We all know it's the festive season when we start noticing the classic red and white colours popping up everywhere - tree ornaments, Scuba Claus at the Aquarium, and good old Father Christmas. Things just wouldn't seem quite the same without the red and white to inspire our yuletide glee!

We know why humans lovered and white, but why do so many sea animals also have Father Christmas colours?

When Scuba Claus comes out of hibernation, it is a clear sign that the Festive Season is upon us!

Did you know that the red and white Santa suit was an invention of Harper's Weekly, a magazine popular during the American Civil War? This version of Santa was a small gnome who could slide down chimneys, and it wasn't until a Coca-Cola ad campaign in 1931 that Santa officially became a full-sized human.

Fish invisibility

Pure sunlight is white, a mixture of all colours on the colour spectrum. If you look at a white object, like a sheet of paper, what you're actually seeing is that the paper is reflecting the whole spectrum - and your brain tells you "Psst! It's white!"

Similarly, if you look at something green, like an avocado, what is happening is that all the wavelengths of light except the ones that make green light are being absorbed by the avo - so your brain interprets that reflected spectrum as "green".

Near the surface, or in artificial light like at the Aquarium, the crown squirrelfish stands out - there's no hiding with these Christmas colours. Fortunately, this fish is a smart hunter - only entering shallow water at night, and preferring dark caves and water as deep as 60m in the day.

Looking at a red and white fish, like the squirrelfish above, we can say that the white stripes are white because they reflect the whole spectrum, and the red stripes are red because they only reflect red and absorb everything else. But that's only true if we look at this fish at the surface in white light...

The jacopever (Helicolenus dactylopterus) might look like a Christmas stocking, but it is nearly invisible in the deep waters, up to 1km, where it hunts. Its white belly provides countershading, making it difficult to spot by animals looking up at it. Credit: NOAA Photo Library [CC BY 2.0]

What does water do to light?

The ocean looks blue to us because the molecules of water are particularly good at reflecting or allowing the transmission of light with short wavelengths. Sometimes this light is reflected back out of the ocean (which is thy the surface looks blue), and sometimes it passes to greater depths, which is why things look blue while you're underwater.

Long-wavelength light, what we see as red, also collides with water molecules, but it is not always reflected and is usually absorbed as energy by the molecules it strikes. The result of this is that the deeper you get, the less red light makes it - Santa would become invisible if he went this deep!

The red-and-white camouflage scheme is not unique to fish. Other marine animals also find this invisibility trick useful to avoid predators, like this South African camel shrimp (Rhynchocinetes durbanensis). Credit: Christian Gloor [CC BY 2.0]

Back to the fish...

Back to our squirrelfish friend - what does it actually look like in its natural habitat?

By a depth of 10m, almost the entire red part of the colour spectrum has been removed. Oranges and yellows are indistinct, and even greens become a bit duller. The squirrelfish begins to fade into the background.
By 30m, almost all colour except blue is removed from the spectrum. Because the squirrelfish doesn't have any blue pigments of its own, it fades almost entirely into the background. As long as this fish stays away from light-coloured surfaces where it will be in contrast, it will be almost invisible.

The red stripes won't look red at all - they will reflect very little light. Unlike a black stripe which would contrast against the water around the fish, the colour red still reflects the tiny amount of red light that is present. This has the effect of making the fish look invisible!

The white markings don't look white either. Depending on the fish's depth, more and more of the colour spectrum will be absent.

This scorpionfish combines its red colouration with a clever hiding spot to become an invisible ambush hunter on the seafloor. Only artificial light gives it away. Credit: NOAA Photo Library [CC BY 2.0]

Why aren't they just blue if camouflage is so important?

You would think that it would make sense for a fish to simply use blue for camouflage, instead of going through the complexity of using reds and whites. Indeed, many shallow-water reef fish are blue-coloured and this is an effective camouflage at shallow depths.

However, at depths, blue becomes a problem. Because a blue fish's skin reflects blue light very effectively, it may actually appear brighter than the surrounding water and make it more visible than a red fish at the same depths.

You might think that the colour blue makes good camouflage in a blue ocean - but not really. Blue fish, like the regal tang (made famous by Finding Dory) invest incredible resources in their blue colours not for camouflage, but for communication. Credit: Przemek Pietrak [CC-BY-3.0]

Blue pigments are also very difficult for plants and animals to produce and require incredibly complex chemical pathways to attain. From an evolutionary standpoint, there is no good reason for fish to have taken this complex, energy-expensive path when the far simpler red pigments would be an equally effective camouflage.

Reds, oranges, and yellows are common in nature because of a group of pigments called carotenoids. Carotenoids are made by thousands of types of plants, algae, bacteria, and fungi (both on land and in the ocean) to help with chemical reactions in their cells. Carotenoids dissolve in fat - so it is very easy for animals to absorb and concentrate these red pigments from their food. Salmon, sea snails, sea stars and crustaceans all make use of these pigments - and humans do too (as Provitamin A which aids our vision). Credit: Oregon State University [CC BY-SA 2.0]

Santa-coloured fish choose their décor to help them survive - please be mindful of your gift wrapping and decor this festive season so that you can help them to survive too. Avoid single-use plastic items where possible and cut up any loops of ribbon, tape or cord before tossing them away. Our fishy friends will thank you!

Not all animals use red and white to camouflage themselves at depth. The bumpy red and white body of the pygmy seahorse (Hippocampus bargibanti) blends in perfectly with the corals in inhabits - one of the most perfect examples of camouflage in the animal kingdom. Credit: Rickard Zerpe [CC BY-SA 2.0]

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