Glow it up

Plants can do it. Fish can do it. Worms can do it. Even single-celled plankton can do it. Why does so much of the natural world get to glow in the dark?

Written by       Illustrated by Giselle Clarkson

Giselle Clarkson

The yacht Ganesha was off the southern coast of Java in August 2019 when it happened. Naomi McKinnon, crewing on her gap year, was on night watch when she noticed that the sea was brighter than the sky—like a film-negative version of reality. She wondered if something was wrong with her eyes. But as the boat sailed on into the calm night, the gleam intensified. “It was a very peaceful thing to watch,” she says. “It wasn’t scary. But it was very mysterious.”

Soon, from horizon to horizon, the sea was lit from within—a soft yellowish-greenish white, like the glow-in-the-dark star stickers that McKinnon had coveted as a child. Even the seawater in the yacht’s toilet bowl had the same ghostly shine. Crew members snapped a few photos on their phones, which became the first known eyewitness pictures of a rare phenomenon known as milky seas.

Sailors have occasionally reported similar events over the centuries. In 1864, captain Raphael Semmes had the impression he was sailing on “a phantom ship lighted up by the sickly and unearthly glare of a phantom sea”. He wrote: “The whole face of nature seemed changed.”

Until recently, it wasn’t clear whether milky seas were real or hallucinatory, akin to “folklore or sea monsters”, says Steven Miller, an atmospheric scientist from the University of Colorado.

Miller is an expert in using satellites to study Earth, and in 2011, the launch of a new, more sensitive low-light sensor made him wonder if it could register bioluminescence. This was of particular interest to the US Navy, since bioluminescence can give away the position of submarines; in 1918, a British ship sunk a German U-boat after spotting its luminous outline.

It took six years to figure out how to spot milky seas, but Miller and his team have now identified around 15 such events. There are one or two per year, mostly in the Indian Ocean or around Indonesia. “These things always happen in remote areas,” he says. “It’s almost like they’re hiding from us.”

One event, in 2019, encompassed an enormous area of water south of Java that glowed from late July to early September. Miller spotted it in satellite images in 2020: a spiralling form 100,000 square kilometres in size—nearly as big as the North Island—and far larger and more long-lasting than milky seas were thought to be. After he published a scientific paper about it, McKinnon contacted him to describe her experience sailing through it—a rare connection between satellite detection and eyewitness report.

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There are still many mysteries about milky seas: what conditions trigger them, which critters are responsible, and why they’re so rare.

We know a lot more about the reasons behind the more common form of ocean-surface bioluminescence—the flashing particles often glimpsed in waves and wakes around New Zealand. When plankton sense movement in the water, they sparkle, startling the tiny copepods that eat them—and revealing the copepods to nearby hungry fish. It’s like a burglar alarm, says Miller. It flashes and then subsides.

Milky seas, on the other hand, shine continuously—and it’s probable the creatures making the light actually want to be devoured. Only once have scientists collected water samples from a milky sea: in 1985 off the island of Socotra in the Arabian Sea. They found the water contained a type of bacteria, Vibrio harveyi, living alongside the microalgae Phaeocystis.

We won’t know for sure whether it’s always these bacteria that cause milky seas, or why they glow, until scientists can study them properly. The leading theory, though, is that the bacteria—whatever they are—proliferate as they munch away on a dying algal bloom. Once they sense they’re running out of food, they glow in order to attract fish.

“If you’re bacteria, you want to go back to where you like it the best,” says Miller. “And that’s in the intestines and gut of animals.” A steadily incandescent milky sea, then, is probably the result of trillions of individual bacteria shouting “Eat me!”

Miller’s passion for the mystery of it all is obvious. “These bacteria that individually are tens of nanometres across can collectively produce a structure that we can see from space,” he says. “It’s an extraordinary expression of life. And we need to understand why that’s happening.”

Ideally, he’ll find out about a milky sea quickly enough to study it in person. When that day comes, he knows exactly what he’ll do: jump in and swim down into the unearthly water until he finds out how deep the glow goes.

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The uniform glow of a milky sea might be a rare occurrence, but every day is a bioluminescent party in the deep sea.

