Fine-tuned for image formation: We typically think of eyes as having one or more lenses for focusing incoming light onto a surface such as our retina. However, light can also be focused using arrays of mirrors, as is commonly done in telescopes. A biological example of this is the scallop, which can have up to 200 reflecting eyes that focus light onto two retinas. Palmer et al. find that spatial vision in the scallop is achieved through precise control of the size, shape, and packing density of the tiles of guanine that together make up an image-forming mirror at the back of each of the eyes. Science , this issue p. 1172

The eyes of some aquatic animals form images through reflective optics. Shrimp, lobsters, crayfish, and prawns possess reflecting superposition compound eyes, composed of thousands of square-faceted eye units (ommatidia). Mirrors in the upper part of the eye (the distal mirror) reflect light collected from many ommatidia onto the photosensitive elements of the retina, the rhabdoms. A second reflector, the tapetum, underlying the retina, back-scatters dispersed light onto the rhabdoms. Using microCT and cryo-SEM imaging accompanied by in situ micro–X-ray diffraction and micro-Raman spectroscopy, we investigated the hierarchical organization and materials properties of the reflective systems at high resolution and under close-to-physiological conditions. We show that the distal mirror consists of three or four layers of plate-like nanocrystals. The tapetum is a diffuse reflector composed of hollow nanoparticles constructed from concentric lamellae of crystals. Isoxanthopterin, a pteridine analog of guanine, forms both the reflectors in the distal mirror and in the tapetum. The crystal structure of isoxanthopterin was determined from crystal-structure prediction calculations and verified by comparison with experimental X-ray diffraction. The extended hydrogen-bonded layers of the molecules result in an extremely high calculated refractive index in the H-bonded plane, n = 1.96, which makes isoxanthopterin crystals an ideal reflecting material. The crystal structure of isoxanthopterin, together with a detailed knowledge of the reflector superstructures, provide a rationalization of the reflective optics of the crustacean eye. Significance Some aquatic animals use reflectors in their eyes either to form images or to increase photon capture. Guanine is the most widespread molecular component of these reflectors. Here, we show that crystals of isoxanthopterin, a pteridine analog of guanine, form both the image-forming “distal” mirror and the intensity-enhancing tapetum reflector in the compound eyes of some decapod crustaceans. The crystal structure of isoxanthopterin was determined, providing an explanation for why these crystals are so well suited for efficient reflection. Pteridines were previously known only as pigments, and our discovery raises the question of which other organic molecules may be used to form crystals with superior reflective properties either in organisms or in artificial optical devices.

The eyes of some aquatic animals form images through reflective optics. Shrimp, lobsters, crayfish and prawns possess reflecting superposition compound eyes, composed of thousands of square-faceted eye-units (ommatidia). Mirrors in the upper part of the eye (the distal mirror) reflect light collected from many ommatidia onto the underlying photosensitive elements of the retina, the rhabdoms. A second reflector, the tapetum, underlying the retina, back-scatters dispersed light onto the rhabdoms. Using microCT and cryo-SEM imaging accompanied by in situ micro-X-ray diffraction and micro-Raman spectroscopy, we investigated the hierarchical organization and materials properties of the reflective systems at high resolution and under close to physiological conditions. We show that the distal mirror consists of three or four layers of sparse plate-like nano-crystals. The tapetum is a diffuse reflector composed of hollow nanoparticles constructed from concentric lamellae of crystals. Isoxanthopterin, a pteridine analog of guanine, forms both the reflectors in the distal mirror and in the tapetum. The crystal structure of isoxanthopterin was determined from crystal structure prediction calculations and verified by comparison with experimental X-ray diffraction. The extended hydrogen bonded layers of the molecules results in an extremely high calculated refractive index in the H-bonded plane, n = 1.96, which makes isoxanthopterin crystals an ideal reflecting material. The crystal structure of isoxanthopterin, together with a detailed knowledge of the reflector superstructures, provide a rationalization of the reflective optics of the crustacean eye. Significance: Aquatic animals use reflectors in their eyes either to form images or to increase photon capture. Guanine is the most widespread molecular component of these reflectors. Here we show that crystals of isoxanthopterin, a pteridine analogue of guanine, form both the image-forming ‘distal’ mirror and the intensity-enhancing tapetum reflector in the compound eyes of some decapod crustaceans. The crystal structure of isoxanthopterin was determined, providing an explanation for why these crystals are so well suited for efficient reflection. Pteridines were previously known only as pigments and our discovery raises the question of which other organic molecules may be used to form crystals with superior reflective properties either in organisms or in artificial optical devices.