How and why do octopus change colour?

​While reviewing some marine survey videos lately I found myself mesmerised by watching an octopus move across my survey area, settling and changing colours before moving again and changing yet again. The colour change was so rapid and the octopus completes three quick changes before scooting off out of the video.  It got me to thinking that colour changing is a pretty cool skill to have. The most us puny humans can manage is to develop a tan, producing melanin in the skin to try and prevent the damage that UV radiation can cause.

Colour in cephalopods (octopus and squid) depends on 4 different types of cells.  The first layer of colour controlling organs in the skin are the chromatophores.  Each chromatophore consists of a small balloon like sac filled with pigment. Each sac is connected to around 20 muscles, and each muscle is controlled by 2-6 nerves linked directly to the brain of the octopus. The octopus can stretch the balloon-like sac and allow the pigment to cover a large surface area, so we get to see the black, brown, orange, red or yellow colour just under the surface of the skin. When the muscles around the sac relax, it shrinks and the colour is hidden.  Chromatophore sacs are individually controlled so the cephalopod can control which colours are displayed and where, hence the patterns seen in cuttlefish.  Deep water cephalopods have very few chromatophores as colour isn’t much use in an environment with little light.

The next layer of colour organs under the chromatophores are the iridophores. Iridescence is the property of luminous colours that change depending on the angle they are viewed from. Iridophores are the key to how cephalopods create the metallic green, blue, silver and gold colours. Iridophores work by reflecting light from stacks of very thin cells.  It’s not certain how iridophores are controlled, but they are slower to respond than chromatophores so it’s unlikely to be controlled by nerves but more possibly by hormones.

Then there’s the leucophores. These are cells that scatter full spectrum light, so they appear white. In fact, they will reflect any light that is shone on them, and the light doesn’t change with the angle that you view at. It’s thought that having leucophores underlying the chromatophores increases the intensity of the colours that we observe. Leucophores also help with the cephalopods ability to colour match because they reflect the surrounding light.

Cephalopods have 3 types of specialised colour creating organs in their skin to mimic their background for camouflage and communicate. The cephalopod eye is remarkably similar to a vertebrate eye consisting of an iris, lens and photoreceptor cells. The similarity is often cited as an example of convergent evolution, both vertebrates and cephalopods need to observe their environment and they have solved how to do this in a similar way. But there is a critical difference, cephalopods are colourblind, so their eyes only see in black and white. How on earth does that make sense? An animal with the ability to make a myriad of colours, metallic sheens and mesmerising patterns can’t actually see in colour?

The explanation for this apparent contradiction is that the cephalopod eyes have wide pupils in a strange variety of shapes, U-shaped, W-shaped or dumbbell shaped. When light passes through the wide pupil, the lens in the eye acts as a prism and splits the light into different colours, a large pupil allows for more splitting, known as chromatic aberration.  Cephalopods use their wide pupils to create the maximum chromatic aberration and focus on these different wavelengths by changing the depth of their eye ball (altering the distance between the lens and the retina). So, cephalopods can detect colour, not by using special proteins embedded in cells in the retina (like we do) but by changing whether the light focusses on the retina at all. They find it easy to focus distinguishing between bright and dark colours, so that probably explains why display patterns are usually colour separated by black bars.
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But if a cephalopod can’t really see so well, how on earth do they mimic their environment? The secret to this lies in the presence of opsin (light detecting protein) in the skin. Its thought that its possible for some cephalopods to sense how much ambient light is present across their periphery and adjust their skin colour and brightness accordingly. To camouflage yourself, you don’t have to be a perfect match for your surroundings, you just have to match it slightly more than your predator can distinguish.