“In certain parts of the ocean, more animals are bioluminescent than aren’t,” says Kat Bolstad from AUT University.

According to recent estimates, as much as 76 per cent of marine life is capable of emitting light—mostly green, blue, yellow, white, and very occasionally red. Or even a combination: the extremely creepy stoplight loosejaw dragonfish, found in deep New Zealand waters, has a green and a red light under each eye.

Since 95 per cent of the possible habitat on Earth is in the deep sea, bioluminescence is probably the most common form of animal communication on our planet, says Bolstad. “And that’s kind of mind-blowing for us, right? It seems so strange and special, like, ‘Wow, some animals can glow.’ But actually, we’re the weird ones.”

Bioluminescence is so useful in the deep that it has evolved a number of times—at least 30 marine fishes have independently developed the ability to glow. It’s crucial to finding food, hiding from predators, and finding mates.

If you have bioluminescence that lets you signal to somebody else in the dark sea that you’re the same species and you’re interested, that’s a strong selection point,” says Bolstad. “If that thing also then lets you somehow find or attract food, that’s a double bonus. And if you can deploy it in some way to help save your life, if somebody else is trying to make you their dinner, that’s a triple win.”

In the places the sun can’t reach, plants don’t grow, so there are no herbivores—everything in the deep sea is either a scavenger or a carnivore. “Everyone’s trying to eat anyone else that they bump into,” says Bolstad.

Bolstad is an expert on cephalopods, the group of animals that includes octopuses, squids and nautiluses. Around a third of them (mostly the squids) can emit light, and in an astounding variety of ways.

Human-sized Humboldt squids, for instance, use a repertoire of 28 pigmentation patterns on a glowing background to signal to each other, even stringing different patterns together to make a kind of sentence.

Vampire squids (which are not actually squids, but more closely related to octopuses) have large light organs on their rear ends. When a vampire squid is being chased, it can squeeze the muscles around its light organ to make the organ appear smaller, like an iris constricting. To predators, it looks like the rear lights of a car zooming away. In reality, the squid does a little backflip and doesn’t go anywhere. “But it made it look like it jetted away into the distance,” says Bolstad.

The light organs of the eight-armed octopus squid (which is definitely a squid, not an octopus) are the biggest in the animal kingdom. The yellow-green, lemon-sized photophores are located on the tips of two of the animal’s arms, which it holds out in front of its body like a pair of floodlights. They’re always on, but the squid can cover and reveal the lights with flaps of skin that open and close like eyelids—a way of startling or momentarily blinding nearby predators.

Then there’s spew bioluminescence. Just as some shallow-water squids and octopuses make a dark cloud of ink as a smokescreen to help them escape from predators, some of their deep-sea relatives eject a sticky, glowing cloud of mucus. “You stick that all over your predator, and now they’re glowing in the dark, and you’re not,” says Bolstad. (Meanwhile, male ostracods—tiny ocean crustaceans—vomit light to impress females. Each species has its own display pattern, like zigzags or swirls, which helps females spot potential mates.)

The best way to defend yourself, of course, is to avoid getting noticed in the first place, and bioluminescence is great for that, too. In the deep sea, enough light filters down from the surface to silhouette an animal’s shape from the perspective of other animals looking up from below. Many sharks, squids, and others therefore have subtly bioluminescent tummies, which enables them to blend into the light from above—a strategy called counter-illumination. But because evolution is an eternal arms race, some predatory squids can see through this trick.

The strawberry squid is named for its beautiful rose-red body, speckled with pale, seed-like photophores. But it’s also called the cock-eyed squid, due to its laughably asymmetric eyeballs. The small, flat right eye looks downward, scanning for bioluminescence below. The left one is twice the size, covered in a see-through yellow dome, and is tuned to a slightly different wavelength of light that allows it to glimpse the signals of animals swimming above.

“It has its own counter-illumination, but it’s also seeing through everybody else’s counter-illumination,” says Bolstad. “It’s just perfect—and it looks ridiculous.”

The enormous eyes of the colossal squid—a creature once thought to be myth—also evolved to detect bioluminescence. Colossal squids spend their lives trying to avoid sperm whales, which can be given away by bioluminescent plankton, just like the German U-boat. The squids’ huge eyes enable them to spot trouble before they get within range of the whales’ echolocation. “A diving sperm whale probably looks like a glowing comet coming down through the water column,” says Bolstad.

