Biological photonic systems composed of anhydrous guanine crystals evolved separately in several taxonomic groups. Here, two such systems found in fish and spiders, both of which make use of anhydrous guanine crystal plates to produce structural colors, are examined. Measurements of the photonic‐crystal structures using cryo‐SEM show that the crystal plates in both fish skin and spider integument are ∼20‐nm thick. The reflective unit in the fish comprises stacks of single plates alternating with ∼230‐nm‐thick cytoplasm layers. In the spiders the plates are formed as doublet crystals, cemented by 30‐nm layers of amorphous guanine, and are stacked with ∼200 nm of cytoplasm between crystal doublets. They achieve light reflective properties through the control of crystal morphology and stack dimensions, reaching similar efficiencies of light reflectivity in both fish skin and spider integument. The structure of guanine plates in spiders are compared with the more common situation in which guanine occurs in the form of relatively unorganized prismatic crystals, yielding a matt white coloration.

ABSTRACT:The metallic luster from the skin of fish is due to a photonic crystal system composed of multilayer stacks of cytoplasm and crystals. The crystals are described as thin (50-100 nm) plates of guanine, with no reference to their hydration state. We established through X-ray diffraction that their crystal structure is that of anhydrous guanine. We noted that their crystal structurefunction relationship is exceptional compared to other purines with similar molecular stacking of the crystal structure. These elongate in the direction of molecular stacking, in contrast to the biogenic anhydrous guanine crystals whose smallest dimension is in the stacking direction. On the basis of the known crystal structure of anhydrous guanine, theoretical growth morphology was calculated. These calculations predict crystals elongated in the direction of the molecular stacking. The exposed molecular plane of the biogenic crystals is the (102) plane, which is composed of densely packed H-bonded guanine molecules. It is known that the in-plane polarizability of guanine molecules is significantly higher than the direction perpendicular to the molecular plane, most likely causing anisotropy of the crystals refractive index. It is therefore conceivable that the unique morphology observed in crystals from the skin of fish is designed to enhance their light reflective properties.

Dedicated to Professor Duilio Arigoni on the occasion of his 75th birthdayThe leaves of some plants, particularly among the Solanacea, contain crystals of calcium oxalate with a peculiar chiral pseudo-tetrahedral morphology, even though the calcium oxalate crystal structure is centrosymmetric, hence achiral. We studied the morphology of these crystals extracted from the leaves of three Solanacea plants: the potato, the hot pepper, and a species of wild Solanum. The crystal morphology was the same in all three species. Based on the examination of more than 100 crystals from each plant, we showed that the crystal morphology is chiral with invariant chirality. We suggest that morphological chirality is induced by macromolecules during nucleation from a specific, genetically encoded crystal plane, and is further established during subsequent controlled crystal growth. This is one of few examples where it is possible to deduce a molecular mechanism for biologically induced breaking of morphological symmetry in organisms. A very high level of recognition is required by the macromolecules to allow them to distinguish between symmetry-related crystal planes. It is also surprising that this finely controlled mechanism of crystal formation, including the chiral morphology, has been conserved during evolution.Introduction. ± Morphological mirror symmetry is commonly observed in nature. When the morphological symmetry is broken, however, the resulting object may exist in two mirror-related, or enantiomorphous, forms. In the latter case, it is often only one particular mirror image that is induced in biology, and not the other. The term breaking of morphological symmetry refers here to permanently and invariantly removing from the morphology of a symmetrical object symmetry elements of the second kind such as mirror planes or centers of inversion [1]. The resulting object has a chiral morphology in the classical sense, i.e., it is not super-imposable on its mirror image [2] [3].Snails are a common and classical example of morphological chirality, having spiral shells that revolve in a clockwise direction for more than 98% of the individuals of a given species [4]. Chirality is, thus, characteristic not only of the particular individual, but of the entire population. Because only one of the mirror-related morphologies exists, we refer to it as invariant chirality. It transpires that the direction of revolution of snail shells is determined genetically, and manifests itself as soon as at the stage of the second cellular division [5]. How this is determined is, however, not yet understood.Other well-known examples of spiral morphology are found in the direction of juxtaposition of the leaves of some plants, or the direction of twining of various climbing plants. Notably, approximately 30% of these twine constantly in one direction [4]. This phenomenon, noted and discussed by Darwin [6], may have originated from environmental stimuli, such as the direction of the sunlight, but was later transformed into a genetically encoded char...