Herring aid: fish subvert laws of light for camouflage

Silvery fish have found a smart way to get around the laws of physics so that they maintain their reflective camouflage in open water, biologists reported on Sunday. Sprats, sardines and herrings have a skin that neutralises the polarisation of light, enabling them to keep their protective silver cloak, they found. Polarisation describes how light waves travel. Light that is reflected becomes horizontally polarised, meaning that the waves are all oscillating horizontally. Under a law called the "Brewster window effect," polarisation leads to a drop in the amount of light that is reflected. This in theory should pose a problem for shoal fish which swim in the mid-water zone, where they have to reflect light from the sky so that they meld into the background and thwart predators. Reporting in the journal Nature Photonics, British researchers found that underneath the scales of these fish lies a remarkable layer of skin called the stratum argenteum. It comprises alernating layers of proteins, one called guanine crystals that highly refract light, and another, called a cytoplasm, which has a low index for refracting light. The investigators found that there were two kinds of guanine crystals, each with slightly different optical properties. Working together, rather like two refractive lenses, they neutralise polarisation. "We measured how different polarisations of light were reflected from the skin and found that all polarisations were reflected the same way," Nicholas Roberts at Britain's University of Bristol said in an email to AFP. "By creating a non-polarising reflector, the fish have found a way to maximise their reflectivity over all the 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." The research opens the way to multilayered mirrors, made from polymers, which mimic the fish skin, the scientists hope. It could be a boost for optical fibres, which use non-polarising reflectors to enhance light transmission, said lead author Tom Jordan. "These man-made reflectors currently require the use of materials with specific optical properties that are not always ideal," Jordan said. "The mechanism that has evolved in fish overcomes this current design limitation and provides a new way to manufacture [them]."