Little marine mollusks called chitons can’t move very fast, but they do have spiky armored shells made of aragonite. When they spot a predator, they just clamp down tightly, even onto the slippery surfaces of rocks. And according to a new Science study, that armor doesn’t just protect them from predators, it also helps them see them better too: Their shells are covered in hundreds of tiny eyes built with the same mineral that makes up the rest of their armor.

Chitons are the only mollusks with living tissue integrated within their hard, outer shells. And while most eyes are made of organic compounds like proteins, chitons’ eyes are made of the crystalline mineral aragonite. Each eye has an outer cornea, a lens, and a chamber that houses about 100 photoreceptor cells that allow them to perceive changes in light. 

MIT’s Christine Ortiz and colleagues studied the intertidal chiton species Acanthopleura granulata using microscopy, X-ray and crystallographic techniques, and computer modeling. They performed experiments on individual, isolated chiton eyes in the shell to determine if each of their tiny, microscopic lenses are able to form images – and see. In the light micrograph to the right, you can see a region of the chiton's shell surface with multiple small, dark-pigmented eyes composed of aragonite. The crystals in the lens are larger than those in the rest of the shell, and they’re specially aligned to gather light up and bundle it. 

"By studying isolated eyes, we identified how exactly the lens material generates a defined focal point within the chamber which, like a retina, can render images of objects such as predatory fish," study author Ling Li of MIT and Harvard says in a statement. The crystal arrangement helps collect light and focus it so that complete (but blurry) images – like that of fish (pictured below) up to two meters away – can be formed with each eye and processed. This gives them just enough time to clamp to the substrate so that they’re not easily dislodged.

Many biological tissues have more than one function, but few of these are optimized to do both jobs well. There’s a tradeoff for the chitons too: As the size, complexity, and functionality of the individual eyes increase, the mechanical performance of the armor at that spot decreases. However, the researchers write, Acanthopleura granulata has evolved several strategies to compensate for its mechanical vulnerabilities to form a multipurpose system with co-optimized optical and structural functions. This includes positioning the mechanically weak, sensory regions in valleys created by protruding, robust, non-sensory regions. Also, it helps to have so many eyes in case some get damaged. 

^{Images in the text: Wyss Institute at Harvard University}