Fireflies blink their lights. In the ocean, it’s more dramatic.

Deep down in the ocean, there is no red.

The ocean is so deep and so filled with water, that even light has to fight to make its way down there. As the rays travel, they bump into the tiny particles of dust and water they find on the way. They get spread out, redirected; and less of them manage to reach the lower levels.

Some light particles are stronger than others. They have more energy in them. They can push their way through. Those particles get less spread out, less thrown off course. They get to travel further down into the ocean.

You can make out how powerful light-particles are just by looking at them. Your eyes and brain colour-code them for you, so you can make out their power by looking at their colour.

The weakest of those colours is red.

Red light is too weak to reach the depths of the ocean. It gets scattered long before it gets there. At the bottom of the ocean live creatures who have never seen red, and who never will see it. They physically cannot. Because there isn’t any red to see, their eyes never adapted to be able to see it.

That’s why, deep down in the ocean, there is so much red.

Things appear coloured because of the coloured light that bounces off them. Green things look green because they absorb all colours of light except green. Red things look red because they absorb all colours except red.

You see things as those colours, because those are the only colours coming out from the thing and into your eyes. But down in the ocean, there is no red light to bounce off. Absorbing colours except red is the same as absorbing all colours.

In the ocean, colouring something red is the same as colouring something black. It will be dark. Camouflaged. Invisible. The small fish that wants to hide, the hungry fangtooth waiting to pounce, they could all be red instead of black. It’s only a matter of chance.

In some parts of the ocean, food is not a major problem. It’s not even a minor problem. You could say it’s not a problem at all.

In shallow seas, where sunlight can easily touch the bottom, the water is full of life. Tiny plant-creatures float all through, feeding on the sunlight. They in turn become food for other, larger creatures. Larger plants grow up from the seabed, adding to the food supply.

In those places, there’s no question of “getting food”. All you have to do is open your mouth and eat.

Elsewhere, things are different. The light supply fades away as you go deeper into the ocean. These are the places where no plants can grow. The creatures living here are few and far between. They eat what little bit falls down from above. Tiny animal-creatures have to make their energy without light.

In these places, fish have evolved to snap up whatever they can get. Anything they can lay their teeth on becomes food. The backswallower, like so many deep-sea fish, has an expandable stomach. With its huge, flexible jaws, it can gulp up creatures larger than itself.

Other fish have even bigger jaws. The teeth of the viperfish are so big, they remain sticking out even when its mouth is closed.

In these places, fish don’t move fast. They don’t waste energy chasing each other around. Instead, they stay in one place, saving their energy, or moving slowly till something interesting drifts by. Then, they pounce.

Deep-sea dwellers have found a way to see in the dark. Not much light can reach from above — but they can still make their own!

Many ocean species make their own light, they same way as fireflies do. But some of their uses are much more dramatic.

When attacked, the green bomber worm lets out a bright blast of light, dazzling its enemy so it can make a quick getaway. The deep-sea squid, on the other hand (or rather, many other hands) disconnects its glowing arms. It escapes and leaves them behind, clinging onto the attacker to distract it.

Having lights makes you visible, but it can also make you invisible. The toadfish lights itself to match the sunlight from above the water. From underneath, it looks like just another bit of water with sun coming from above. This trick of the toadfish is known as “counter-illumination”.

The cookie-cutter shark takes it even further. Part of its underside glows in a small, fish-shaped pattern. When other fish get attracted to the glowing shape, they become easy prey for the shark to pick off from.

What about the ocean depths? Yes, fish use light there, too. They use it to find each other. They use it to see what’s ahead. And they use it to trick people into thinking they’re what they aren’t.

The deep-sea dragonfish has a glowing ‘barble’ sticking out from its chin as bait. When things come to eat it, they’ll swim straight into the dragonfish’s waiting mouth.

To make themselves invisible, some dragonfish have see-through bodies. They’re almost invisible. But what about the fish they eat? A lot of them will be glowing, too. Will you see them, going through the dragonfish’s body, as they get digested as food? The black dragonfish has a black stomach, to prevent just that.

One kind of dragonfish is unusual not because of how it looks, but because of how it sees.

Many deep-sea creatures have a light shining ahead of them like a torch. The Malacostus, or “stoplight loosejaw”, is no exception. Most of these torch-lights are bluegreen and greenblue, the same colour as what shines down from above.

The Malacostus dragonfish, however, has another trick up its sleeve. It can also make another kind of light, a kind otherwise unseen in the ocean depths. When it switches on this light, the dragonfish can see all and none can see it. This invisible light is what the dragonfish uses when it wants to travel in secret.

Interestingly, the dragonfish can’t see that special light directly. Its eyes need to use a special thing called chlorophyll, which catches the light and lets the eyes detect it’s there.

Other creatures use chlorophyll to catch light too, but it’s usually for a different reason: it’s what plants use to catch sunlight. The stoplight loosejaw and a few other dragonfish are the only ones known to use it for something else. They’re the only ones using it to look at lights they can’t see.

And what colour is it? What colour is this invisible light? It’s one of the deep ocean’s most invisible colours: Red.

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