The same material that shields the West India fuzzy chiton from its enemies also endows it with the ability to see.
Scientists have discovered that the main difference between the eyes scattered on the tough shell of this mollusk, named Acanthopleura granulata, and the eyes implanted in our own heads is that theirs are made of the mineral aragonite, whereas ours are protein-based. The latter type is far more common in the tree of life, but mollusks in particular have developed very advanced and creative ways to see.
In 2011, researchers discovered that the chiton’s eyes work just like human eyes in that its mineral lenses focus light on the retina, which is covered in photoreceptors. But it has been unclear whether each eye forms an image or if the image is stitched together using the information compiled by all the eyes individually.
Now, materials scientists Ling Li of Harvard University and Christine Ortiz of MIT have demonstrated exactly what a chiton sees through its many rock-hard lenses. Here’s Susan Milius, reporting for Science News:
They attached a chiton lens to the end of a microscope objective in a water bath. Looking directly through the aragonite lens at a fish-shaped silhouette, the researchers detected a somewhat blurred, but recognizable, shape.
The lens achieves its clarity via tweaks in structure, the researchers found. A chiton lens is made of basically the same material as the unseeing armor around it. But in the lens, the component grains of aragonite are bigger. This means incoming light has to pass through fewer grain-to-grain transitions and less of the material between grains. Reducing grain-to-grain jumps means less scattering of light into a dim haze.
There aren’t many photoreceptors in the retina, so each eye probably only picks up a blurry figure. But since the mollusk has eyes all over its shell, an ominous blur is enough warning to protect against predators.
One thing the scientists don’t yet know is how the chiton processes this deluge of information. But they’re starting to see the benefits of such a dual-purpose system; the chiton has turned what would appear to be a weakness (that their eyes lessen the physical infallibility of their shells) into an advantage. Because the eyes are made of essentially the same material as the shell—and because the eyes sink into valleys on the armor’s surface—the mollusk is able to multi-task: it can both see and defend at the same time.
Scientists are eager to invent technologies—for armor, vehicles, buildings—that embed sensory elements inside structural elements in the same way. This finding could put them one more step in that direction.