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What Makes Some Deep-Sea Inhabitants Glow?

Bio-luminescent jellyfish swimming in aquarium

You don’t have to go deep underwater to find animals that glow (though the deeper you go, the more fascinating things become). What makes these critters glow is the same as what makes other animals glow, including the firefly.

What Is Bioluminescence?

That thing that makes some critters glow is called bioluminescence. Bioluminescence is seen above water through the fireflies mentioned above, but it is far more common underwater in vast oceans. According to Ocean Exploration and Research, the majority of organisms with this glowing ability are found in the water column, or pelagic zone, which is about 656 to 3,280 feet under the water.

The most common undersea creatures with bioluminescence are jellyfish and some fish. Others you may not be familiar with are siphonophores and comb jellies. It’s common for any underwater animal that is made up of mostly water to have some bioluminescence capabilities.


Here’s a more diverse list of bioluminescent organisms living under the water, aside from those previously mentioned:

  • Dragonfish
  • Dinoflagellates (algae)
  • Squid
  • Octopus
  • Anglerfish
  • Sea Salp

Bioluminescence is a chemical process and requires the molecule luciferin. The chemical reaction that happens when luciferin is brought into contact with oxygen is what produces the light, which is most often blue (which seems to travel the best in water, and the creatures glowing under there want to be seen for various reasons). Still, it can also be anywhere from a greenish-yellow to violet, and sometimes even red.

If you’re interested in the step-by-step scientific process of bioluminescence, this video walks you through the steps of how it works:

What Is the Purpose of Bioluminescence?

The purpose of bioluminescence varies from creature to creature. Some, like the firefly, use their ability to light up for attracting mates. Much like peacocks and turkeys fluff out their feathers to show off to a mate, the glowing of an appendage may tell a potential mate you’re interested.

The angler fish uses its dangling light to attract a meal. The fish dangles its light, but the rest of him (including his giant teeth) can’t be seen in the vast darkness of the ocean), and by the time a smaller fish realizes it’s been lured into a trap, it’s too late.

Aside from finding food and mates, another use of bioluminescence is for scaring away predators, as the sudden glow might look distasteful to a predator—creatures above and below the water use this defense mechanism.

Here’s a fun video that gives some examples of how bioluminescence is used and the process in which it works (since it can differ for different organisms):

What We Don’t Know About Bioluminescence

There’s a lot that still isn’t known about bioluminescence. It’s hard to study fish and other creatures in the deep, dark sea. And our inability to get down there and do much observing probably helps these creatures survive even more than their glowing lights alone do.

Many of the organisms equipped with bioluminescence are transparent, and only easily visible when they are glowing, so seeing them is difficult with the naked eye. Being used to dark space, shining a light on them will often chase the creature away. Some underwater creatures are light-sensitive, so shining a bright light into their dark world can permanently blind them and leave them for certain death with the inability to see food or predators.


Even though the science of bioluminescence is still somewhat of a mystery, it is interesting to view these strange and alluring creatures when able. You can sometimes catch a glimpse of glowing jellyfish in aquariums, and fireflies are a common visual experience in the early summer each year.

Yvonne Glasgow Yvonne Glasgow
Yvonne Glasgow has been a professional writer for almost two decades. Yvonne has worked for nutritionists, start-ups, dating companies, SEO firms, newspapers, board game companies, and much more as a writer and editor. She's also a published poet and a short story writer. Read Full Bio »