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In the late spring of 1843, the naturalist Ernst Dieffenbach was starving and soaked to the skin. He was attempting to scale Mount Taranaki, but the rain poured down in torrents, the wood was too wet to make a fire, his party had no tents in which to shelter, and they had run out of provisions.

As darkness fell, and Dieffenbach huddled in the forest trying and failing to stay dry, there was but one consolation, he wrote. “During these nights the forest assumed a beautiful appearance: the fallen trees, and almost the whole surface of the ground, sparkled in a thousand places with the phosphorescence of the decayed matter;—we seemed to have entered the illuminated domain of fairy-land.”

What Dieffenbach calls phosphorescence is really bioluminescence—the result of a chemical reaction inside a living organism’s cells that produces light. (Phosphorescence generally refers to a material that absorbs light and emits it later, like glow-in-the-dark stickers, while fluorescence is when something absorbs light and immediately shines it back. For instance, more than a hundred species of mammals—from rabbits to echidnas to bats—have fur that fluoresces under UV light.)

Bioluminescence is much rarer on land than in the ocean, but New Zealand has a handful of standout examples. Two years before Dieffenbach’s encounter with glow-in-the-dark fungi, another colonist, William Colenso, described decaying wood in the Far North “shining with such a peculiar silent luster, (if I may so speak) in the depths of the forest”.

They’re still out there, if you keep your eyes open. In 2021, the bioluminescent glow of the native fungus Mycena roseoflava was photographed for the first time on Rakiura/Stewart Island.

There are also our glowworms, of course (which are not actually worms, but gnats), and our glowing earthworms (which are definitely worms). Octochaetus multiporus is pinky-grey, as fat as a finger, and reaches 30 centimetres long. It burrows deeply into the soil in tussock country, forests, and pastures in the South Island and southern North Island.

Earthworm expert Nicole Schon from AgResearch went looking for them one night under the gorse bushes on her rural Manawatu property. She sunk her spade into the soil and quickly found a few of the worms. When she picked one up, a glowing orange-yellow fluid oozed out of its mouth, anus, and the pores along its back. The slimy stuff coated her hands, like liquid oozing from a broken glow stick. She reckons she could have read a newspaper by the worm’s bright light for the few minutes the glow lasted.

Why would worms need to glow like this? Scientists aren’t sure, but it’s possible it functions like spew bioluminescence—a way to startle a probing kiwi, for instance. Sometimes, though, it gives the worms away: in 1922, an Italian naturalist described chickens in New Zealand feeding on Octochaetus worms at twilight—he thought they looked like “glowing macaroni”.

Māori have long known about Octochaetus: a 1952 glossary gives several names for them—piritaua, puratoke, titiwai—and there are records Māori used bundles of the luminous worms as lures for night-fishing. Octochaetus might not be the only glowing worm, either. “Quite a lot of native earthworms excrete a milky fluid,” says Schon. “So it wouldn’t surprise me if there was another one out there.”

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Humans have long found ways to turn nature’s lights to our advantage. The Roman philosopher Pliny the Elder advised rubbing the slime of a luminous jellyfish onto a walking stick to make a torch. Coal miners once filled jars with fireflies. Indigenous peoples in Indonesia used glowing mushrooms to light their way in the forest. And during World War II, the Japanese army harvested vast quantities of bioluminescent ostracods and issued packets of their dried-up bodies to soldiers. When the soldiers added water, the ostracods would begin to glow: a cold, blue light strong enough for the men to read by, but too dim to reveal them to their enemies.

Bioluminescence has fascinated and captivated humans for millennia. Is it because it’s a skill we don’t possess? “We don’t have this superpower that even the simplest organisms seem to have,” says Miller.

He believes that if we can learn exactly when, where and why phenomena such as milky seas take place, it might lead to greater understanding of the infinitely intricate Earth system—how the atmosphere is connected to the ocean and in turn to the microscopic animals within it.

“I can’t let go of the idea that there’s something that they’re trying to tell us,” he says, “these little critters that are glowing in unison.”

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