THE skin of silvery fish such as sardines and herring acts as an “invisibility cloak” against predators and may offer scientists useful tips on designing optical devices, according to new research.
The fish produce a natural optical camouflage by reflecting light thanks to iridescent scales made out of a chemical called guanine found in DNA.
The crystals in the scales of the fish prevent light reflected from their surfaces from becoming polarised, which would ruin their camouflage.
Fish such as sardines and herring possess not one but two types of the “guanine” crystals, researchers have discovered.
Each crystal has different optical properties. By mixing them together, the fish ensure that light bouncing off their skin is not polarised and they remain highly reflective.
This helps them hide from predators by matching the background light flickering through the water.
Dr Nicholas Roberts, of the University of Bristol, said: “We have discovered a generic and novel optical mechanism in silvery fish like herring, sardine and sprat that seemingly breaks this basic law of reflection, enabling non-polarising reflections to occur.
“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.
“What we have discovered in these species of fish is their mechanism of camouflage exhibits polarisation neutrality, thus maximising their reflectivity over all angles. This would help the fish best match the open-water background light field and aid their ability to camouflage themselves against predators.
“Our work suggests that by having a particular mixing ratio of these two types of guanine crystal, these species of fish have evolved a structure that enables near constant reflectivity over all angles of incidence. This creates an optimal solution for camouflage purposes.”
As a result of this ability, the skin of silvery fish could hold the key to better optical devices, such as light emitting diodes.
Researcher Tom Jordan, a PhD student in Dr Roberts’s lab, said: “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 manmade 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,” Dr Roberts explained.
“Many aquatic animals, such as squid, cuttlefish and mantis shrimp, are sensitive to the polarisation of light and have well-developed polarisation vision.
“We are very interested in the polarisation properties of the reflectors that they use, and any novel optics that have evolved as a result of evolutionary adaptations.”
The findings of the University of Bristol team are published in the journal Nature Photonics.