How silvery fish become invisible to predators

Silver-colou­red fish use an optical illusion by bending the laws of physics to become invisible to predators, a new study has found.

Researchers explained that reflective surfaces polarise light, a phenomenon that fishermen or photographers overcome by using polarising sunglasses or filters to cut out glare. Tom Jordan and Julian Partridge from Bristol University, however, found that silvery fish such as herring, sardines and sprat have overcome this basic rule of reflection.

The fish’s skin contains multilayer arrangements of reflective guanine crystals, the Discovery News reported.

It was previously thought that fish skin would fully polarise light when reflected. As the light becomes polarised, there should then be a drop in reflectivity. But that’s not what always happens, as it turns out.

The researchers found that the skin of sardines and herring contain not one but two types of guanine crystal. Each has different optical properties.

By mixing these two types, the fish’s skin doesn’t polarise the reflected light and maintains its high reflectivity, the report said. The result is an optical illusion that can make the fish at times seem invisible to other marine dwellers.

“We believe these species of fish have evolved this particular multilayer structure to help conceal them from predators, such as dolphin and tuna,” researchers said. “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,” they said.

In future, fish skin might inspire inventors to create better optical devices. Jordan explained that 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,” he concluded.