In a world without sun, the animals of the deep sea have no choice other than to illuminate their own lives and glow in the dark.

Light in the abyss:
Sunlight only penetrates the top 100 metres (330 feet) of the ocean, and animals that live below the reach of the Sun’s rays experience constant darkness. There are several zones to the ocean from the sunlight or photic zone at the surface all the way down through the twilight and midnight zones to the abyssal zone at 4,000 to 6,000 metres (13,100 to 19,700 feet) deep. This water is completely devoid of light, but is home to hundreds of weird and wonderful marine species that humans have had to develop highly specialised equipment to even glimpse. Animals have even been found living below the abyssal zone, at such dark depths that it’s almost unsurprising that some have evolved to generate their own light.

“At such dark depths it’s almost unsurprising that animals have evolved to generate their own light”

Bioluminescence, a natural glow produced by an organism, is a chemical process involving a substance called luciferin. When this molecule comes into contact with oxygen it produces a chemical called oxyluciferin and the by-product is light. While only a handful of terrestrial animals have this capability, it’s relatively common in the sea. where it has evolved numerous times. This demonstrates how crucial it is to be able to produce light, and for some species it’s the difference between survival and death.

Who glows there?
From recycling the Sun’s rays to complex chemical reactions, there are various types of glow

Bioluminescence: Creating light from darkness
The is used to describe light than is produced by a chemical reaction inside an organism. The vital substance in this reaction is luciferin, which can react with oxygen to produce light. The process is aided by the enzyme luciferase, but doesn’t need an external source of light, which is why deep-sea creatures like anglerfish can use it to lure their prey. There are at least four different types of luciferin in animals, each with a corresponding luciferase. Some bioluminescent species rely on a supply of these molecules within their diet, as they are unable to create them.

Fluorescence: Simple light exchange
Unlike bioluminescence, this process relies on an external light source. The light is absorbed by atoms and re-emitted again almost immediately. If the light source disappears, so does the fluorescence. While being reflected, the photons lose energy, meaning the visible glow differs in colour to the light source. Butterfly wings contain fluorescent pigments that intensify their colour, to attract mates or repel predators.

Phosphorescence: The light that lingers
Like a glow-in-the-dark sticker, phosphorescent animals absorb light and use it to give off an eerie glow. The process is similar to fluorescence, but phosphorescent animals can ‘save up’ light and release it slowly. Like fluorescence, this depends on a light source, such as the Sun. However, the electrons become excited to a higher degree than in fluorescence and they release light slowly, at a lower intensity. Animals like jellyfish absorb light at the water’s surface, giving them a glow that alarms and repels predators.
Barbeled dragonfish use invisible light
“Prey species cannot see the predatory dragonfish conning”
The light this fish produces is on the border of the infrared spectrum. This is outside the visible spectrum of most fish, so prey species cannot see the predatory dragonfish coming. Along with hunting by invisible light, the dragonfish uses it to recognise friends. The dangling barbel structure plays a role in identification, and members of a shoal can distinguish one another simply by seeing the light.
At depths of 2,000 metres (6,500 feet) there are few other options for visual communication, and as their light is invisible to other species, they can shine to their friends as bright as they like.

Flashlight fish borrow bioluminescent bacteria
Without the ability to talk like humans, flashlight fish communicate by dazzling one another with repeated bursts of light. Their glow actually comes from a species of bacteria that lives in the fish’s light organ, located beneath each eye socket. They change the message they are conveying by increasing or decreasing the frequency of the flashes, covering or exposing the light organ by moving an eyelid-like flap. These headlights can also be used to escape attack; by shining at full brightness in front of a predator’s eyes, the fish can then dart away while the hunter is stunned.

Comb jellies reflect light like an underwater rainbow
Also known as sea gooseberries, comb jellies aren’t actually that closely related to jellyfish. They belong to a family of their own, known as ctenophores.
Glowing bright neon in the dark depths of the ocean, these oval jellies propel themselves through the water using a group of eight combs, or cilia, which they operate like the paddles of an oar. The combs shimmer in all colours of the rainbow in the dark as light hits and refracts from the surface.

Cockatoo squids cast no shadow
“They have opaque eyes that can give away their presence as they swim”
These incredible creatures are almost completely transparent, but they have opaque eyes that can give away their presence as they swim through the ocean. To prevent any shadows revealing them to predators passing below, the squids cast a bioluminescent blue glow from U-shaped cells beneath their eyes. This eliminates the silhouettes created by their eyes and makes it much more difficult for a predator to detect their exact position.

The bigfin reef squid has special light-bending cells
The bigfin reef squid is covered in special cells to make them stand out. Their heads and bodies are covered in chromatophores, which are light-reflecting cells giving them a metallic iridescent sheen. They are also one of the only squid species to possess leucophores, which are the cells responsible for reflecting the colour of the ambient light surrounding the squid. These two types of cells work in unison meaning the squid is able to produce a variety of vivid colour combinations and visually striking patterns.