Many fish such as the Herring have a silvery reflective appearance. What is the purpose of this silvery color? What does it do for the fish? Silvery fish such as herring, sardine and sprat may be "breaking" a basic law of physics, according to new research from the University of Bristol published today in Nature Photonics. Reflective surfaces polarize light, a phenomenon that fishermen or photographers overcome by using polarizing sunglasses or polarizing filters to cut out reflective glare. However, PhD student Tom Jordan and his supervisors Professor Julian Partridge and Dr Nicholas Roberts in Bristol's School of Biological Sciences found that these silvery fish have overcome this basic law of reflection — an adaptation that may help them evade predators.
Light reflected by shiny transparent materials is partly or fully polarized, except when the light is perpendicular to the surface. A polarizing filter, such as a pair of polarizing sunglasses, can be used to observe this effect by rotating the filter while looking through it at the reflection off of a distant horizontal surface. At certain rotation angles, the reflected light will be reduced or eliminated. Fully polarized and the image becomes clearer and more visible.
Previously, it was thought that the fish's skin — which contains "multilayer" arrangements of reflective guanine crystals — would fully polarize light when reflected. As the light becomes polarized, there should be a drop in reflectivity.
The Bristol researchers found that the skin of sardines and herring contain not one but two types of guanine crystal — each with different optical properties. By mixing these two types, the fish's skin doesn't polarize the reflected light and maintains its high reflectivity.
Dr Roberts said: "We believe these species of fish have evolved this particular multilayer structure to help conceal them from predators, such as dolphin and tuna. 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."
As a result of this ability, the skin of silvery fish could hold the key to better optical devices. Tom Jordan said: "Many modern day optical devices such as LED lights and low loss optical fibers use these non-polarizing types of reflectors to improve efficiency. 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 non-polarizing reflectors."
For further information see Reflectivity.
Herring image via Wikipedia.