A multilayer crystalline structure used by silvery fish to evade ocean predators bends the laws of physics and could be the key to developing better optical devices. Reflective surfaces polarize light, but silver-colored fish such as sardines, herring and sprat do not polarize light in the way that most reflective surfaces do. They have overcome this basic law of reflection to conceal themselves from predators, according to new research from the University of Bristol. Previously, it was thought that the fish’s skin — which contains multilayer arrangements of reflective guanine crystals — would fully polarize light and become less reflective. The shiny skin of fish such as sardines has two types of guanine crystal — each with different optical properties — that do not polarize reflected light, but rather maintain its high reflectivity. The silvery fish skin could hold the key to better optical devices, University of Bristol scientists say. Courtesy of National Oceanic and Atmospheric Administration/National Marine Fisheries Service (NOAA/NMFS). The researchers discovered that the skin of herring and sardines contains not one but two types of guanine crystal — each with different optical properties. By mixing these two types, the fish’s skin does not polarize the reflected light and maintains its high reflectivity. “We believe these species of fish have evolved this particular multilayer structure to help conceal them from predators, such as dolphin and tuna,” said Dr. Nicholas Roberts of Bristol’s School of Biological Sciences. “These fish have found a way to maximize their reflectivity over all angles they are viewed from. This helps the fish best match the light environment of the open ocean, making them less likely to be seen.” The fish’s silvery skin could hold the key to better optical devices, the researchers said. “Many modern-day optical devices, such as LED lights and low loss optical fibers, use these nonpolarizing types of reflectors to improve efficiency,” said doctoral candidate Tom Jordan. “However, these man-made reflectors currently require the use of materials with specific optical properties that are not always ideal. The mechanism that has evolved in fish overcomes this current design limitation and provides a new way to manufacture these nonpolarizing reflectors.” The findings were reported in Nature Photonics (doi: 10.1038/nphoton.2012.260) For more information, visit: www.bris.ac.uk