Volume 95 Issue 48 | p. 11 | Concentrates
Issue Date: December 4, 2017

Reflective guanine crystals lend sight to scallops

Tightly tiled square plates create a mirror to concentrate light on the mollusk’s retina
Department: Science & Technology
Keywords: Materials, guanine crystals, scallop eyes, mirror
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This micrograph reveals tightly tiled plates of reflective guanine crystals.
Credit: Science
A photomicrograph depicts neatly organized square plates tiled together.
 
This micrograph reveals tightly tiled plates of reflective guanine crystals.
Credit: Science
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Scallop eyes use a guanine-based mirror, not a lens, to focus light on the retina.
Credit: Ceri Jones/Haven Diving Services & Benjamin Palmer
A photo of a scallop on the seafloor shows several eyes peeping from under its shell; an inset shows a close-up of one eye.
 
Scallop eyes use a guanine-based mirror, not a lens, to focus light on the retina.
Credit: Ceri Jones/Haven Diving Services & Benjamin Palmer

What do scallop eyes have in common with iridescent crustaceans and shimmery fish scales? Guanine crystals. The shape of the reflective crystals, it turns out, is an important part of natural mirrors found within scallop eyes. Scallops use mirrors instead of lenses to concentrate light on their retinas. Scientists had suspected these mirrors were composed of alternating layers of guanine and cellular cytoplasm, a strategy used by crustaceans and fish to create glimmering surfaces. However, the delicacy of scallop eye mirrors made elucidating the nanoscale structure challenging. Researchers led by Lia Addadi of Weizmann Institute of Science decided to take a closer look by using cryogenic scanning electron microscopy, which rapidly freezes water in tissue samples to create near-lifelike visualization conditions. “The first time we saw the eye mirror in the electron microscope, our mouths fell open,” says colead author Benjamin A. Palmer. The images revealed a tightly organized mirror structure, with 20 to 30 layers of tiled square plates made of β-guanine, with each layer perfectly aligning the plates in a vertical stack (Science 2017, DOI: 10.1126/science.aam9506). The mirror best reflects blue-green light, the same wavelengths that reach the scallops’ seafloor habitat.

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society

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