The most sophisticated eyes in the animal kingdom belong to mantis shrimps a group of extraordinary species that live on coral reefs around the world. In a brand new paper just out, a team of scientists examine just what lies behind these complex eyes and they’ve uncovered some tricks of nature that could find applications in the cutting edge of modern technology, perhaps spawning a new generation of DVDs and CDs.
Last year, a paper in the journal Current Biology announced the discovery that mantis shrimp can see both linear polarized and circular polarized light. Now a team led by Nicholas Roberts from the University of Bristol have discovered that mantis shrimp eyes do this using special light-sensitive cells that act as a device known as a quarter-wave plate. Essentially these plates convert circular polarized light into linear polarised light. Manmade quarter wave plates are vital components of DVD players and some camera filters, but they don’t work nearly as well as the ones that have evolved in the eyes of mantis shrimps.
Normal light behaves like a wave that can vibrate in any direction. In contrast, linear polarized light only vibrates in only one plane. We usually don’t see polarized light, although we are aware of it, for example the light that reflects off glass or the surface of a pool of water (putting on polarizing sunglasses cuts out those polarising rays and lets you see through the water).
Circular polarized light behaves like a spiral or a helix. The remarkable mantis shrimps can even tell the difference between circular polarized light spins to the left or to the right.
Not only that, but unlike manmade quarter wave plates that only work well in one colour, mantis shrimp eyes work almost perfectly across the whole visual spectrum from near ultra violet to infra-red.
Leading on from their earlier work, the research team discovered that the receptors that detect circular polarized light are located in a central band across the mantis shrimp eye – their eyes are divided into 3 distinct regions, giving them trinocular vision in each eye and are packed with light-detecting units called omatidia, similar to other invertebrates including insects. They did this essentially by shining polarized light of different types through thin sections of parts of the eye and measuring how well those cells absorbed the light.
Part of the reason why these biological structures work so much better than manmade structures is because they are so tiny – on the nano scale – with components that are smaller than the wavelength of even ultra violet light (10-400nm). That is very challenging to create in manmade structures.
The big question is why on earth do mantis shrimps need such incredibly complex eyes?
There is lots of polarized light bouncing around the clear shallow waters of coral reefs where most mantis shrimps live. Their favourite prey includes small slivery fish whose scales polarize light.
Scientists also think that mantis shrimps might use circular polarized light as a way of secretly communicating with each other. Parts of their bodies reflect circular polarized light, and we so far don’t know that any other animal can detect this type of light. So perhaps they send out signals – flashing mating colours maybe - without any fear of being detected by anyone else except another mantis shrimp.
So when it comes to solving technical problems like how to see well in a bright watery world, natural selection has come up with an elegant and simple solution. And perhaps the mantis shrimp will teach us a thing or two about how to build the optical devices of the future.