Short answer: bioluminescence is light made by a living thing through a chemical reaction, and in the open ocean it is the rule, not the exception. A study of more than 350,000 deep-sea video observations found that 76 percent of the animals seen could make their own light. On land, glowing is rare. In the sea, the dark is full of lights.
What is bioluminescence?
NOAA defines it as light produced by an organism using a chemical reaction (NOAA Ocean Exploration). It is a form of chemiluminescence, which just means visible light made by a chemical reaction. When the reaction happens in a living thing, scientists call it bioluminescence (NOAA fact sheet).
Like fire, the reaction uses oxygen. Unlike fire, it gives off very little heat. Almost all of the energy comes out as visible light (NOAA Ocean Exploration fact sheet). Cold light. In the cold dark.
How do animals make light?
Two ingredients: an enzyme called luciferase and a substrate called luciferin. Luciferase speeds up the reaction between luciferin and oxygen. The luciferin gets oxidized, light comes out, and what is left is an inactive molecule called oxyluciferin. The luciferase gets recycled, so the light keeps coming as long as there is luciferin and oxygen (NOAA fact sheet).
Where the reaction happens varies wildly. Some animals do it inside their own cells. Some outsource it to bacteria living in their bodies. Some shrimp do it outside their bodies entirely, by spewing the chemicals into the water. And different species use different luciferin molecules, which suggests the ability to make light evolved separately many times (NOAA fact sheet; NOAA). Evolution kept reinventing the flashlight.
How common is it?
Very. In 2017, Séverine Martini and Steven Haddock published a study in Scientific Reports using 17 years of video from remotely operated vehicles off the California coast, from the surface down to 3,900 meters. They classified more than 350,000 observations. 76 percent of the individual animals they saw had the ability to make light. More than 97 percent of cnidarians (the jellyfish crowd) did, and the share of glowing animals was remarkably uniform across depths (Martini & Haddock, 2017).
NOAA's numbers line up: about 80 percent of animals living between 200 and 1,000 meters deep are bioluminescent (NOAA Ocean Exploration). Fish, squid and gelatinous zooplankton, including jellyfish, siphonophores and comb jellies, are the most common glowers.
Why is it usually blue?
Because blue light travels best through water (NOAA). Most bioluminescence is blue or blue-green. Some is nearly violet, some green-yellow, and less often, red (NOAA fact sheet). If you are going to signal in the deep, you use the color that carries.
What do they use it for?
Honestly, scientists do not fully know. NOAA says the main purpose is still not understood. But the list of likely uses is long: finding food, attracting or detecting prey, defending against predators, hiding, attracting mates and communicating (NOAA fact sheet). A few of the strangest tricks, all from NOAA Ocean Exploration:
- Invisibility by glowing. Some squid and deep-sea fish have light organs packed on their undersides. From below, their glow blends with the dim light coming from above, so they have no shadow. This is called counter-illumination.
- The burglar alarm. The jellyfish Atolla wyvillei flashes a bright ring of light when attacked. Scientists think it may be calling in bigger predators to eat whatever is eating it.
- Vomiting light. The deep-sea shrimp Heterocarpus ensifer spews glowing fluid from glands near its mouth, likely to distract predators while it escapes.
- The vampire squid. It is covered in light organs called photophores and uses a disorienting light show to throw off attackers.
- Rented light. Some anglerfish grow bioluminescent bacteria in special light organs. The fish feeds the bacteria. The bacteria make the light the fish uses to lure prey.
Still a mystery
Part of the problem is that glowing animals are hard to watch. Many live where we rarely go, and many types of bioluminescence are hard to see under ordinary light (NOAA fact sheet). Turn on a big white light to film the deep and you blind the show you came to see.
So the deep is not a dark, empty void. It is blinking. We just have not learned to read most of it yet.
Sources: NOAA Ocean Exploration, "What is bioluminescence?" · NOAA Ocean Exploration, Bioluminescence fact sheet (PDF) · Martini & Haddock (2017), Scientific Reports 7:45750