One of a Kind: The Animal with One of the Most Extraordinary Eyes on Earth
The peacock mantis shrimp possesses one of the most unusual visual systems ever discovered, allowing it to detect forms of light that humans cannot see and process visual information in ways that scientists are still trying to fully understand.
More Than Just Colour Vision
Humans have three types of colour-sensitive photoreceptors, which allow us to distinguish millions of colours by comparing signals from each type. Peacock mantis shrimp, by comparison, possess up to sixteen different photoreceptor types.
For many years, this led to the assumption that they must see far more colours than humans. But research suggests the reality is more complicated.
Rather than carefully comparing information from different photoreceptors as humans do, mantis shrimp may identify colours more directly. This appears to allow them to recognise colours extremely quickly, although not necessarily with greater precision than our own visual system.
In other words, the mantis shrimp's eyes seem to prioritise speed over detail.
Seeing Light Humans Cannot
What makes mantis shrimp truly remarkable is not simply the number of photoreceptors they possess. Their eyes can detect aspects of light that humans are completely unable to perceive.
One of these is polarised light. Light is an electromagnetic wave, and as it travels, its electric field vibrates. Under normal conditions, these vibrations occur in many different orientations. Polarised light is different because the vibrations become organised into a specific orientation or pattern.
A familiar example is a pair of polarised sunglasses. These work by blocking light vibrating in certain directions, reducing glare reflected from surfaces such as water, roads, and snow. While humans can use technology to filter polarised light, we cannot naturally see the patterns it creates.
Many marine animals produce polarised reflections from their bodies, creating visual signals that remain hidden from species unable to detect them.
Peacock mantis shrimp can detect both linear polarised light and circular polarised light. Linear polarisation occurs when light waves are restricted to a single plane, rather like a rope being shaken only up and down or side to side. Circular polarisation is more complex, with the orientation of the light rotating as the wave travels, creating a spiral-like pattern.
The ability to detect linear polarisation has been found in several groups of animals, but the ability to detect circularly polarised light is exceptionally rare. Mantis shrimp are among the few animals known to possess it. As a result, they may be seeing patterns and visual signals on a reef that are entirely invisible to us.
Three Eyes in One
The structure of the mantis shrimp's eye is just as unusual as its ability to detect light.
Each eye contains a specialised central region known as the midband, which divides the eye into separate sections. This allows a single eye to observe the same object from multiple perspectives at the same time. As a result, each eye can estimate depth independently, something that most animals normally require two eyes to achieve.
This unusual arrangement has led some researchers to describe each mantis shrimp eye as functioning almost like three eyes combined into one.
Eyes That Never Stop Moving
If you watch a mantis shrimp closely, one of the first things you notice is that its eyes are almost constantly moving. Each eye sits on a mobile stalk and can rotate independently of the other. While one eye is looking in one direction, the other may be focused somewhere completely different.
This allows the animal to scan a large area without moving the rest of its body, an advantage for both hunting and avoiding predators on a busy coral reef.
Built for Life on a Reef
Coral reefs are visually complex environments filled with colours, patterns, moving shadows, and reflections. Scientists believe the mantis shrimp's visual system may help it identify prey, recognise rivals, communicate with potential mates, and navigate this complex environment.
Exactly how a mantis shrimp experiences the world remains uncertain. What is clear, however, is that it is not simply seeing a sharper or more colourful version of the world that humans experience. Instead, it appears to be detecting information that is completely inaccessible to us, making its visual system one of the most unusual ever discovered.