Some fish appear to be less visible to predators because their silvery skin does not polarize reflected light, say researchers in the UK. The researchers studied three types of fish and found that their skins contain two types of "guanine crystal", each with different optical properties. The team says that the mechanism could be easily applied to man-made optical devices that require non-polarizing reflectors to improve their overall efficiency.

Many fish have silvery iridescent skins and this has intrigued researchers for many years. "We have known that they try to be as silvery as possible to camouflage [themselves]," explains Nicholas Roberts of the University of Bristol. Roberts, along with other colleagues from the university, have been trying to understand the exact purpose of the silvery cloaks of fish such as the European sardine and Atlantic herring. When light reflects from a surface, it usually becomes polarized. It was thought that the fish's skin would fully polarize light when reflected, so there would be a drop in overall reflectivity. This is disadvantageous to the fish because it would appear more visible to a predator.

Shiny cloaks

Fish skin is a layered construct – if you chip off the first layer that is made up of scales, the silver sheen remains. Underneath the scales is a tissue layer – known as the "stratum argenteum" – that consists of guanine crystals and cytoplasm. It is the guanine crystals that are interesting – they are of two types, each with different optical properties and each present in different ratios in the skin.

The two types of guanine crystal have optical axes that are either parallel to the long axis of the crystal or perpendicular to the plane of the crystal. And it is this arrangement that effectively neutralizes the polarization of reflection and makes the reflected light polarization-neutral across a range of angles, according to the researchers.

As the two types of crystal are present, light is reflected at every angle, and the drop in reflectivity usually caused by polarization is avoided, so their skin maintains its high reflectivity. Because it does not suffer from a drop in reflectivity, it no longer stands out from its surroundings. This polarization-neutrality must be especially handy for the fish because several aquatic animals are known to have vision that is not sensitive to colour but can detect differences in polarization, and they use this to their advantage while hunting their prey.

Following the fish...

This simple optical trick could prove of great use in man-made devices where high polarization independence is required, according to the team. Dielectric multilayer reflectors that are non-polarizing are already essential to optical devices and have many varied applications in optical fibres, dielectric waveguides and LED back reflectors. The reflective mechanism of the fish "is distinct from existing non-polarizing mirror designs in that, importantly, there is no refractive index contrast between the low-index layers in the reflector and the external environment. This mechanism could be readily manufactured and exploited in synthetic optical devices", the researchers say in the paper.

In the months to come, Roberts and his colleagues are keen to look further into other aquatic animals that possess "polarization-sensitive vision" and to study their "intriguing optics".

The work is published in Nature Photonics.

  • The upcoming November special issue of Physics World is devoted to "animal physics". You can download a free PDF of the issue from from Wednesday 7 November 2012.
  • David Hu from Georgia Institute of Technology's laboratory for biolocomotion presents a special online lecture at 3.00 p.m. GMT on Thursday 8 November 2012, which you can view by registering here.