Mantis shrimp have at least 12 types of colour receptors. Humans have just three.

Human colour vision relies on three types of cone cells, sensitive to red, green and blue light. Our brains compare signals from these cones to see millions of colours. Mantis shrimp eyes are far more complex, with 12 colour receptor types, plus sensitivity to ultraviolet and to polarized light, which humans cannot detect.

You might expect that to give them incredible colour vision. But a 2014 study published in Science tested mantis shrimp and found they struggled to tell apart colours that were close together, doing worse than humans. The researchers suggested their eyes may recognise colours directly, each receptor acting like a quick colour label, rather than comparing signals as our brains do.

This fast system could help them react instantly in busy coral reefs.

Mantis shrimp eyes sit on mobile stalks and can move independently. Each eye is divided into three sections, giving it depth perception from a single eye. Biologists, including Justin Marshall and Thomas Cronin, spent years mapping the receptor types in these eyes, revealing their unusual complexity.

In the 2014 Science study, Hanne Thoen and colleagues trained mantis shrimp to pick specific colours for a food reward, then tested how similar colours could be before the animals got confused. The answer surprised many, overturning the idea that more receptor types automatically mean better colour discrimination.

Each mantis shrimp eye has a central band of specialised rows that handles colour and polarization, flanked by two halves.

Because the three regions look at the same spot, one eye alone can judge depth, a trick humans need two eyes for.

Colour vision varies hugely across animals. Many mammals, including dogs, have only two cone types. Birds, reptiles and many fish have four, often including ultraviolet. Mantis shrimp are the extreme case, showing that evolution can build colour vision in very different ways.

Polarized light detection is useful underwater. It can reveal transparent animals, reduce glare and may help mantis shrimp communicate with each other through patterns on their bodies that only they can see.

Engineers have studied mantis shrimp eyes to design cameras that detect polarized light, with possible uses in medical imaging such as spotting cancerous tissue.