Perhaps you didn’t realize, like reader Gregory (who sent me the Science paper), that scallops have eyes. But they do indeed—up to 200 tiny eyes lining the mantle, each a millimeter across: about the size of an “o” on a printed page.
Here’s what the array looks like in the scallop Pecten:
And a close up of the miniscule baby blue eyes:
Why do they need them? Because scallops aren’t sedentary molluscs: they swim actively by “jet propulsion,” flapping their shells to get away from predators or to find new resting sites. To wit:
It’s been known for a while that these eyes probably involve mirror reflection of incident light onto a retina, but how that reflection was achieved wasn’t clear, except that the mirror probably involved guanine crystals (guanine is one of the four nucleotide basis that make up the “code” of DNA). But a new paper in Science by Benjamin Palmer et al. (free access, reference below), elucidates how the eye works, and it’s amazing. The mirror, formed of overlying sheets of guanine crystals, reflects light back on retinal tissue that sits in front of the reflector, and there is not one but two retinas, each giving information about different parts of the scallop’s environment. And the mirror, besides functioning very efficiently, is a thing of beauty: a marvel of natural selection.
First, though, another picture (from the paper) of the eyes lining the mantle (captions of all figures come from the paper):
So here’s how the eye works. Figure “A” below is an image produced by the technique that enabled this research to be done: cryogenic scanning electron microscopy (cryo-SEM), in which a frozen sample is scanned. (A Nobel Prize in Chemistry was awarded this year to the researchers who developed the method.) This enabled the researchers to visualize not only the entire structure of the eye, as in “A” below, but also sections of it, so they could look at the fine structure of the guanine “mirror” as well as make computer models of how light would travel after entering the eye.
Image “A” is analyzed in “B”, with different colors represent the parts of the eye and the directions of light. Incoming light (red lines) hits the mirror (green) after passing through the cornea (black), the iris (navy blue), the lens (light blue), and the transparent retinas (gray cloud). After hitting the mirror, light rays (now yellow) are reflected through the guanine layers, eventually striking the two retinas. One retina is proximal (closer to the scallop’s body) and the other distal (closer to the front of the eye):
Here’s a cross section of the tiny eye with the elements labeled. You can see the two retinas (iii and iv), with the bowl-shaped mirror (v) right below the retinas. As I said, the retinas are transparent so they don’t block incoming light. Yellow arrows show the direction of light entering the eye:
Here’s a colored cross section that makes identification of the parts easier (see the caption). The two retinas are olive-green and orange, and the mirror is bright green:
What’s truly remarkable about the eye is the “mirror”, composed of a tiled “floor” of guanine crystals in the shape of squares (not their natural crystalline configuration—how does the scallop do this?). Each “floor” is a sheet, and there are 20-30 of them set one above the other, interspersed with cytoplasm. Here’s a cryo-SEM image (see caption), and isn’t it remarkable?
Why the multiple layers? As this site from Duke University explains:
. . . .by carefully choosing the thickness of each layer, one can arrange for light that reflects at each interface to interfere constructively such that all incoming light (within a certain range of wavelengths) is reflected back toward its source, that is the layers act as a high quality mirror. It is fairly easy for biological creatures to secrete such alternating clear layers with slightly different properties, rather harder for humans to do so using chemical and mechanical engineering.
That is a remarkable feat of natural selection.
Equally remarkable is the calculation (from the authors’ simulation) that the light best reflected is in the blue-green spectrum, with a wave length of about 500 nanometers: just the wavelength of light that reaches the sea-floor environment of the scallop.
But wait! There’s more! As suggested by another of the authors’ models, light from different parts of the eye reaches the two retinas differentially. Light entering the center of the eye is preferentially directed toward the distal, or outer retina, while light coming in from the sides of the eye goes to the proximal or inner retina. Thus the two retinas give information about different parts of the habitat. Why would this be useful? As the authors suggest, the peripheral vision could help the scallop guide its movement while swimming and help it to find a new settling site, while the central vision could give information about a predator approaching them.
Finally, the data coming from the different eyes is integrated and sent to a scallop “brain”, or, as the authors describe it, “the lateral lobes of the parieto-visceral ganglion (PVG), the site of visual processing in scallops.” Thus there’s no independent data from each eye, which isn’t really needed here given that the retinas single out different parts of the scallop’s environment.
The mirror reflecting light onto an image-detector is precisely the way reflecting telescopes work, though human-constructed mirrors are very different from those of the scallop. In fact, I don’t think humans are capable of making mirrors like this bivalve does. As Leslie Orgel once said, evolution is cleverer than you are.
Palmer, B. A., G. J. Taylor, V. Brumfeld, D. Gur, M. Shemesh, N. Elad, A. Osherov, D. Oron, S. Weiner, and L. Addadi. 2017. The image-forming mirror in the eye of the scallop. Science 358:1172-1175. (pdf here)