We’ve all seen those hypnotic videos of colour-changing animals – a cuttlefish pulsing stripes across its body, a chameleon melting from green to gold, or an octopus vanishing into coral like a magician’s smoke bomb. Their skin shifts hues like it’s nothing. But how do they actually do that?

Take starfish, for instance. They don’t seem to have eyes, yet somehow they “know” what their surroundings look like. Cephalopods, your octopuses, squids, and cuttlefish, go even further, creating patterns that match their environment with uncanny precision. How can they pull that off if they can’t even see any details around them?

Seeing Without Eyes?

A chromatophore is a specialised cell found in animals, and even some bacteria, that contains pigment or reflects light. You’ll find them across the animal kingdom: in fish, frogs, chameleons, and even in certain bacteria (yes, microbes get to have fun too). Depending on the species, chromatophores come in different flavours. Some are pigment-based, like those filled with melanin (the same as in human skin), while others use microscopic structures to bend and reflect light, acting like natural nanotech (1).

Under white light, chromatophores are often classified by the colour they show off – red, brown, blue, green, and the iridescent in-betweens. In vertebrates like fish and reptiles, these cells sit in neat layers under the skin, filtering and bouncing light to produce a kaleidoscope of shades.

Chromatophores 101: Nature’s Colour Cells

In creatures like octopuses and cuttlefish, chromatophores are tiny, elastic sacs filled with pigment. These sacs are surrounded by radial muscle fibres which are wired to the nervous system. When the animal wants to display a colour, it sends a signal that contracts those muscles, pulling the pigment sac open like an umbrella. The expanded pigment becomes visible on the surface. Relax the muscle and the sac snaps shut – colour gone!

So instead of pigment just sitting there passively, the cephalopod is actively controlling its skin colour with muscle contractions, at speeds fast enough to create those mesmerising rippling patterns. All these changes are actively, neurally controlled; they're not automatic like blushing. They're often voluntary, and dynamic, responding to things like light, mood, temperature, and stress (2). 

In fact, cephalopod chromatophores are sensitive to direct electrical stimulation. One study found that when researchers applied oscillating electrical patterns to the squid Sepioteuthis lessonia, the pigment sacs expanded and contracted in synchronised, wave-like patterns under 1.5Hz; essentially, we can rhythmically ‘play’ these cells like an instrument! (1)

Chromatophores in vertebrates work a bit differently. Instead of opening and closing sacs, the pigment inside the cell moves around, spreading out when the colour needs to be more visible, clustering together when it doesn't. Still responsive, still cool, just a little less… flashy.

Layers, Pigments, and Light Tricks

Here’s where things get really interesting. Chromatophores aren’t all for show. They’re sensitive to light, chemistry, and electrical signals, which makes them incredibly valuable for science and technology! Some fish chromatophores, for example, visibly change colour in the presence of toxins like cholera and pertussis. They detect these threats in real time, with the colour change varying with concentration, meaning you can even tell how much of a toxin is there, not just whether it is present (3). That makes them powerful candidates for biosensors, living tools that can monitor environmental or biological conditions.

Why is it a big deal?

Unlike traditional sensors made of synthetic materials or inert components, chromatophore-based systems are made of living cells. They keep reacting, adapting, and functioning over time, giving them an edge in sensitivity, flexibility, and longevity (2).

While chromatophores already act as living, colour-changing pixels, researchers are exploring how to use them in adaptive camouflage technologies. Imagine a bandage that shifts colour when it detects infection, the moment bacteria start to grow, not just after the infection has spread. Or ocean sensors that monitor salinity and pollution, while blending seamlessly into coral reefs so as not to disturb marine life. All of these possibilities are made an achievable reality by these remarkable sacs of pigment! 

These amazing cells offer a glimpse at what happens when evolution builds something both beautiful and functional. Next time you see a chameleon vanish into a leaf, or an octopus ripple with light like a living mood ring, take a second to think about what’s really going on under the surface. Behind every colour shift is a tiny symphony of biology and physics, all working together in real time. And the best part? It’s still magic. It doesn't stop being magic when we figure out how it works! 

References

1. Lei Y, Chen W, Mulchandani A. Microbial biosensors. Analytica chimica acta. 2006;568(1-2):200-10. doi: 10.1016/j.aca.2005.11.065

2. Tan L, Schirmer K. Cell culture-based biosensing techniques for detecting toxicity in water. Current opinion in biotechnology. 2017;45:59-68. doi: 10.1016/j.copbio.2016.11.026

3. Plant TK, Chaplen FW, Jovanovic G, Kolodziej W, Trempy JE, Willard C, Liburdy JA, Pence DV, Paul BK. Sensitive-cell-based fish chromatophore biosensor. InBiomedical Vibrational Spectroscopy and Biohazard Detection Technologies 2004;5321;265-274. doi: 10.1117/12.528093

   
   

4. Kim T, Bower DQ, Deravi LF. Cephalopod chromatophores contain photosensitizing nanostructures that may facilitate light sensing and signaling in the skin. Journal of Materials Chemistry C. 2025;13(3):1138-45. doi: 10.1039/D4TC04333B