More Than Meets the Eye: How the Mantis Shrimp Sees a World We Can't Even Imagine!

As humans we tend to assume that everything we see around us is the real, complete picture. We can distinguish the colours and shapes in front of us. It’s objective, surely? But in reality, everything we as a species see is entirely shaped, and limited, by our biology. 

What we see is only ever what our eyes have evolved to detect, and some animals have evolved to detect much more than others. The most impressive vision system of them all belongs to a family of kooky crustaceans living in burrows on coral reefs around the world—the mantis shrimp!

First, an important clarification. Despite the name, this animal is neither a mantis nor a shrimp—it's a stomatopod (a weird distant relative of crabs and lobsters). It's also, famously, capable of throwing a punch fast enough to boil the water around it. But physics-defying underwater combat aside, another incredible feature of this animal is its eyes.

To appreciate what makes the mantis shrimp's eyes so extraordinary, it helps to understand what they've evolved to detect in the first place: light itself. Light travels in waves of different sizes (wavelengths), ranging from the long waves used for radio signals to the tiny, high-energy waves of gamma rays. Scientists call this entire range the electromagnetic spectrum. The part that we humans are actually capable of seeing is a narrow slice sitting in the middle, called visible light.

That little segment is hugely important to us. It’s what we’ve evolved our entire existence around, and it drives almost every food chain on Earth. But it’s worth noting that while visible light is our ‘human’ segment, every species on the planet has had to find its own way of dealing with light, or the lack thereof—and no two species do it quite the same way! 

Our eyes rely on two kinds of light-sensing cells: rods for seeing in dim light, and cones for seeing colour. We have three types of cones (colour-sensing cells), tuned roughly to red, green and blue wavelengths, and every colour we've ever experienced comes from our brain blending signals between just those three.

Other animals are working with an entirely different toolkit. For example, dogs have only two types of cones, but far more rods, which gives them better night vision! Birds have four types of cones, letting them see ultraviolet light, which is completely invisible to us. Vision, as it turns out, isn't one universal experience shared across the animal kingdom. It has evolved differently, animal by animal, need by need.

Which brings us to the mantis shrimp, an animal that has taken this idea to unfathomable heights!

Mantis shrimp eyes are widely considered the most complex in the animal kingdom. Each eye sits on its own stalk and works independently of the other, constantly scanning for prey and threats. Where humans have three types of colour-sensing cells, the mantis shrimp has up to 12 (some of which allow them to see ultraviolet light). Now you’d think this would make them the best colour-spotters on the planet. But curiously, it doesn't!

Researchers trained mantis shrimp to recognise particular colours in exchange for food, then tested how well they could tell colours apart. The results were surprising, as they found that the mantis shrimp could only distinguish colours that were quite far apart on the spectrum. We humans, on the other hand, working with only three cone types, can pick apart colours far more similar to each other, and can distinguish between millions of different shades.

So why so many receptors for such unremarkable colour vision? Scientists haven’t quite figured that one out yet. One theory is that each receptor type only recognises one specific colour outright, rather than blending signals the way our brains do. So instead of seeing blue-green, they see only blue or green. It could be a trade-off between specificity and speed (pretty useful when you need to instantly tell the difference between predator and prey).

It's a nice reminder that having more receptors doesn’t automatically mean better vision.

Colour is only half the story. Mantis shrimp also have a further four photoreceptor types dedicated entirely to detecting polarised light, bringing their grand total to 16. That is more than any other animal known to science!

What’s so special about polarised light? It is light that vibrates along a single, uniform plane—it is perfectly organised! Normal (unpolarised) light is chaotic, vibrating wildly in every direction. Humans can't actually perceive this alignment at all, but we do notice its side effects. For example, the blinding glare reflecting off water is largely polarised light. But our eyes aren't detecting the direction. We just notice the intense brightness. To combat this, polarised sunglasses act like a fence, blocking that specific direction to instantly wipe out the glare.

Mantis shrimp can even detect polarised light that spirals in a circle (circularly polarised light), an incredibly rare superpower in the animal kingdom. But it is far more than just a cool party trick. The practical implications become exciting when you consider what else reflects polarised light differently: cancer. 

Cancerous tissue reflects polarised light differently to healthy tissue, making it a promising way to spot cancer early, non-invasively. The catch has always been that human eyes, and ordinary cameras, simply can't see polarisation to take advantage of this fact.

It's this exact ability that inspired engineers at the University of Illinois, led by Viktor Gruev, to build cameras modelled directly on the mantis shrimp's eye. His team's technology, paired with drugs that stick to cancer cells and glow under a particular kind of light, now helps surgeons see precisely where cancer sits inside a patient's body, in real time!

A visual trick that evolved on a coral reef is now helping save lives in operating theatres. The mantis shrimp never "meant" to inspire medicine, but by building eyes for a very different job, evolution accidentally created a blueprint that human engineers could borrow.

The mantis shrimp isn't alone in inspiring brilliant inventors, either! Nature has spent millions of years testing solutions to problems, and biomimicry, the practice of learning from nature to design human technology, lets us borrow the best of them. Velcro was inspired by burrs clinging to fur, and high-speed trains have been reshaped by studying how a kingfisher dives smoothly into water. Each began with someone looking at the natural world through different eyes. Our own view is just one slice of what's out there, so the next time you watch a mantis shrimp, or any animal, consider what it can do that we can't. 

Siôn Davies

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