Three Quarters of Deep-Sea Creatures Glow in the Dark

Video screen capture. NOAA

A new study counts the ocean’s animals that make their own light, resulting in a profound conclusion.

Mother Nature performs all kinds of magic, hovering near the top of the list is the appearance of fireflies, punctuating summer evenings with their bioluminescence-powered fairy lights. But what if more insects came with their own glow? The world inhabited by a bevy of bioluminescent creatures may seem farfetched, but in fact, such is the way of the sea.

Marine biologists have long been intrigued by the volume and variety of glowing animals in the ocean – yet documenting the numbers has proven challenging. But now, researchers Séverine Martini and Steve Haddock from the Monterey Bay Aquarium Research Institute (MBARI ) have taken on the task. And what did they find? In their new study they show that three quarters of the animals in the area they researched – Monterey Bay waters between the surface and 4,000 meters deep – can produce their own light.

Bioluminescent sea creatures have been tough to quantify because few cameras are sensitive enough to capture the softer glow of many of the animals – creatures that live deeper than 1,000 feet exist in an almost pitch black world where not a lot of bioluminescence is required. Add to that the fact that animals don’t keep their lights on full-time – it takes energy and makes them more conspicuous to predators – and the task is even more difficult. Until now, estimates of how many animals make their own light have been mostly based on “qualitative observations made by researchers peering out the windows of submersibles,” notes MBARI. “Martini and Haddock’s study is the first ever quantitative analysis of the numbers and types of individual glowing animals at different depths,” the organization adds.

The researchers compiled data on every animal larger than one centimeter that appeared in video from 240 dives by MBARI’s remotely operated vehicles (ROVs) in and around Monterey Canyon. They counted over 350,000 individual animals, each of which had been identified by MBARI video technicians using a vast database known as the Video Annotation and Reference System (VARS). The VARS database contains over five million observations of deep-sea animals, and has been used as a source of data for more than 360 research papers.

The authors compared the animals observed during the 240 ROV dives with a list of known bioluminescent animals. And from there the animals were further organized.

One surprising aspect of the data was that the proportion of glowing to non-glowing animals was basically alike from the surface to depths of 4,000 meters. “Although the total number of glowing animals decreased with depth (something that had been previously observed),” MBARI notes, “this was apparently due to the fact that there are simply fewer animals of any kind in deeper water.”

Even so, they discovered that different groups of animals were largely responsible for the light produced at different depths. In the range between the surface and 1,500 meters, for instance, jellyfish and comb jellies were the primary light-up animals. From 1,500 meters to 2,250 meters down, worms were the animals lighting the way. Even further down, small tadpole-like animals known as larvaceans accounted for some 50 percent of the creature softly illuminating the depths.

Within specific animal groups, they found that some groups are more predominantly bioluminescent. A whopping 97 to 99.7 percent of the cnidarians (jellyfish and siphonophores) have the ability to glow; meanwhile half of the fishes and cephalopods produce their own light.

In the end, it's fascinating to imagine a watery world so full of swimming creatures that glow in the dark. But what's so profound is what it means for the Earth as a whole, to those of us bound to the terra firma, at least.

“I’m not sure people realize how common bioluminescence is. It’s not just a few deep-sea fishes, like the angler fish. It’s jellies, worms, squids...all sort of things,” Martini says. “Given that the deep ocean is the largest habitat on Earth by volume, bioluminescence can certainly be said to be a major ecological trait on Earth.”

The research was published in Scientific Reports.