Three quarters of deep-sea animals make their own light!


A siphonophore, Frillagalma vityazi, lit by lights on a remote-controlled vehicle (top) and emitting bioluminescence in the lab (bottom). Top image: 2015 MBARI. Bottom image: Steve Haddock, MBARI.

A new study has shown that there are many more bright (bioluminescent) animals in the ocean than we initially thought.

Ever since explorer William Beebe descended to the depths of a metallic sphere in the 1930s, marine biologists have been stunned by the number and diversity of brilliant animals in the ocean. Yet few studies have actually documented the number of bright animals at different depths. In a new study published 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 4000m depth can produce their own light.

You would think that it would be easy to count the number of bright (bioluminescent) animals in the ocean, just by watching 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 mostly black, so animals don’t need to shine very brightly. In addition, most animals do not glow continuously because producing light requires additional energy and can attract predators.

Due to the difficulty of counting glowing animals at depth, most previous estimates of the proportion of glowing animals were based on qualitative observations made by researchers looking out the windows of submersibles. The Martini and Haddock study is the very first quantitative analysis of the number and types of individual luminous animals at different depths.

Researchers have compiled data on every animal larger than one centimeter appearing in video of MBARI’s 240 remote controlled vehicle (ROV) dives in and around Monterey Canyon. They counted more than 350,000 individual animals, each of which had been identified by MBARI video technicians using a large 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 data source for over 360 research articles.

Martini and Haddock were surprised to find that the proportion of shiny and non-shiny animals was quite similar from the surface down to 4000m. Although the total number of bright animals decreases with depth (which has already been observed), this is apparently due to the fact that there are simply fewer animals of all kinds in the deeper waters.

Even though the proportion of bright and non-bright 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 up to 1500 m, most of the bright animals were jellyfish (jellyfish) or comb jellies (ctenophores). From 1,500 m to 2,250 m deep, worms were the most abundant bright animals. Below, the small tadpole-like animals known as larvae made up about half of the bright animals seen.

The analysis also showed that certain groups of animals were much more likely to glow than others. For example, 97-99.7% of cnidarians (jellyfish and siphonophores) in videos are able to produce their own light. In contrast, only about half of fish and cephalopods (squid and octopus) are bioluminescent.

Commenting on the importance of his research, Martini said, “I’m not sure people realize how common bioluminescence is. It’s not just a few deep-sea fish, like monkfish. It’s jellies, worms, squid… all kinds of things. In fact, she and Haddock concluded their article by writing, “Given that the deep ocean is the largest habitat on Earth by volume, bioluminescence can certainly be considered a major ecological trait on Earth. “


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