Eerie green pinpoints of light dancing in a forest. Sparkling cyan waves that wash ashore in the dead of night. Many organisms have found ways to emit light, allowing them to glow brightly even in the darkest parts of our world.
On a chemical level, we know how bioluminescence works. A pigment called a luciferin undergoes a chemical reaction, generally triggered by an enzyme, that emits light.
The ability to emit light is thought to have evolved independently up to 50 times among living organisms. They use it to flirt, to defend themselves and even to deceive one another.
For reasons that will become clear, there is no definitive answer to the question "what is the brightest living thing". But here are the best candidates.
"The intensity range of bioluminescence is enormous, spanning at least nine orders of magnitude above the lower threshold of human vision," says oceanographer Edith Widder of the Ocean Research & Conservation Association in Fort Pierce, Florida. She has used submersibles to study glowing deep-sea creatures.
"The dimmest is that of a single bioluminescent bacterium, which is actually too dim to see with the naked eye," says Widder. "Bacteria don't produce light unless there are enough of them together that they can produce visible light."
Some of the brightest light emitters are jellyfish
But when enough bacteria glow at once, the results can be spectacular.
For centuries sailors have reported "milky seas", an ethereal glow that stretches from horizon to horizon. Charles Darwin personally saw a milky sea from HMS Beagle. But the reports were often dismissed as salty sailors' tales.
That changed in 2005. A team led by Steven Miller, then at the Naval Research Laboratory in Monterey, California, spotted a milky sea in satellite images. The sparkling patch of the north-west Indian Ocean was bright enough to be seen from space for three nights in a row. The satellite data corroborated an account from a ship's captain.
It seems likely that milky seas are created by masses of bioluminescent bacteria. At a critical density in the ocean they could produce an electric blue glow.
Being visible from space is pretty impressive, but the bacteria can only manage that in huge groups. What about individual organisms?
"At the other end of the intensity spectrum, some of the brightest light emitters are jellyfish," says Widder. For example, the crystal jelly produces a unique green fluorescent protein, which biologists now use as a research tool.
Ostracods produce one of the most concentrated, distantly visible lights
"The brightest bioluminescence I ever got a picture of was a siphonophore chain that was at least 20ft [6m] long," says Widder.
In 1985 Widder took a submersible down into the Monterey Submarine Canyon, where she encountered a giant siphonophore (Praya dubia). Reaching 131ft (40m) in length, it is one of the longest animals in the ocean. It looks like a giant underwater rope light, and glows blue when disturbed.
But again, this was not a single creature. A siphonophore is actually a colony of thousands of individuals. Each specialises in a particular function, from propelling the colony through the water to lighting up.
Another candidate for the brightest marine creature spends its time in shallower waters.
Gretchen Gerrish of the University of Wisconsin-La Crosse studies small crustaceans called ostracods. In the Caribbean they are also known as "blue tears", because on moonless nights they put on sparkling mating displays.
"Based on my personal observations in shallow tropical Caribbean habitats, ostracods produce one of the most concentrated, distantly visible lights that appear in the coral reef environment," Gerrish says.
Males of the species use their light-emitting organs in monthly courtship displays, blazing a trail of bright dots to attract the attention of females.
But Gerrish has found that the threat of attack provokes the most blinding bursts of light.
"The brightest luminescence of the ostracod is produced when they are preyed upon," says Gerrish. "The ostracods release large amounts of both luciferase and luciferin, which mix and light up the outline of the predatory fish."
This flash-bomb tactic could be some of the brightest bioluminescence in the ocean. Gerrish's colleague Trevor Rivers of the University of Kansas in Lawrence is now in the process of measuring its waveform and intensity.
There are also some rather bright organisms living on land.
Both Aristotle and Pliny the Elder wrote about luminous damp wood. We now know this is the result of fungi in the decaying plant. So-called foxfire is not just a folk tale: more than 70 species of bioluminescent fungi are now known to science.
The most famous bioluminescent species on land are the glowworms and fireflies.
For centuries sailors have reported "milky seas", an ethereal glow that stretches from horizon to horizon
Glowworms are not worms but the larvae of a number of different insects, including flies in Australia and New Zealand, gnats in North America, and beetles across Europe and Asia.
Fireflies are also misleadingly named: they are not flies, but rather beetles in the family Lampyridae. The larvaeglow to warn off predators. The adults use the light-emitting organs on their rears to coordinate mating.
"Some species produce brighter bioluminescent signals than others," says Marc Branham of the University of Florida in Gainesville, who first documented the fireflies' use of bioluminescence for sexual selection.
"This is often due to differences in size between species, as well as differences in the number and size of their light organs. Also, in most species males produce more light than do females."
The trouble with all these examples is that we do not have comparable measures of how bright they are.
Many studies of bioluminescence have measured the wavelength of the light, because this can help to identify the species emitting it. However, only a few researchers have studied the luminous intensity.
Both Aristotle and Pliny the Elder wrote about luminous damp wood
Their figures need to be taken with a pinch of salt, because they only measure light travelling in a single direction. That is fine for measuring the brightness of a directed beam of light, like a flashlight, but it might not give the full picture for glowing creatures that shine light in all directions.
With that in mind, a review published in 1998 tentatively identified the headlight elater as the brightest bioluminescent insect, based on the surface brightness of its light-producing organs.
The beetle is clearly pretty bright, but given the lack of data it seems likely that there are brighter organisms out there. Beyond that, it does not seem fair to compare a large mass of organisms – such as a cloud of bacteria, or a siphonophore – to a single insect.
When you consider the complications that arise from colour, distance, size of the light source, its direction and how long it lasts, you can understand why the brightest life on Earth still remains to be seen.
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