Light in the Darkness: The Mystery of Bioluminescence
Imagine diving into pitch-black waters, thousands of feet below the surface—and then seeing lights—a myriad of moving lights from the strangest creatures you ever saw. Flickers, glows, pulses of eerie blue and green. Like stars scattered in the deep sea. Welcome to the world of bioluminescence, nature’s living light show.
What Is Bioluminescence?
Bioluminescence is the ability of living organisms to produce light through a chemical reaction. In the ocean, it’s everywhere—from microscopic plankton near the surface to bizarre fish that haunt the deepest trenches. It’s estimated that over 75% of animals in the deep ocean are bioluminescent in some form. In this dark world, light is language, lure, and lifeline.
The basic chemistry involves a light-emitting molecule called luciferin, and an enzyme called luciferase. When combined with oxygen, they produce a cold light—meaning that it generates almost no heat.
But the magic is not in how it's made. It’s in why.
Why Do Sea Creatures Glow?
Light is survival in the deep. Some fish glow on their undersides to match the faint light from above—a trick called counterillumination. It helps them disappear from predators below. Others use light to attract prey. Some, including certain fish and squid, use flashing patterns to find mates or to signal to others of their kind. And others use it for defence. For example, deep-sea shrimp eject glowing clouds to confuse or distract predators:—an underwater flash bang.
The variety of bioluminescent life is staggering. For example dinoflagellates are microscopic plankton that create glowing waves in tropical seas when disturbed. It is often the first bioluminescence that divers see.
Comb jellies
Though comb jellies (ctenophores) use light, producing shimmering, rainbow-like pulses, it is not true bioluminescence. The light is reflected in a phenomenon called structural colouration, caused by diffraction. Comb jellies have rows of tiny, hair-like structures called cilia arranged in a pattern that resembles combs; thus their name. As they swim, these cilia beat in coordinated waves. When light from the environment strikes them, such as the light from bioluminescent animals near them, a diver’s torch, or faint sunlight in shallower depths, the movement of the cilia diffracts the light, breaking it up into different wavelengths—much like a prism or the iridescent sheen on soap bubbles or oil slicks. The result is a beautiful, shifting rainbow effect that looks like it’s glowing, even though it does not come from an internal chemical reaction.
Anglerfish – The Deep-Sea Lurker with a Lantern
Perhaps the poster creature for deep-sea bioluminescence is the female anglerfish. She looks like something from a nightmare with her wide, toad-like mouth filled with translucent needle-sharp teeth, her bloated body, and the dangling spine that protrudes from her forehead. At the tip of that spine is a bulb of glowing bacteria—a living fishing lure. She waves it gently to attract prey in the deep blackness, then strikes with terrifying speed. Males, by contrast, are tiny and often fuse to the female’s body, becoming little more than reproductive appendages.
Firefly Squid – A Living Light Show
Only about the size of your hand, the firefly squid lives in the waters around Japan and is famous for its electric-blue light displays. Tiny glowing dots, called photophores, cover its body—especially on the tentacles and around the eyes. In the spring, they gather in the millions near the surface to spawn, turning the ocean into a shimmering, pulsating galaxy. The light is used for communication, camouflage, and for dazzling would-be predators.
Lantern fish – Twinkling Travellers of the Twilight Zone
Lantern fish are small, silvery fish that inhabit the mesopelagic zone, 200 to 1,000 meters down. They are named for rows of glowing photophores along their sides and bellies, which emit a soft blue-green glow. These lights help them blend in with the faint light from above, making them invisible to predators below. At night, they rise to the surface in vast numbers, which is the largest daily migration of vertebrates on Earth.
At night, many species migrate upward to feed, then return to the safety of deeper water by day.
Atolla Jellyfish – The Alarm Bell of the Deep
This jellyfish, also called the “alarm jelly”, is deep red with a crown-like shape and a trailing tentacle. When threatened, it flashes rapid rings of blue light in a circular pattern, like a deep-sea siren. This bioluminescent “burglar alarm” is believed to attract larger predators to scare off whatever is attacking it. Visually, it’s like watching an underwater UFO light up and spin away into the darkness.
Tomopteris – The Glowing Bristle Worm
This delicate, transparent marine worm drifts gracefully through the open ocean with paddle-like appendages. When disturbed, it releases brilliant golden-yellow bioluminescence—a rare colour in the deep sea. Its glowing segments flash like tiny lanterns, possibly confusing predators or warning them off. Scientists still aren’t entirely sure how this worm produces yellow light, making it a subject of ongoing fascination.
Ostracods – Tiny Architects of Light Displays
Barely larger than grains of sand, these crustaceans create complex light patterns during courtship. In Caribbean waters, males of certain species release glowing blue trails in rhythmic bursts, creating patterns in the water to attract females—essentially an underwater mating dance in Morse code. These patterns are precise and species-specific, almost like living calligraphy in the sea.
The Vampire Squid (Vampyroteuthis infernalis)
Despite its fearsome name this creature is neither a squid nor a bloodsucker. It belongs to its own ancient lineage and is actually a gentle scavenger of the deep. It is a deep red or purplish-black colour with a soft, gelatinous body about the size of a football, and webbed arms that form a cloak-like membrane. Its eyes are among the largest in the animal kingdom relative to its body size, glowing electric blue or red depending on the surrounding light. Its name comes from the cloak it can pull over itself, which looks like a cape.
Instead of squirting ink when startled like other cephalopods, the vampire squid uses bioluminescent displays to confuse predators. Its body is covered in light-producing photophores, which it can pulse in mesmerising patterns. When threatened, it performs a dramatic defensive behaviour called “the pineapple posture,” turning inside-out and revealing flashing arm tips that eject glowing mucus laced with blue light that looks like underwater fireworks.
The vampire squid lives in the oxygen minimum zone, between 600 and 900 meters in depth, where few animals can survive. Instead of chasing prey, it extends two long, sticky filaments to gather “marine snow”—bits of organic material drifting down from above.
Hatchetfish
The deep-sea hatchetfish is named for its distinctive hatchet-shaped body, with a flattened silver form that gleams like polished chrome. It is small—usually only a few inches long—with large, upturned eyes for spotting silhouettes above them. Their flattened, silver sides act like mirrors, helping them vanish in the dim, blue twilight of their mid-water world. They drift with a slightly tilted posture, often motionless, like metallic leaves suspended in a vast, blue-black void. Hatchetfish use rows of photophores on their bellies to perform counterillumination—matching the faint light from above so that they disappear when viewed from below. They can adjust the brightness and distribution of this light in real time, depending on the ambient light levels—a remarkable example of evolutionary precision.
Hatchetfish inhabit the mesopelagic zone, 200 to 1,000 meters deep, where light is minimal but not entirely absent.
In the deep sea, where light is rare and predators are everywhere, survival often depends on mastering the dark with light.
Science, Art, and Mystery
Scientists have only just begun to understand bioluminescence. How do some animals control it so precisely? How did such complex systems evolve so many times across different species? Researchers are now using these natural lights to inspire everything from medical imaging to bioluminescent sensors that can track environmental changes. Artists and film makers use it to evoke wonder and mystery—and it’s easy to see why.
Every flash of light in the ocean is a message, a defence, or an invitation. But above all, it’s a reminder that even in the darkest places, life finds a way to shine.
author of The Shark Sessions
