Silvery fish. (Photo: Monterey Bay Aquarium/Rick Browne)

Silvery fish bend a law of physics: study

UNITED KINGDOM
Tuesday, October 23, 2012, 04:20 (GMT + 9)

Silvery fish such as herring, sardine and sprat are "breaking" the basic physics law of reflection, according to a study from the University of Bristol published this week in Nature Photonics.

Reflective surfaces polarize light, but PhD student Tom Jordan and his supervisors Professor Julian Partridge and Dr Nicholas Roberts in Bristol's School of Biological Sciences have discovered that these silvery fish have defeated this basic law of reflection, which helps protect them from predators.

Until now, it was believed that the fish's skin, which contains "multilayer" arrangements of reflective guanine crystals, would fully polarize light when reflected, thereby reducing reflectivity and making them more visible to predators.

However, the Bristol researchers found that the skin of sardines and herring contain two types of guanine crystal instead of one – each with different optical properties. As these two types work together, the fish's skin does not polarize the reflected light and keeps 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,” Roberts commented.

“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," he elaborated.

As a result of this ability, the skin of silvery fish could hold the key to better optical devices. 

"Many modern day optical devices such as LED lights and low loss optical fibres use these non-polarizing types of reflectors to improve efficiency,” Jordan said.

“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," he added.

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