To live at 2000 m beneath the surface of the sea is to drift through a boundless, cold night. Animals are born, live and die without ever seeing the sea floor or the sun, floating in a sort of starless cosmos.
The deep is the world’s largest habitat. Waters deeper than 1000 m make up nearly 80 per cent of the Earth’s biosphere, but life doesn’t teem here—even bumping into your own kind is a contingency.

Finding sustenance—or a mate—is a daily trial for most life, but for creatures in this abyssal dark, it’s a significant challenge. They must still be able to find food, and recognise, attract or avoid one another. The solutions they have found are nothing short of wondrous.

All up, around 90 per cent of life here employs bioluminescence, which occurs when a light-emitting molecule, called a luciferin, oxidises with some catalysing enzyme—either a luciferase or photoprotein. At the surface, luciferin in algae can light up the wake of a boat.

In the deep, however, bioluminescence allows for cues, communication, camouflage and countermeasures. In short, it lets life proceed. Organisms use light to hunt (as a lure, as a searchlight, or to confuse or stun prey) and to hide (as counter-illumination, as a smokescreen, or to startle attackers). Bioluminescence is such an important prerequisite for life in the darkness that marine biologists figure it has evolved inde­pendently at least 40 times.

But in the uncompromising realm of the deep, life has become an evolutionary arms race. Night vision, cloaking devices, stealth technology—what the human military has spent billions of dollars devising, these creatures have produced with little more than DNA and time.

Deeper than 200 m, sunlight is too feeble for photosynthesis to occur, so this unique ecosystem is founded on organic matter falling from above. As light fails in the deep, it does so wavelength by wavelength: red is the first to dim (which is why sea water looks blue), while deep blue persists to around a kilometre down. Consequently, the bulk of creatures at depth are black or blood red, making them all but invisible to predators or prey.

Some hunters have developed acutely sensitive, upward-facing eyes that can pick out the faintest of silhouettes against the barely perceptible glow from the surface.

Their prey have responded, either by becoming transparent—like many of the jellies and ctenophores—or by evolving banks of light-emitting organs (photophores) along their underbellies. They can switch them on to match the exact hue and shade of the faint blue surface light, so that their narrow bodies simply blend into the ambience. Studies of the midshipman fish, as well as certain squid, have revealed that they can match not just the colour of surface light, but its angle and intensity as well.

Even this disguise has been pre-empted by some predators, which employ modi­fied yellow lenses sensitive to the slightest mismatch between the colour of biolumines­cence and ambient light.

Most bioluminescence is emitted in a rather narrow band of blue-green wave­lengths, around 470 nanometers, which transmit furthest in water, so the majority of deep-sea fish have corresponding, mono­chromatic, colour sensitivity to those wave­lengths. But there are intriguing exceptions, such as the Malacosteus dragonfish, which uses a chlorophyll derivative to perceive infrared light, an otherwise imperceptible wavelength. No vertebrates are known to synthesise chlorophyll derivatives—instead, marine biologists believe Malacosteus obtains them from the copepods it eats. It hunts using a red-emitting photophore, like head­lights that are invisible to all but itself. Hapless prey have no idea they’re being stalked.

Little wonder, then, that animals have invested heavily in equally elaborate defences. Some startle pursuers with bril­liant flashes of bioluminescence. The green bomber worm launches grenades of bright light that explode like fireworks, blinding and confusing an attacker, or fooling it into lunging at the wrong target. Others take the ploy beyond simple evasion into reprisal. Some sea cucumbers bind predators in sticky threads that go on shining brightly for minutes, turning the erstwhile attacker into a brightly advertised menu item.

A more drastic variation is the “sacrificial tag”, an ejected tissue that can go on glowing for hours, even inside the predator’s stomach. In the deep sea, where invisibility means survival, that last meal can be the last. Even this cunning ruse has been countered, though: many predators have developed black- or red-pigmented guts (most red and orange pigments absorb blue light) to conceal such beacons, even when the rest of their body is transparent.

Another application of bioluminescence is the fishing lure. Anglerfish are the best-known practitioners, dangling a glowing bait (a colony of bioluminescent bacteria) in front of their cavernous mouths, but dragonfish­ along with some octopuses and squid—use a similar ploy. Certain planktonic jellies “jig” with bioluminescent tentacles, flicking them up and down in front of dangling thickets of stinging cells.

The cookie-cutter shark feeds by taking an opportunistic bite out of whatever it can, but researchers have never fully understood how it gets close enough to fast-moving prey such as fish and squid. Recently, they proposed an ingenious use of counter-illumination. Most of the cookie-cutter is illuminated by photophores, except for a dark band below its mouth. Seen from below, that lightless patch might resemble the silhouette of some prey species. When the fish or squid closes in on this black hole, the shark makes its move.

But bioluminescence isn’t always about pretending to be what you’re not—some­times it advertises what you really are. Carib­bean ostracods—small crustaceans—signal with light shows specific to each species, performing elaborate three-dimensional displays to court a mate. So-called “sneaker males” shadow displaying males, trying to exploit their efforts while saving themselves the energy of producing light of their own. Fire‑worms, pelagic octopods and ponyfishes also produce synchronised, gender-specific group displays.

Crushing pressure, numbing cold, weak oxygen loads, a paltry pantry: the demands of life at depth ensure this place will never host abundant life. But this harsh environ­ment hasn’t dimmed the inventiveness and resourcefulness of natural selection. Rather, it has resulted in some of Nature’s most inspired work.