A new study has shown that there are many more glowing (bioluminescent) animals in the ocean than we first thought.
Ever since explorer William Beebe descended into the depths in a metal sphere in the 1930s, marine biologists have been astounded by the number and diversity of glowing animals in the ocean. Yet few studies have actually documented the numbers of glowing animals at different depths. In a new study in Scientific Reports, MBARI researchers Séverine Martini and Steve Haddock show that three quarters of the animals in Monterey Bay waters between the surface and 4,000m deep can produce their own light.
You would think it would be easy to count the number of glowing (bioluminescent) animals in the ocean, just by looking at videos or photographs taken at different depths. Unfortunately, very few cameras are sensitive enough to show the pale glow of many marine animals. Below 300m the ocean is essentially pitch black, so animals don’t need to glow very brightly. Also most animals don’t glow continuously because making light takes extra energy and can attract predators.
Because of the difficulty in counting glowing animals at depth, most previous estimates of the proportion of glowing animals were based on qualitative observations made by researchers peering out the windows of submersibles. Martini and Haddock’s study is the first ever quantitative analysis of the numbers and types of individual glowing animals at different depths.
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.
Martini and Haddock were surprised to find that the proportion of glowing to non-glowing animals was pretty similar from the surface all the way down to 4,000m. Although the total number of glowing animals decreased with depth (something that had been previously observed), this was apparently due to the fact that there are simply fewer animals of any kind in deeper water.
Even though the proportion of glowing to non-glowing animals was similar at all depths, the researchers found that different groups of animals were responsible for the light produced at different depths. For example, from the sea surface down to 1,500m, most of the glowing animals were jellyfish (medusae) or comb jellies (ctenophores). From 1,500m to 2,250m down, worms were the most abundant glowing animals. Below that, small tadpole-like animals known as larvaceans accounted for about half of the glowing animals observed.
The analysis also showed that some groups of animals were much more likely to glow than others. For example, 97 to 99.7 percent of the cnidarians (jellyfish and siphonophores) in the videos are able to produce their own light. In contrast, only about half of the fishes and cephalopods (squids and octopuses) are bioluminescent.
Commenting on the significance of her research, Martini said, “I’m not sure people realise 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.” In fact, she and Haddock concluded their paper by writing, “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.”