The scallop sees with space-age eyes – hundreds of them

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It’s hard to see what’s so special about a scallop. It looks a lot like a clam, a mussel or any other bivalve. Inside its hinged shell hides a muscular creature that is best eaten pan-fried in butter.

But there’s something more to this ubiquitous starter: the scallop sees its world through hundreds of eyes. Arranged across the opening of its shell, the eyes glow like an underwater necklace. Each sits at the end of its own tentacle and can be extended beyond the edge of the shell.

While some invertebrate eyes can only perceive light and dark, scientists have long suspected that scallops can distinguish images, perhaps even recognize predators fast enough to fly away safely. But the scallops’ eyes – each about the size of a poppy seed – are so small and delicate that scientists are struggling to figure out how they work.

Now, a team of Israeli researchers has uncovered the hidden sophistication of the scallop’s eye, thanks to powerful new microscopes. On Thursday, they reported in the journal Science that each eye contains a miniature mirror made up of millions of square tiles. The mirror reflects incoming light onto two retinas, each of which can detect different parts of the scallop’s environment.

Our own eye has been compared to a camera: it uses a lens to focus light onto the retina. The new research suggests that scallop eyes are more akin to another type of technology: a reflector telescope of the type first invented by Newton. Today, astronomers build gigantic reflector telescopes to look into deep space, and they also build their mirrors out of tiles.

“For me, Newton and Darwin come together in those eyes,” said Gáspár Jékely, a neuroscientist at the University of Exeter who was not involved in the new study.

Previous studies had given scientists clues that the scallop’s eye was oddly complex. Each has a lens, a pair of retinas, and a mirror-like structure on the back. The scientists suspected that the light passed through the lenses and retinas, which are mostly transparent, bounced off the mirror and hit the retinas on the way back.

But no one knew how the mirror worked, or why scallops needed two retinas when other animals only needed one.

Benjamin A. Palmer, a postdoctoral researcher at the Weizmann Institute of Science in Israel, and his colleagues recently used a powerful new tool known as a cryo-electron microscope to observe the eyes of scallops.

He and his colleagues froze eye slices, allowing the tissue to be inspected down to fine molecular detail. (Last month, three pioneers in cryo-electron microscopy won the 2017 Nobel Prize in Chemistry.)

Researchers have long known that the mirror in a scallop’s eye is made up of a molecule called guanine. It is best known as one of the main ingredients of DNA, but in some animals guanine is packed into crystals that reflect light.

Some fish have a silver tint to their scales from guanine crystals. Chameleons use guanine crystals to help them change the color of their skin. But no one knew how guanine helped scallops see.

Using cryo-electron microscopes, Dr. Palmer and his colleagues discovered that scallops form a kind of guanine crystal never before seen in nature: a flat square. “We were amazed,” he said. “We knew it would be something cool.”

The researchers found that the mirrors are made up of twenty to thirty layers of guanine, each containing millions of squares that fit together perfectly like tiles on a wall.

“To see that square tiling is completely new,” said Daniel I. Speiser, a visual ecologist at the University of South Carolina who was not involved in the study.

Dr. Palmer and his colleagues took X-rays of the scallops’ eyes to determine that these layers form a flat-bottomed bowl. The scientists created a computer model of the entire eye based on these findings, allowing them to trace the paths light takes as it bounces off the mirror.

Paradoxically, the squares of guanine do not reflect light by themselves, they are transparent. But their arrangement makes it a collective mirror.

The layers of tiles are separated by thin layers of fluid, and as a ray of light passes through them, it deforms more and more from its original direction. Eventually, the light turns around completely, moving towards the front of the eye.

This arrangement is well suited for underwater viewing, the researchers found, because it is more effective at bouncing certain colors of light than others. “You have a mirror that basically reflects one hundred percent of the blue light it receives,” Dr. Palmer said. “It makes a lot of sense that he reflects all the light he has in his surroundings.”

The model created by Dr. Palmer and his colleagues could also solve the mystery of the two retinas. The researchers found that each retina receives highly focused light from different parts of the animal’s field of vision.

A retina can create a sharp image of what is right in front of the eye. The other retina gives a better view of the periphery.

Dr. Palmer hypothesized that scallops could use each retina to deal with a different challenge in their lives.

The retina that sees the central field of vision could allow scallops to quickly recognize oncoming predators, allowing them to beat a hasty retreat by swimming.

Scallops may instead pay attention to their peripheral vision when looking for a place on the seabed where they can settle to feed.

The new study demonstrates that each eye is extremely complex. Additionally, the hundreds of eyes on a scallop all send signals to a single set of neurons, which can combine that information to create a rich picture of the outside world.

For Dr. Speiser, this all seems exaggerated. Why does a fairly ordinary bivalve need Star Wars vision technology? “It’s always a puzzle why they see so well,” he said.

Dr Palmer said scallop eyes could inspire new inventions. There is certainly precedent: NASA built X-ray detectors to study black holes that mimic the eyes of lobsters. Maybe an artificial scallop eye could take pictures in dark sea water.

But Dr. Palmer is more excited about the prospect of creating new materials for engineering. His study shows that the scallops have developed a mastery of crystal formation, guiding them into shapes the researchers didn’t think possible.

At this point, no one has any idea how they do it yet. “Understanding this could open the door to much bigger things than just making a single device,” Dr Palmer said.

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