Reflective guanine crystals lend sight to scallops

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.