In a first ever study that quantifies the number of deep-sea animals capable of producing their own light, scientists have estimated that three quarters of deep-sea animals are bioluminescent.
Published in Scientific Reports is a new study by MBARI researchers Séverine Martini and Steve Haddock who looked at hundreds of hours of footage captured during 240 dives by MBARI’s remotely operated vehicles (ROVs) in and around Monterey Canyon. The duo estimated that three quarters of the animals in Monterey Bay waters between the surface and 4,000 meters deep can produce their own light.
The study is basically a compilation on every animal larger than one centimeter that appeared in video from 240 dives. The duo was able to count 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 then compared the list of animals seen during the 240 ROV dives with a list of animals and animal groups that were known to be bioluminescent. This list was based on a review of previous scientific papers, as well as firsthand observations by Haddock and others. As an indication of the lack of research in this area, the most complete source of bioluminescence information for marine animals was a paper published in 1987, 30 years ago.
Martini divided the observed animals into five categories:
- Definitely bioluminescent
- Highly likely to be bioluminescent
- Very unlikely to be bioluminescent
- Definitely not bioluminescent, and
- Undefined (not enough information was available to determine if an animal is bioluminescent or not).
Because scientists know so little about deep-sea animals, 20 to 40 percent of the animals seen below 2,000 meters were classed as “Undefined.”
Looking through the data, 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 4,000 meters. 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 1,500 meters, most of the glowing animals were jellyfish (medusae) or comb jellies (ctenophores). From 1,500 meters to 2,250 meters 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.