THE ocean is dark and full of terrors, and the black dragonfish is the darkest of them all. Its surface, new measurements reveal, is as black as the blackest material known—the result of an abyssal arms race.

“The trick to being really dark is to control the scattering of light,” says Sönke Johnsen of Duke University, in North Carolina, who studies the dragonfish. “You have to let light into a material and let it bounce around a lot.” Black velvet, for instance, appears darker than other fabrics because photons (the particles of light) skip between its fine hairs and do not escape. Similarly, Vantablack, the least reflective artificial material, traps photons in a forest of carbon nanotubes standing on their ends. It absorbs 99.965% of visible light. Objects coated in it seem to disappear, leaving behind an inky silhouette.

Karen Osborn of the Smithsonian National Museum of Natural History, in Washington, DC, noticed a similar effect when she tried to photograph deep-sea fish, many of which are coated in a fragile black film that has to be removed before a picture can be taken. Under a scanning electron microscope, she discovered that this film is made of millions of microscopic melanin granules shaped like drug capsules, capped by a thin gelatinous layer.

The absorbing effect of the film is so great that instruments calibrated in the usual way cannot detect any light reflected from the fish at all. Dr Osborn’s attempts to measure the light inside an empty pitch-black room yielded the same result. Eventually, she and her colleagues worked out that the dragonfish reflects just one in every 2,000 photons incident upon it—an absorbance of 99.95%. Similar measurements hold true for a whole range of fishes brought up from the abyss.

In the deepest depths of the ocean, what light could these animals be trying to avoid? The photic zone, which is bathed in sunlight during the day, and starlight and moonlight at night, reaches down a few hundred metres. Yet blacker-than-black fish are found much deeper than that. The black dragonfish, for instance, lives up to 2,000 metres beneath the surface.

The deep ocean has other sources of light than astronomical bodies, though. Three-quarters of marine organisms off the coast of California produce their own, and that is probably true in most oceans, says Steve Haddock of the Monterey Bay Aquarium Research Institute. Recently, Dr Haddock brought back the first full-colour high-definition videos of bioluminescence in the abyss. At a meeting in Monterey, in September, he offered a preview. Sea cucumbers, normally pale and beige, rippled with waves of blue bioluminescence. A shimmery gold viperfish, when disturbed by Dr Haddock’s remotely operated submarine, suddenly switched its lights on, covering every detail in its skin. A jellyfish displayed swirling blue pinwheels. A brittleworm glowed yellow.

Dr Haddock hopes his new submarine-borne camera will grant him a better understanding of how animals use bioluminesce in the deep. Some predators, for instance, employ glowing lures to attract prey. Others produce pulses of light to illuminate their targets—in which case, from the prey’s point of view, having an invisibility cloak has obvious advantages.

Some organisms use bioluminescence as a defence mechanism. Lighting up an attacker can make it more vulnerable to the attentions of others. And there are times when switching the lights on is a better camouflage than absorbing light.

Watases lanternfish are generally hunted by predators that strike from below. It may seem surprising, therefore, that among the light-producing cells distributed across their bodies they have a set that point downward from their bellies towards the sea floor. They also, however, have light-sensing cells pointing upwards on their backs. At the meeting in Monterey José Paitio, of Chubu University in Japan, described how the two sets of cells work together. The dorsal ones sense the colour and intensity of light filtering downwards. The ventral ones respond to that signal, generating exactly the amount of light required to blend in, so that the fish disappears from view when seen from underneath. A truly bright idea.