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

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2024-02-12 02:30:02

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.

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