What Animal Has 200 Eyes? This Shellfish Uses Tiny Mirrors to See

This strange marine animal has the the most eyes in the animal kingdom.

A scallop may look like little more than a pair of shells sitting on the seafloor. But look closely at the soft tissue along the edge of its shell, and you’ll discover something extraordinary: dozens, sometimes as many as 200, tiny, glittering eyes.

And whereas most animals rely primarily on lenses to see the world, scallops have a different kind of visual system. Along the edges of their shells, the eyes of these marine mollusks use mirrors similar to astronomical telescopes. 

A telescope hidden inside each eye

In a landmark 2017 study, scientists revealed how these extraordinary structures work, uncovering one of nature’s most unusual solutions to the problem of sight.

Most animal eyes work by bending light. A lens focuses incoming rays onto a retina, where light-sensitive cells convert them into signals that the nervous system can interpret.

Scallops take a different approach.

Although their eyes contain lenses, the real optical work happens at the back of each eye, where a concave mirror reflects incoming light toward two separate layers of light-sensitive cells. It’s essentially the same trick used by reflecting telescopes, which rely on curved mirrors to collect and focus light.

Except scallops evolved their version hundreds of millions of years before humans built the first telescopes.

Scientists have known about these mirrors since the 1960s, when biologist Michael Land first investigated the scallop’s unusual visual system. 

×

Get smarter every day...

Stay ahead with ZME Science and subscribe.

Daily Newsletter
The science you need to know, every weekday.

Weekly Newsletter
A week in science, all in one place. Sends every Sunday.
No spam, ever. Unsubscribe anytime. Review our Privacy Policy.

Thank you! One more thing...

Please check your inbox and confirm your subscription.

But the exact structure of the mirror remained a mystery. How could a living animal construct something reflective enough, smooth enough, and precisely curved enough to form an image?

A team of researchers from Israel’s Weizmann Institute of Science set out to investigate these questions. Their findings published in 2017, revealed an astonishingly sophisticated structure, assembled from millions of tiny components.

Millions of crystals make one remarkable mirror

The secret ingredient is guanine, a molecule most familiar as one of the four chemical bases in DNA. But guanine has another useful property: when it crystallizes, it can reflect light extremely effectively.

Scallops exploit this property to build their mirrors.

Using cryogenic scanning electron microscopy (cryo-SEM), which allows delicate biological structures to be examined at extremely low temperatures, the researchers discovered millions of square guanine crystals, each roughly one micrometer wide. That’s about one-hundredth the width of a human hair.

These crystals fit together like tiles on a bathroom floor. But the mirror isn’t just one layer of tiles. It contains approximately 20 to 30 layers stacked one above another, separated by thin layers of cellular fluid called cytoplasm.

Light reflects at the boundaries between the crystals and the surrounding fluid. With so many precisely arranged layers, these reflections reinforce one another, producing an efficient biological mirror.

The engineering goes even deeper. Guanine crystals don’t naturally grow into these tidy squares. Earlier research showed that the tiles contain smaller crystalline domains joined in a particular arrangement, a phenomenon called crystal twinning. This helps explain how scallops achieve such an unusual shape.

The result is a surprisingly uniform mosaic, with relatively few gaps that could scatter light and blur the image.

The mirror is also especially good at reflecting blue-green light. That’s no coincidence. Seawater absorbs many wavelengths, particularly red light, relatively quickly. Blue-green wavelengths travel farther underwater, making them especially useful for a marine animal.

In other words, scallops have biological mirrors that are remarkably well suited to the light available in their habitat.

One mirror, two different views

But it gets even more interesting; the crystals are only a part of the trick. The mirror’s overall shape is just as important.

They discovered that the scallop’s mirror has an unusual shape. Rather than forming a simple bowl, its central region is relatively flat, while its outer edges curve more sharply. This design affects where light falls inside the eye. Light coming from directly ahead is focused mainly onto the upper retina, while light entering from the sides reaches the lower retina.

It’s a clever arrangement. Instead of using one retina to process everything, the scallop has two light-sensitive layers that may specialize in different visual tasks.

Earlier experiments found that the upper retina responds to dark, moving objects, a useful ability for detecting approaching predators.

And predators are something scallops have good reason to worry about. Starfish, crabs, and other animals can threaten them. Some scallops escape by rapidly clapping their shells together, forcing jets of water outward and propelling themselves away from danger.

Their eyes help detect the danger before it’s too late.

Scallops can even adjust their pupils

As though having hundreds of tiny reflecting telescopes weren’t strange enough, scallops can also adjust how much light enters their eyes.

A 2019 study in Current Biology found that scallop pupils constrict in response to bright light, shrinking to approximately 60% of their fully dilated area. Unlike humans, whose pupils can respond within seconds, scallop pupils take several minutes to adjust.

They also lack the familiar iris found in human eyes. Instead, the researchers identified arrangements of muscle-associated fibres in the cornea that may help control the opening. The exact mechanism still requires further investigation.

The adjustment involves a familiar trade-off. A wide pupil lets in more light, making it easier to see in dim conditions. But it can also produce a blurrier image. A narrow pupil sacrifices some light sensitivity in exchange for sharper vision.

Using optical models, the researchers calculated that fully dilated pupils could make scallop eyes approximately three to five times more sensitive to light than fully constricted pupils, depending on which retina was considered. The downside was reduced spatial resolution.

“It’s just surprising how much we’re finding out about how complex and how functional these scallop eyes are,” Todd Oakley, an expert in evolutionary biology and a professor at the University of California, Santa Barbara, told the Smithsonian Magazine.

What scallop eyes could teach us

The scallop is the animal with the most eyes, and the biological engineering behind those eyes is stunning. So stunning, in fact, that engineers are looking to learn from it.

RelatedPosts

Mind-Blowing $1.4 billion Thirty Meter Telescope begins construction in Hawaii
Previously unseen huge structure located within our galaxy
Liquid telescope could help us see the very first stars
A Long Skinny Rectangular Telescope Could Succeed Where the James Webb Fails and Uncover Habitable Worlds Nearby

Engineers already use multilayered reflectors and segmented mirrors in sophisticated instruments. Understanding how scallops control crystal growth and arrangement could offer new ideas for manufacturing reflective materials and miniature optical systems.

But this is still being explored, it’s not a proven technology. The immediate scientific achievement is understanding how evolution produced such an elaborate structure from biological materials.

There are still mysteries, too. Researchers don’t fully understand how scallops process the images produced by their two retinal layers, or how their many eyes contribute to their overall perception of the world.

For an animal that spends much of its life on the seafloor, a scallop has an extraordinary way of looking at the world.