New research explains the metallic shine of Gin Rin.
Japanese Koi carp are famous for their iridescent shimmer, but Lia Addadi at the Weizmann Institute of Science in Rehovot, Israel, and her colleagues have discovered why some glimmer more brightly than others.
They used correlated optical and electron microscopy to compare scales and skin of a common variety of koi with those from a shinier type called Gin Rin. In the scales and skin of both types are cells called iridophores, which contain reflective stacks made up of layers of guanine crystals and cytoplasm. The Gin Rin fish had a much greater density of iridophores, each containing more stacks of crystals than in the common type. The Gin Rin crystals were also oriented differently — almost parallel to the surface of their scales instead of at an angle of around 30 °, as in the common variety.
Fish have evolved biogenic multilayer reflectors composed of stacks of intracellular anhydrous guanine crystals separated by cytoplasm, to produce the silvery luster of their skin and scales. Here we compare two different variants of the Japanese Koi fish; one of them with enhanced reflectivity. Our aim is to determine how biology modulates reflectivity, and from this to obtain a mechanistic understanding of the structure and properties governing the intensity of silver reflectance. We measured the reflectance of individual scales with a custom-made microscope, and then for each individual scale we characterized the structure of the guanine crystal/cytoplasm layers using high-resolution cryo-SEM. The measured reflectance and the structural-geometrical parameters were used to calculate the reflectance of each scale, and the results were compared to the experimental measurements. We show that enhanced reflectivity is obtained with the same basic guanine crystal/cytoplasm stacks, but the structural arrangement between the stack, inside the stacks, and relative to the scale surface is varied when reflectivity is enhanced. Finally, we propose a model that incorporates the basic building block parameters, the crystal orientation inside the tissue, and the resulting reflectance and explains the mechanistic basis for reflectance enhancement.
Research can be found here: J. Am. Chem. Soc. http://doi.org/xd8 (2014)