Tuesday, May 15, 2012

Scientists’ Fascination With… A Shrimp

For some years now, scientists have been equally baffled and astounded by the eyesight of one particular creature above all others... the mantis shrimp.

The eyes of insects are made up of optical units known as ommatidia, and the mantis shrimp possesses remarkably few of these in each eye (amazingly, we can call 10,000 per eye small here). Unlike animals such as the dragonfly however, each row of ommatidia in the mantis shrimp dedicates itself to performing a different function; some are used to detect light, others distinguish colour etc. This becomes even more astounding when we consider that the human eye contains three colour receptors whereas those of the mantis shrimp contain a whopping twelve, giving them the ability to see ultra violet, infra red and polarised vision.

No need for the brain
These outstanding creations are housed on the ends of stalks which can be moved completely independently from each other to up to 70 degree angles, and what they do see is processed by the eye itself rather than the brain.

Even more bizarrely than this, it seems that each eye of the mantis shrimp is divided into three sections providing ‘trinocular vision’ and perfect depth perception, meaning that even if one were to lose an eye the other would be just as adept as normal in judging depth and distance.

Polarised vision
Perhaps the most amazing feature of a mantis shrimp’s eyes is their ability to see polarised light, which requires very complex physics and has only been achieved by computers in the last decade. 

The basic notion is that perceiving colour comes from the ability to tell how fast the electric field in a light wave is oscillating (blue light oscillates faster than green, which is faster than red) and the direction of the oscillation is known as polarisation. Many animals have some form of polarisation but it is usually very limited, whereas the mantis shrimp appears to be able to very accurately determine the direction of the oscillation as well as how polarised the light is. A change to the amount of polarisation is a very rapid means of telling the animal that something is going on.

The consequences of this are far reaching. Because of their spectacular vision, mantis shrimp can see in twelve primary colours compared to our measly three, and are suspected to even have a form of communication which other animals (including us) cannot see. Whilst we can only speculate on the usage of this profound ability, it is suspected that, like a lot of amazing things in nature, the most common usage for it will revolve around sex. On a more practical level, much of the mantis’ food comes from transparent animals which are difficult to see in water but which become much more easily noticed with an adept polarised vision.

There are some fascinatingly complicated reports following investigations into the eyes of this amazing animal, and they are thoroughly worth a read if you think you can stomach that much condensed physics. As for me, I’m content just wondering what the world must be like for the mantis shrimp, and whether I can coerce one into being my ‘I Spy’ partner in the future...

Robin is interested in eyesight of the planet's animals and writes for glasses retailer Direct Sight.

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