Iridescence—a key component of certain makeup, paints, coatings of mirrors and lenses—is also an important feature in the natural world. Both fish and spiders make use of periodic photonic systems, which scatter or reflect the light that passes against their scales or skin to produce an iridescent camouflage. Previously, scientists knew that animals like the Koi fish and the silvery spider used guanine crystals of a specific size and orientation to achieve this reflectivity. However, a recent study by researcher Avital Levy-Lior and her colleagues at the Weizmann Institute of Science and York University have discovered how these structures came together through evolution.
Thanks to advanced techniques, the researchers were able to study the skin and scales intact without damaging their structure. Their findings show that while both fish and spiders used different crystal arrangements, they were both able to evolve effective structures for achieving reflectivity using the same base material (guanine crystals).
The fact that both fish and spiders were able to evolve a unique but equally effective structure for photonic camouflage is particularly intriguing. Says Prof. Peter Fratzl of the Department of Biomaterials at the Max Planck Institute of Colloids and Interfaces, "It is very surprising that fish and spiders, pertaining to completely different taxonomic groups, independently acquired through evolution the ability to generate mirror-like reflections on their skin by depositing guanine crystals. This suggests that the solution must be quite efficient and it is, therefore, extremely promising for the material scientists to try and understand the structural principles of these photonic crystals working as (colored) mirrors."