Looks like scallops are the FBI of the marine world.
Using a technique called cryo-electron microscopy, scientists have been able to take a closer look at the inside of these bivalves’ bizarre eyes — all two hundred of them. In a report published in Science, the researchers from the Weizmann Institute of Science identified tiny, perfectly flat, crystal squares of guanine (one of the four basic units that make up DNA) that sit inside the scallops’ eyes, creating a kind of mirror that lets them see.
What’s so strange about this is that most animals have eyes that use lenses to reflect light, but scallop eyes have crystal mirrors that focus light instead. A few other animals have mirrors too, but Benjamin Palmer, coauthor of the study, says that few of them form such a clear image like scallops’ do.
“It’s such an unusual visual system,” says Daniel Speiser, a marine ecologist at the University of South Carolina who wasn’t part of the study. “The closer you look, the more puzzling it gets.”
Guanine doesn’t typically form crystal shapes that pack together evenly. Palmer suggests that this means the scallop is controlling the crystallisation process. “That’s really weird,” he added. “It’s the first time we’ve seen a perfect square!”
In addition to this, the mirror has an unusual 3D shape that lets the scallop focus light on one of two retinas, depending on the angle of the incoming light. One retina is tuned to dimmer light coming from its peripheral vision, while the other can capture objects and motion in bright light.
“This is a new idea for how the scallop can make use of both retinas in the same eye,” says Speiser.
The sheer complexity within their eyes remains a puzzle for experts. Usually, such complex and hard-working eyes would indicate that the species evolved to better hunt its prey, but scallops don’t exactly need 20/20 vision to strain food particles from the water.