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.

J. Am. Chem. Soc. http://doi.org/xd8 (2014)