The most incredible eyes in the animal world can be found under the sea, on the head of the mantis shrimps. Each eye can move independently and can focus on object with three different areas, giving the mantis shrimp “trinocular vision”. While we see in three colours, they see in twelve, and they can tune individual light-sensitive cells depending on local light levels. They can even see a special type of light – ‘circularly polarised light’ – that no other animal can.
But Nicholas Roberts from the University of Bristol has found a new twist to the mantis shrimp’s eye. It contains a technology that’s very similar to that found in CD and DVD players, but it completely outclasses our man-made efforts. If this biological design can be synthesised, it could form the basis of tomorrow’s multimedia players and hard drives.
Previous studies have found that mantis shrimps can detect polarised light – light that vibrates in a single plane as it travels. Think of attaching a piece of string to a wall and shaking it up and down, and you’ll get the idea. Last year, scientists discovered that they can also see circularly polarised light, which travels in the shape of a helix. To date, they are still the only animal that can see these spiralling beams of light.
Its secret lies at a microscopic level. Each eye is packed with light-sensitive cells called rhabdoms that are arranged in groups of eight. Seven sit in a cylinder and each has a tiny slit that polarised light can pass through if it’s vibrating in the right plane. The eighth cell sits on top and its slit is angled at 45 degrees to the seven below it. It’s this cell that converts circularly polarised light into its linear version.
In technical terms, the eighth cell is a “quarter-wave plate”, because it rotates the plane in which light vibrates. Similar devices are also found in camera filters, CD players and DVD players but these man-made versions are far inferior to the mantis shrimp’s biological tech.
Synthetic wave plates only work well for one colour of light. If you change the wavelength slightly, they become ineffective, so designing a wave plate that works for many colours is exceptionally difficult. But the mantis shrimp has already done it. Its eyes work across the entire visible spectrum, from ultraviolet to infrared, achieving a level of performance that our technology can’t compete with.
What’s more, the same eighth cell not only manipulates circularly polarised light, but it can sense ultraviolet light too. It’s a detector and a converter – a two-for-one deal that nothing man-made shares.
Why the mantis shrimp needs such a sophisticated eye is unclear. It could help them to see their prey more clearly in water, which is rife with circularly polarised reflections. It needs good eyesight to be able to hit its prey accurately. Like a crustacean Thor, mantis shrimps shatter their victims with devastating hammer blows inflicted by the fastest arms on the planet. Their forearms, which end in clubs or spears, can travel through water at 10,000 times the acceleration of gravity and hit with the force of a rifle bullet.
Another option is that their super-eyes allow them to send and receive secret messages. A mantis shrimp’s shell reflects circularly polarised light, and males and females produce these reflections from different body parts. Their ability to see this type of light could give them a hidden channel of communication that only they can see, for use in courtship or combat.
Whatever the reason for it, Roberts thinks that the eye’s structure is “beautifully simple”. It’s all in the shapes of the cells, their size, and the amount of fat in their membranes. For all its outstanding performance, the eye’s abilities were probably easy to evolve, requiring only small tweaks to the basic blueprint of the light-detecting cells.
Now that we know about the microscopic structures behind the mantis shrimp’s amazing eye, Roberts is hopeful that engineers can mimic it using liquid crystals. “The cool thing is I think it’s actually something you could make and it would improve the workings of current technologies such as Blu-Ray, which uses multiple wavelengths of light, and of future data storage devices,” he said. It wouldn’t be the first time that crustaceans have inspired technology. A new type of X-ray telescope, for example, was based on the eye of the lobster.
Reference: Nature Photonics DOI: 10.1038/NPHOTON.2009.189
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