How a fish 'broke' a law of physics

Silvery fish such as herring, sardine and sprat are "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 polarising sunglasses or polarising filters to cut our 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.

Previously, it was thought that the fish's skin – which contains "multilayer" arrangements of reflective guanine crystals – would fully polarizse light when reflected.  As the light becomes polarised, 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 polarise the reflected light and maintains its high reflectivity.

Dr Roberts says, "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 maximise 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 says, "Many modern day optical devices such as LED lights and low loss optical fibres use these non-polarising 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-polarising reflectors."