A creature that glows to survive—not to be seen, but specifically to destroy—has a subtly unsettling quality. The Mariana Trench, located miles below the Pacific where sunlight completely disappears and pressure is measured in tons rather than pounds, is engaged in its own arms race. No noise, no bullets. Just light, used with icy accuracy.
In April 2016, NOAA’s research ship Okeanos Explorer sent its remotely operated vehicle Deep Discoverer down close to the Enigma Seamount, and the footage it returned was truly bizarre. There, roughly 2.3 miles below the surface, a red-and-yellow jellyfish that science had never seen before held its outer tentacles wide and motionless, ensnaring any unfortunate creature that happened to drift into range. Its bell’s bioluminescent glow wasn’t ornamental. It was a trap. Scientists speculate that the vivid coloring along its radial canals and close to its gonads may attract prey to what appears to be safety but is actually the opposite.
Most textbooks don’t even try to explain what that one sighting revealed about the deep ocean. Light does not mean that there is no danger down in the trench. The threat is light.
To appreciate how unusual this is, it’s worth taking a step back. Animals that glow on the surface usually do so to attract mates, communicate, or deter predators with poisonous coloring. The reasoning is recognizable, almost homegrown.
However, bioluminescence has been pushed in unexpected directions in the Mariana Trench, where organisms have evolved in almost complete isolation for millions of years and the water column extends down nearly seven miles. Certain animals use it to blend in with the dim remnants of light from above. Others create flashes to confuse or blind assailants. Some lure prey with something glowing at the end, much like a fisherman drags a lure through water.
The escalation inherent in this is what gives it the appearance of an actual arms race. Light-producing organisms have compelled the evolution of more accurate light-detecting organisms. eyes that are deeper. enhanced photoreceptor sensitivity.
The ability to discern between the steady glow of a bacterial colony and the blue-green flash of a dinoflagellate, which turns out to be very important when one indicates harmless plankton and the other may be attached to something that wants to eat you. Over geological time, a slow counter-move appears to be taking place as each biological adaptation draws another into existence.
During the Cold War, researchers were aware of the military ramifications of all this. The possibility of using bioluminescent marine organisms to track submarines was thoroughly investigated by both the US and the USSR. The reasoning was simple: bioluminescent organisms are disturbed when a submarine moves through water, and surface ships or airplanes may be able to see this disturbance.
The claim that a German U-boat in 1918 revealed its location close to Gibraltar by passing through a bioluminescent patch is contested. Defense organizations on both sides invested actual funds in researching the larger concept because it was still deemed credible. The research itself is a sort of acknowledgment that whatever is happening with light in the deep ocean is sophisticated enough for governments to take seriously, even though neither effort resulted in a practical detection system.
Because we haven’t done much research on bioluminescence, it’s still unclear how many species in the Mariana Trench use it. Less than a tenth of a percent of the deep ocean floor has been directly observed by humans, according to a sobering discovery made by scientists after cataloging over 43,000 deep-sea dives since 1958. In actuality, the area we’ve seen is about the size of Rhode Island. Everything else is ROV footage and inference, which rarely captures the whole picture but captures moments. When this article was written, the jellyfish discovered close to the Enigma Seamount had not yet been given names. Perhaps it’s still not.
The fact that bioluminescence is commonplace down there has been verified by science. Perhaps the exceptions are the organisms that don’t make light. The list of glowing species, which includes bacteria, jellyfish, worms, crustaceans, and sharks, is similar to the majority of the phyla you would find in a marine biology survey, but it has been moved into a register that is illuminated from the inside. Additionally, the strategies get more complex the deeper you go. There’s a feeling that the Mariana Trench in particular has had time to establish its own internal logic and set of rules regarding the meaning and cost of light.
It’s difficult to avoid feeling like we’re very late to something that has been going on for a very long time when watching this unfold through ROV footage—the jellyfish with its tentacles spread, the unidentified bioluminescent organism recently spotted at 6,100 meters. Without us, the trench has been carrying out this task. Long after the research vessel turns back toward shore and the cameras go dark, it will continue to do this.
