A shrimps eye view

This may seem a little off-topic, but nature often abounds with solutions to problems we have yet to dream of and, since nature often has a smarter way of doing things, developed through the trials and errors of evolution, we can learn a lot from the living world around us. It turns out, for example, that the mantis shrimp has evolved powerful vision that includes the ability to detect polarised light, and not just linearly polarised light; they can also sense circular polarisation. This could help improve our understanding of how best to design machine vision for use in environments where information on these light properties may be of great benefit.

Linearly polarised light is light whose electric field is aligned in one direction only, whereas circularly polarised light has an electric field whose direction rotates, either clockwise or anticlockwise. There are many creatures that demonstrate the ability to detect linearly polarised light, which can be a powerful aid to navigation for example, because sunlight is partially polarised by the earth's atmosphere and therefore it is possible to know where the sun is in the sky, even on a cloudy day. Circularly polarised light, however, is less abundant in nature and therefore it was surprising to find an animal that could detect it. Researchers in Germany and in Australia recently discovered that mantis shrimp, whose vision was already known to be extremely powerful, have this capability.

Eyes of the mantis shrimp

Being able to sense both linear and circularly polarised light, expands the mantis shrimp's range of polarisation vision, allowing it to better distinguish the state of the light's polarisation. It can do this because it has evolved a part of the eye with a structure that causes it to behave like an optical component known as a quarter wave plate, which turns circularly polarised light into linearly polarised light.

The advantage of being able to sense the polarisation properties of light is that it gives the mantis shrimp enhanced vision suited to life on the sea floor, improving contrast, cutting out glare caused by the scattering of sunlight, helping it navigate and enabling it to detect its prey more easily. It may even be involved in signalling, which is suggested by the fact that certain parts of the mantis shrimp actually reflect polarised light.

Source: Kleinlogel S, White AG (2008) The Secret World of Shrimps: Polarisation Vision at Its Best. PLoS ONE 3(5): e2190 doi:10.1371/journal.pone.0002190.