Mantis Shrimp: proud owners of the strangest eyes in the animal kingdom The human eye is one of the many organs which separates us from the animal kingdom. Its evolution over hundreds of thousands of years allows us to detect minute changes of colour in our surroundings, lending us unique perceptive abilities compared to many of the species we share the world with. But there are plenty of species whose eyesight would make very little sense to us, the most unusual of which is the mantis shrimp.

The eyes of the mantis shrimp have taught us about underwater sight, compound eyes, hexnocular vision and has even led to the reverse-engineering of cameras which detect polarised light. All of that while maintaining its reputation for being not only beautiful, but one of the most creatively violent animals on earth!

So, what is this critter, and why does it have such unusual eyes?

What is the Mysterious Mantis Shrimp? The mantis shrimp is an aggressive, carnivorous shrimp species which predominantly lives in the tropical seas of the Pacific. Residing in sand burrows, it comes in around 50 varieties, but these are split into two groups: the ‘smashers’ and the ‘spearers’ - determined by the way they kill their prey.

Compound Eyes As territorial and highly active hunters, their vision is pivotal to their survival. They spend a large portion of each day surveying their surroundings for the faintest movements, responding instantaneously. They have compound eyes similar to a bee or fly, with thousands of small openings, each with tiny ocular organs inside. These feed directly to the mantis shrimp’s nervous system, rather than their brain - reducing processing time, and making their reaction times incredibly fast.

Hexnocular Stalks Their eyes extend from two independently controlled stalks, meaning the shrimp can look in two opposing directions at the same time. Each eye has the equivalent of three pupils, giving the shrimp remarkable hexnocular vision.

In fact, the word hexnocular only applies to this creature. A one-of-a-kind term, for a one-of-a-kind visual system!

The Colour Question Their eyes can interpret a wider dynamic-range of colours, because they possess 12 photoreceptors, compared to our measly 3. These are the channels through which we see different kinds of light. We can’t see ultraviolet light, or polarised light, but the mantis shrimp sees multiple varieties of each.

Interestingly however, the mantis shrimp is not as good as humans at detecting small differences between similar colours. For example, we can see differences in shades of yellow between lemon, amber and gold, whereas the mantis shrimp will only begin to see a change once the wavelength matches orange. Seeing more colours is advantageous to our perception of the world, but the mantis shrimp must be able to detect life as quickly as possible, so those small colour differences don’t matter as much.

Using Polarised Light Polarised light sensitivity is difficult to understand, and even more difficult to imagine seeing ourselves. Polarised lightwaves only vibrate in one direction - perfect for seeing reflections underwater. Humans only see a fraction of this light, and we interpret it as glare, making it useless to us. Mantis shrimp see both linearly polarised light and circularly polarised light, which has amazed scientists since its discovery. This allows the shrimp to see prey which is further away and communicate with their neighbours or mating partners more accurately.

In recent years, marine biologists and computer scientists have collaborated to develop cameras with mechanical layouts based on the biology of the mantis shrimp. They have already emerged in two impactful areas of medical research, where the polarised light reflecting off human tissue reveals more biomechanical information than light from the visible spectrum. Initially, they have been useful for detecting cancerous tissue in hard-to-reach areas inside the body. Furthermore, they are being used to test the strength of knee-ligaments and their replacement materials, in order to optimise patient care after an operation.

Marvels of the Natural World The mantis shrimp is an extraordinary example of the evolutionary process. It has the widest ranging vision of any animal so far discovered in the animal kingdom and has used that to cultivate a unique role in multiple environments.

While we wouldn’t want to lose our incredible colour dexterity, the already wide-ranging medical applications of polarised light, show us just how important it is to be curious about the creatures we share the world with.

Who knows what else is waiting to be discovered? Whatever it is, we’ll certainly be using our own amazing eyes to find it.

Sources:

1. NHM.
2. KQED.
3. The Atlantic.
4. Scientific American.
5. Science Direct.