The Structural Basis for Enhanced Silver Reflectance in Koi Fish Scale and Skin

J. Am. Chem. Soc., Just Accepted Manuscript

DOI: 10.1021/ja509340c

Publication Date (Web): November 13, 2014

Copyright © 2014 American Chemical Society

Abstract

Fish have evolved biogenic multilayer reflectors composed of stacks of intra-cellular 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.

The Structural Basis for Enhanced Silver Reflectance in Koi Fish Scale and Skin