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Study details mollusk with eyes built into its shell

The eyes are hard and true, with lenses that focus light on a photoreceptor-covered retina.

By Brooks Hays
A closeup image of the chiton species, Acanthopleura granulata, shows its ceramic eyes embedded in its shell. Photo by MIT/Ling Li
A closeup image of the chiton species, Acanthopleura granulata, shows its ceramic eyes embedded in its shell. Photo by MIT/Ling Li

BOSTON, Nov. 19 (UPI) -- A new study describes a unique mollusk species with eyes installed directly in its shell.

With ceramic lenses made of the same material as its shell, the Acanthopleura granulate has found a way to sidestep the evolutionary trade-off that forces most mollusks to sacrifice sight for protection.

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Acanthopleura granulate is a species of chiton, a class of large armor-plated marine mollusks.

In a new paper in the journal Science, an international team of researchers described the species' unique visual system. Unlike almost every other species, whose eyes are made of proteins, this unusual chiton uses aragonite, a mineral -- the same material its shell is made of.

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"[The eyes] allow the animal to monitor its environment with the protective armor shell," study author Ling Li, MIT graduate and Harvard researcher, told MIT News. "The majority of the shell is opaque, and only the eyes are transparent."

A previous study proved the species' eyes to be true eyes, similar to human eyes, with lenses that focus light on a photoreceptor-covered retina, producing a real image.

To measure the eyes' focusing potential, Li and her colleagues popped a few eyes out of the shell and put them under the microscope. The lenses themselves focus a rather crisp image, but the retinas are so small they can host but so many receptors. Each chiton eye produced a tiny pixelated image.

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Chitons don't need to see all that well, they just need to know if a predator is nearby. If so, they wedge themselves between rocks until the danger has passed.

Researchers hope their analysis might lead to material science applications in the military and heavy industry fields, where soldiers and workers operating under hazardous conditions could benefit from heavily protected eyewear.

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