If you could switch off the sun and drop into the open ocean a few hundred metres down, you would not find darkness. You would find flickers, sparks and glowing patterns all around you, and many of them would belong to fish. Bioluminescence is one of the most common ways deep-sea fish solve the problems of finding food, avoiding predators and finding mates in a world without sunlight. This page introduces the main groups of glowing fish, explains how and why they make light, and highlights some of the strangest examples. For the wider topic, see our deep-sea bioluminescence guide.

How many fish glow?

Roughly 1,500 known fish species are bioluminescent, and the true number is probably higher because so much of the deep sea remains unexplored.

A 2016 study by Matthew Davis and colleagues, published in PLoS ONE, mapped bioluminescence across the family tree of ray-finned fishes and concluded that it had evolved independently about 27 times. That is a remarkable number. It means that light production is not a one-off invention passed down from a single glowing ancestor, but a solution that fish have arrived at again and again. The same study found that lineages with bioluminescence used for communication tended to split into more species, which suggests that light signals can help drive the evolution of new species in the deep sea.

Most glowing fish live in the mesopelagic or twilight zone, from about 200 to 1,000 metres deep, where a little sunlight still reaches but not enough for photosynthesis. Some live deeper, in the midnight zone, and a few live in shallow water on coral reefs.

Lanternfish: the most numerous glowing fish

Lanternfish, family Myctophidae, are small fish, usually under 15 centimetres long, with large eyes and rows of round light organs, called photophores, along their bellies and sides. There are around 250 species, and together they are thought to be among the most abundant vertebrates on Earth. Estimates of their total mass vary widely, but they may make up a large share of all fish biomass in the ocean.

The arrangement of photophores differs from species to species, and in many species between males and females, so biologists use the patterns to identify them. The fish themselves probably do the same. Lanternfish also use their belly lights for counterillumination, matching the faint light from above so that predators looking up cannot see their silhouettes.

Lanternfish are famous for their daily vertical migration. By day they stay hundreds of metres deep; at dusk, vast numbers rise toward the surface to feed on zooplankton, then sink again before dawn. Their schools are so dense that they show up on sonar as a false bottom, the “deep scattering layer,” which puzzled naval sonar operators during the Second World War. They are a key food for tuna, squid, seabirds, seals and whales, and an important link between plankton and larger animals.

Dragonfish: lures and red searchlights

Dragonfish, family Stomiidae, are among the most fearsome-looking fish in the deep. Most are long, dark and slender, with huge mouths full of fang-like teeth. Many species have a long barbel hanging from the chin, tipped with a glowing lure that they use to attract prey within striking range. They also have rows of photophores along their bodies.

A few dragonfish have one of the most remarkable adaptations in the ocean. Most deep-sea animals can only see blue light, because blue is the color that travels farthest in seawater, and many deep-sea animals have lost the ability to see red altogether. The loosejaws, genus Malacosteus, and a few related dragonfish produce red light from a light organ below the eye, as well as the usual blue. They can also see red, partly thanks to a pigment derived from chlorophyll, which they are thought to obtain from their diet. The result is a private searchlight: a dragonfish can illuminate prey with red light that the prey cannot see. The stoplight loosejaw takes its name from its red and green light organs.

Group	Light source	Where the light is	Main use	Typical depth
Lanternfish	Own chemistry	Belly and flank photophores	Camouflage, recognition	Migrate 0–1,000 m
Dragonfish	Own chemistry	Chin barbel, body, cheek organs	Luring prey, red searchlight	200–2,000 m
Anglerfish	Symbiotic bacteria	Lure (esca) on the head	Luring prey	Mostly 1,000 m+
Hatchetfish	Own chemistry	Belly photophores	Counterillumination	200–1,500 m
Flashlight fish	Symbiotic bacteria	Organs under the eyes	Hunting, signalling, escape	Shallow reefs at night

Anglerfish: light borrowed from bacteria

Deep-sea anglerfish are the classic glowing fish. In most species, the female has a modified spine on her head, tipped with a fleshy bulb called the esca. The esca is packed with bioluminescent bacteria, and the female uses the glowing lure to attract prey close to her large, toothy mouth. The fish provides the bacteria with a home and nutrients; the bacteria provide the light. Research has shown that anglerfish acquire their bacteria from the surrounding water rather than inheriting them directly.

Males of many anglerfish species are tiny by comparison and lack a lure. In some families, the male finds a female, bites onto her and fuses with her body, becoming a permanent partner that supplies sperm. Our guides to the deep-sea anglerfish and the triplewart seadevil cover these strange fish in more detail, and bioluminescent bacteria explains the partners that light them up.

Hatchetfish and the art of disappearing

Marine hatchetfish are small, silvery, very flat fish whose bodies look like the blade of a hatchet when seen from the side. They live in the twilight zone and are masters of counterillumination. Their bellies are lined with downward-pointing photophores, and their large eyes face upward to watch for prey and predators against the faint light from the surface. By adjusting their belly glow to match the light from above, they erase their shadows. Their mirror-like sides reflect the surrounding blue light, making them even harder to see from the side. Many other deep-sea fish, squid and shrimp use the same approach, but hatchetfish are one of its most refined examples.

Glowing fish in shallow water

Not every glowing fish lives in the deep. Flashlight fish, family Anomalopidae, live on coral reefs and come out at night. They carry large light organs beneath their eyes, filled with bioluminescent bacteria that glow continuously. To switch the light off, depending on the species, the fish either raises a black lid over the organ or rotates it downward into a pocket. They blink the lights to find plankton, communicate with one another and confuse predators with a zig-zag “blink and run” escape. Flashlight fish occur in parts of the Red Sea, the Indo-Pacific and the Caribbean.

Other shallow examples include ponyfish, which use bacterial light inside their bodies, and the plainfin midshipman of the North American Pacific coast, which builds its own light-producing chemical from the tiny glowing crustaceans it eats. Pinecone fish, armoured little fish found in the Indo-Pacific, also carry bacterial light organs on their jaws.

Why fish make light

Across all these groups, fish use light for a few main jobs. The first is camouflage through counterillumination, used by lanternfish, hatchetfish and many others. The second is hunting, whether by luring prey toward a glowing bait, as anglerfish and many dragonfish do, or by lighting up prey, as flashlight fish and red-light dragonfish do. The third is communication: species-specific patterns that help fish recognise mates or keep schools together. The fourth is defence, from sudden flashes that startle a predator to glowing clouds released into the water as a decoy. Some fish combine several of these uses. The same toolbox appears in squid and shrimp too; see firefly squid for an example.

Making light has a cost. Fish that produce their own luciferin must spend energy and raw materials on it, and fish that rely on bacteria must house and feed their partners. The fact that so many deep-sea fish pay that cost shows how valuable light is in a place where the sun barely reaches. It also explains why so many glowing fish have unusually large or specialised eyes: in the twilight zone, being able to see faint flashes is as important as making them.

From my side of the glass

I have dived a lot of reefs, including the Red Sea, and kept reef systems at home since 1996, but deep-sea glowing fish are a world most divers never enter. Recreational diving stops at around 40 metres; lanternfish and dragonfish spend their days hundreds of metres below that. Everything we know about them comes from nets, submersibles and remotely operated cameras, and much of it is surprisingly recent.

What strikes me most is how many separate times fish have invented the same trick. A coral reef at noon is a riot of color reflecting the sun. A few hundred metres down, color comes only from what animals make for themselves, and fish have built that ability from scratch over and over. For more of the creatures living in that world, see our deep-sea creatures page, and for the sharks that glow alongside these fish, see bioluminescent sharks.