Why do mantis shrimp see colours humans cannot imagine?

Their eyes pick up parts of light we barely notice.

Mantis shrimp live on bright, shallow reefs where light keeps changing. It bounces off sand, flashes off fish scales, and scatters through water. To deal with all that visual noise, their eyes are built to collect more kinds of information than ours.

A special band in the eye does the heavy work

A strip across the middle of the eye is packed with receptors that handle different parts of the spectrum and other light cues.

Humans rely on three main colour channels. Many mantis shrimp species have far more photoreceptor types, including ones that can detect ultraviolet. That means they can register parts of the spectrum that are invisible to us. On top of that, some mantis shrimp can detect polarised light. Polarisation is not a colour. It is a property of light waves. Underwater, it can help reveal outlines and shiny surfaces, and it can carry signals between animals.

Polarised light can act like a hidden contrast tool

It can make certain surfaces and edges stand out even when normal colour contrast is weak.

The interesting twist is that having many receptors does not necessarily mean they make finer shade comparisons than humans. Evidence suggests their system is tuned for quick recognition. Instead of carefully comparing tiny differences, they may sort visual input fast, which is useful when you need to react in a split second.

Ultraviolet adds another channel of detail

In shallow seas, UV can help with spotting reflective prey and reading markings on other animals.

So when people say mantis shrimp see colours humans cannot imagine, the point is not that they see a prettier rainbow. They pick up extra layers of light information, and their eyes are built to read it quickly.