Three-Quarters Of Deep-Sea Creatures Light Up The Ocean With Their Own Glow

The open oceans are filled with animals that produce their own light. In surface waters these are often jellyfish. Olgalngs/Shuttestock

The world is filled with more beauty than we knew. A survey of species living down to 3,900 meters (13,000 feet) below the ocean's surface has found three-quarters of the inhabitants produce their own light.

Bioluminescence is usually associated with deep-sea creatures living where sunlight doesn't penetrate, or dinoflagellates that occasionally light up bays with a gorgeous glow. These have blinded us, however, to how common the phenomenon is near the surface but far from shore.

Although widespread bioluminescence has been known for almost a hundred years, since submarines started being used for research purposes, little work has been done establishing the proportion of light-producing creatures at different depths. Those animals that don't make their own light are obviously harder to spot, and even some bioluminescent animals glow so faintly they're frequently missed, so taking a census is a challenge.

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Dr Séverine Martini and the appropriately named Dr Steve Haddock of the Monterey Bay Aquarium Research Institute (MBARI) examined video footage collected over 17 years by MBARI's remotely operated vehicles near Monterey Canyon, off the California coast. In Scientific Reports they analyze more than 350,000 animals over 1 centimeter (0.4 inches) in length illuminated in the vehicles' lights through 240 dives.

Using published descriptions of species, and knowledge so extensive Haddock featured in David Attenborough's Life That Glows, Martini and Haddock classified each species by its likeliness to be luminescent. Those about which we know almost nothing were classed as undefined.

Instead of luminescence increasing with depth, it was the undefined category that became more common in deeper waters. Towards the bottom of the dives, almost 40 percent of animals detected were so unfamiliar Martini and Haddock didn't even want to hazard a guess.

On the other hand, in the top 100 meters (330 feet) of the ocean, most creatures were familiar, and glowing. Although these waters had the largest proportion of definitely non-bioluminescent animals, the near-elimination of doubt meant they also contained the most creatures classed as definitely light-producing, more than 50 percent.

Surface water fish seldom produce their own light, but shallow waters are teeming with jellyfish, at least 97 percent of which glow. At a greater depth, the light is more likely to come from worms and creatures known as larvaceans.

“I’m not sure people realize how common bioluminescence is,” Martini said in a statement. “It’s not just a few deep-sea fishes, like the angler fish. It’s jellies, worms, squids… all sort of things,”

If the undefined animals can be allocated correctly, a relationship may be established between depth and frequency of luminosity. This could be useful for determining the richness of life at different depths, helping us work out how many species we are missing in the dark.

Image in text: The ratio of luminescent to non-luminescent animals barely changes with depth, even as the undefined numbers grow. Severine Martini/MBARI 

A deep-sea tomoptorid worm lit by lights on a remotely operated vehicle (left) and emitting bioluminescence in the lab (right). Tomoptorids are rare among deep-sea animals in that they emit yellow, not blue light. MBARI/Shutterstock

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