What Scallops’ Many Eyes Can Teach Us About the Evolution of Vision

The word "scallop" usually evokes a juicy, round adductor muscle—a seafood delicacy. So it isn't widely known that scallops have up to 200 tiny eyes along the edge of the mantle lining their shells. The complexities of these mollusk eyes are still being unveiled. A new study published in Current Biology reveals that scallop eyes have pupils that dilate and contract in response to light, making them far more dynamic than previously believed.

“It's just surprising how much we're finding out about how complex and how functional these scallop eyes are,” says Todd Oakley, an evolutionary biologist at the University of California, Santa Barbara.

The optics of scallop eyes are set up very differently than our own ocular organs. As light enters into the scallop eye, it passes through the pupil, a lens, two retinas (distal and proximal), and then reaches a mirror made of crystals of guanine at the back of the eye. The curved mirror reflects the light onto the interior surface of the retinas, where neural signals are generated and sent to a small visceral ganglion, or a cluster of nerve cells, whose main job is to control the scallop's gut and adductor muscle. The structure of a scallop's eye is similar to the optics systems found in advanced telescopes.

For many years, the physics and optics of the scallop eye posed a perplexing problem. "The main retina in the eye gets almost completely unfocused light because it's too close to the mirror," says Dan Speiser, a vision scientist at the University of South Carolina and the senior author of the new study. In other words, any image on the proximal retina would be blurry and out of focus. “That just seems so unreasonable to me,” Speiser says.

The new study sheds some light on this mystery. The researchers found that the scallop pupils are able to open and contract, though their pupillary responses aren’t as quick as our own. A scallop pupil's diameter changes by about 50 percent at most, and the dilation or contraction can take several minutes. Their eyes don’t have irises like our eyes do, and instead, the cells in the cornea change shape by going from thin and flat to tall and long. These contractions can change the curvature of the cornea itself, opening the possibility that the scallop eye might change shape and respond to light in a way that makes it possible to form crisper images on the proximal retina.

“It really changes the ability of that eye and ultimately the organism to be able to have the type of resolution to see its environment,” says Jeanne Serb, a vision scientist at Iowa State University.

Now, Speiser is working to understand if the scallops are able to change the curvature of the mirror and the eye as a whole, which would enable it to adjust the focus of the image even further. "The eyes' dynamic structures open up some new possibilities for what you can do with a mirror-based eye like this," Speiser says.

Hubble in a bubble: Scallop eyes act like tiny telescopes

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Adaptive mirrors aren’t the scallop eye’s only mystery. “It turns out that scallop eyes have three times as many opsins as we do,” Serb says. Opsins are light-sensitive proteins found in the photoreceptor cells of the retina that mediate the conversion of light into electrochemical signals. Scientists don’t know whether all 12 scallop opsins are expressed in every single scallop eye or if the eyes subspecialize in different channels of the visual spectrum. Some opsins may be expressed in the proximal retina while others are in the distal retina.

Serb’s team at Iowa State studies the opsins in scallops, clams and other animals. Bivalves—mollusks that live inside two matching cupped shells connected by a hinge—have evolved some form of eye multiple times. Some clams even have compound eyes, or eyes with multiple visual units, though they differ from the better-known compound eyes of insects. By studying the different opsins outside of the animals, Serb can measure their absorption and ultimately understand how they work in the different animals.

Eyes have probably evolved at least 50 or 60 times across all animals, and in many cases, the molecular underpinnings of vision—the proteins that translate light signals to electrical signals—vary quite a bit. “The big evolutionary question for me is, how do these proteins evolve to sample light? And then, how does it become specified to the different types of light environments that the animals can occur in?” Serb asks. She believes that the opsins, in most cases, are being repurposed from some other function within the animal to be used in the eyes.

Although there is a diversity of eye morphologies and of photoreceptors across animals, the building blocks—the genes that control eye development—are remarkably similar. For example, Pax6 is a developmental gene that is critical for eye development in mammals, and it plays a similar role in the development of scallop eyes. In a recent study preprint, Andrew Swafford and Oakley argue that these similarities belie the fact that many types of eyes might have evolved in response to light-induced stress. Ultraviolet damage causes specific molecular changes that an organism must protect against.

“It was so surprising that time and time again, all these components that are used to build eyes, and also are used in vision, have these protective functions,” Oakley says. In the deep history of these components are genetic traits that trigger responses to light-induced stress, such as repairing damage from UV radiation or detecting the byproducts of UV damage. Once the suite of genes involved in detecting and responding to UV damaged are expressed together, then it may be just a matter of combining those parts in a new way that gives you an eye, the researchers suggest.

“The stress factor can bring together these components maybe for the first time,” Swafford says. “And so the origins of the interactions between these different components that lead to vision are more attributable to this stress factor. And then once the components are there, whether it be pigments or photoreceptors or lens cells, then natural selection acts to elaborate them into eyes.”

However they were made, scallop eyes have some impressive functionality, warping their internal mirrors to bring light into focus like a telescope. So next time you are enjoying some garlic scallops, try not to imagine the mollusks staring back at you.