We Have No Idea Why

Most animals on earth are bioluminescent, but almost all of them live in the ocean - and scientists aren’t sure why.

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To learn more about this topic, start your googling with these keywords:
- Bioluminescence: the biochemical emission of light by living organisms such as fireflies and deep-sea fishes.
- Biofluorescence: The emission of previously absorbed light by fluorescent proteins in a living organism.
- Luciferin: an organic substance, present in luminescent organisms such as fireflies, that produces light when oxidized by the action of the enzyme luciferase.

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Cameron Duke | Script Writer, Narrator and Director
Sarah Berman | Illustration, Video Editing and Animation
Aldo de Vos | Music

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REFERENCES
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Haddock, Steven. Senior Scientist, Monterrey Bay Aquarium - Personal Communication.

[tone]

I'm no expert, so it's very possible I'm missing something, but it seems like there'd be a significant difference between nocturnal/polar darkness levels and deep-sea darkness levels. Most nocturnal animals can adapt to lower light levels by simply improving their night vision, but at some point, that'd probably start showing diminishing returns, and when you get deep enough in the ocean the level of darkness could be high enough that those sorts of adaptations wouldn't really work, forcing other approaches. That doesn't necessarily answer all of the problem (I assume I'm not the first person to think of that, after all) but it feels like it'd at least go a long way toward starting to explain the discrepancy, right?

[ionic]

My guess is it has to do with the poupulation density of places. The ocean is vast and I’d imagine it could be hard to find anything in the deeper sections of it without the bioluminescence. However, in land you normally can find mates and prey without traveling too far. Not only that, but prey having bioluminescence could make them much more noticeable in more populated areas while it may be safer in the oceanic depths where predators are scarce

[candiman]

Well, the part of the animal that glows must be transparent to light, so maybe land animals need to prioritize protection against the sun's UV rays, or protection from other things, like hair or scales. It seems that most glowing bits are mucus membranes or soft tissue, which is much more vulnerable on land. Just a guess, and doesn't explain the darkness of lakes.

[xyzest]

My best guess is that this has to do with all animals actually emitting light all the time, not in the visible, but in the infrared spectrum. Many insects, reptiles, and some mammals evolved to be able to detect IR light because air (but NOT water!) is transparent to it. So even in the dark, organisms on land have no need to develop bioluminescence: air allows them to be already detectable!

[axkee]

I don't think you properly ruled out darkness as a major reason. Land is always being lit up, even during a new moon the stars still give off light which animals can adapt to using. The ocean is completely dark after a certain depth though making the small amount of light gained from bioluminescence worth much more. Unless there are just as many bioluminescent ocean creatures in the low light levels of the ocean as the no light levels, then it's still a mystery.

[TheAgamemnon]

It might be just a simple question of geometry. The ocean has one more dimension to get lost in, while any organism on land can efficiently search mates in only two dimensions. (Height in air doesn't count, because nothing can stay at altitude forever) Thus not enough benefit from being glowy.

[ordinaryorca]

I think necessity plays an important role. To find a mate on land animals use flashy colours or pheromones but smelling is much more difficult in the ocean, sight is useless at great depths and taste is useless in general.
Meaning animals need to specialise their hearing (echolocation), sight or create a new sense (electroreception).
Using bioluminescence allows you to use your existing senses without upgrading them to their extremes. It is also very versatile such as finding mates or prey and even as a defense. This versatility means that many pressures can lead you down this path explaining in part why they love it so much.

[MagnificentCreature]

Do also note that the land animals with biolumi are insects who adapt rather fast. And seems like a good portion of then can fly and spread vast distances.

I suppose to find similar animals on land, if you’re walking you preferably want other ways to leave/track traces, and smell/scent seems easier than light (to find mates) which may draw unnecessary attention.

But usually you are in similar ecosystem that isn’t as vast in 3 dimensions as the ocean.
That explains the caves thing, and the explanation above (and with the moon I guess) explains the no light on land thing.

[yeasstt]

I always thought it evolved as a camouflage method, like how hatchetfish use it today. That would kind of rule it out for mammals and reptiles, as they wouldn't be able to make much use of it due to a lack of predation from below.

Bioluminescence is fascinating to me, as an icthyologist, and I love reading about cool bioluminescent fish. My favorite examples are probably the Malacosteus (stoplight loosejaws) that use red bioluminescence to detect prey. Many deep sea fish are red (and cant see red) since red light doesn't penetrate to their depth. Stoplight loosejaws basically shine an invisible flashlight on their pray, which is awesome

[bryrusmi]

By no means am I a chemist. But the first question that pops into my mind when I heard the premise of the question was, "what's the chemical makeup of the bioluminescence both the land and the sea"? Is the bioluminescence similar or vastly divergent?

[brianpso]

Maybe it also has something to do with how light travels through water. Light can reach a lot farther in air than in water, so a creature that can generate light on its own can attract whatever it wants to while not broadcasting its existence to possibly unwanted beings.

[cldNclr]

Firefly:*Doesn't live in the ocean*
MinuteEarth:"I can't believe you've done this"

[brandonpiel]

It might be that the bar for evolving bioluminescence is lower in the ocean than on land. There are several species of marine bacteria that bioluminesce (such as Vibrio harveyi). For a marine animal to evolve bioluminescence, all it has to do is culture these bacteria somewhere in it's body. Since these bacteria are only found in marine environments, non-marine creatures can't form a symbiosis with them, and therefore, must evolve bioluminescence on their own.

[BmanX]

I feel like there’s a difference between the darkness at the bottom of the ocean than the darkness at night with the moon, stars and human made light

[missnaomi]

1) It's okay to not have all the answers.
2) Thank you for a video featuring one of my favorite words/phenomena, bioluminescence.

[KnowArt]

Yeah! Negative result videos! I have a soft spot for "yup, we just don't really know, but it's super interesting nonetheless"

[colorado]

Maybe it is because of a funding issue? The devs haven't rolled out the mod to everyone yet as short qualified programmers. They should open source it.

[stevehiler]

I’m not anywhere near being a professional in any of the areas of bioluminescence, evolution, or bodies of water, but a lot of animals in these really deep lakes like eels and catfish have almost like a sixth electrical sense that can detect things in the murky depths and I’d like to think that probably works better for them in freshwater as opposed to the ocean if not only for the fact that it doesn’t work as far as bioluminescence (especially with how cloudy freshwater is in my experience)

[ReimaginedSkies]

Maybe it isn't just salt but some sort of microorganism that has a mutual relationship with many sea creatures. These organisms evolved and depend on saltwater to live which is why it only appears in salty water but adding salt to water changes nothing. If the microorganisms weren't there to begin with or are usable for freshwater creatures it wouldn't matter.

It would be like how termites have special gut bacteria to break down wood that is different from our own gut bacteria. If humans started eating wood unless we somehow came into contact with that bacteria and it evolved for a mutual relationship we still wouldn't be able to eat wood. If we are in an environment like a desert that doesn't support wood and so that bacteria it would be impossible for that exposure to ever occur.

[cfv]

I am outraged that this video titled "we have no idea why" has not given me a definitive answer to this question.