the shell plates. Courtesy of Anne DuPont.

Researchers at Duke University in Durham, N.C., led by Daniel I. Speiser, studied the mollusk vision of creatures collected in the Florida Keys to determine how well these rock eyes can see. Their results were reported in Current Biology on April 26, 2011.

Chitons have the first aragonite-based lenses ever discovered, according to the report. Most animals make their eyes from cells with proteins and chitin.

Researcher Daniel I. Speiser holds what is commonly called a mossy chiton, an animal easy to find in California. This species of mollusk has no eyes. Courtesy of George Foulsham, Office of Public Affairs, University of California, Santa Barbara.

The researchers studied the vision of 3-in. mollusks – Acanthopleura granulata, also known as the West Indian fuzzy chiton. These creatures have hundreds of eyelike structures with aragonite lenses embedded in all eight of their shell plates, which are made of the same rock substance. The tiny lenses cover clusters of light-sensitive cells.

The researchers tested the chitons to determine whether they use their eyes to see objects overhead or simply to detect changes in light. Chitons open their shells to breathe, but clamp down when they are disturbed.

This optical model of the chiton eye shows the way light would bend and focus in the eye, depending on whether the creature is above or below water. The “n” refers to refractive indices. Rhabdoms are transparent, crystalline receptive structures found in the eye. Courtesy of Daniel I. Speiser and Current Biology.

The scientists showed individual chitons either a black disk ranging from 0.35 to 10 cm in diameter or a corresponding gray slide that blocked the same amount of light – each appeared 20 cm above the chitons. The chitons did not respond when the gray slides appeared, but they clamped down when shown a black disk 3 cm or larger in diameter.

This lined chiton lives about 50 feet below the water’s surface in Whidbey Island, Wash. Courtesy of Kirt L. Onthank.

Because the mollusks responded to the larger disk and not to the gray screen, University of Sussex, UK, biologist Michael Land, an animal vision expert who was not involved in this research, believes it is likely that the creatures are seeing the disk and not just responding to a change in light. This is still to be determined, however.

The experiments also indicated that the chiton lenses probably can focus light differently, depending on whether the creature is above or below water.

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