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Fireflies, crustaceans, jellyfish -- lots of living things glow, and they do it for all kinds of reasons, some of which we haven’t even discovered yet.

Hosted by: Hank Green
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(Intro music)

Nature puts on some spectacular light shows, from the sight of fireflies on a summer evening to astonishingly blue beaches, glowing creatures are everywhere. But light is a form of energy, and living things don't waster energy for nothing, so why do they glow? 

Bioluminescence, which is what it's called when living creatures give off light, has evolved separately dozens of times over the course of the history of the Earth. So there must be some advantage to glowing, they're not just doing it 'cause it's pretty. Well actually the fireflies kind of are. Adult fireflies use their glowing bodies to find mates, and to make sure they find the right species, different kinds of fireflies use different patterns of flashes. They glow using a protein called luciferase, which transforms oxygen, energy from the cell's energy storage molecules ATP, and a special starting chemical called luciferin, into light. Super small changes to the firefly luciferase protein can shift the color of light it makes and fireflies glow from yellow-green to orange and red. Why would they wanna make different colors? Well one possibility is that they evolved to contrast with the background colors of different habitats so their mates can see them clearly.

A bunch of other species use different type of luciferase proteins too, like a tiny crustacean called a ostracod which has a frankly hilarious defense mechanism that you might have seen on your Tumblr dash or your Facebook feed. When a fish spots an ostracod, it thinks that's a nice little snack, but the instant the ostracod is swallowed it squirts our a bunch of luciferase. Now these are tiny tiny fish and their bellies are transparent so suddenly the fish's stomach is glowing bright blue, which is a signal to larger predators that want to eat that fish, that there's a tasty snack right there. Now the little fish doesn't want that so it barfs up the ostracod and the glowing stuff and swims for its life. A lot of times when I've seen people share this, they think that the fish is actually spitting out glowing-like stuff like a night fury in How to Train Your Dragon but it's not, it's spitting out a glowing organism that doesn't want to be eaten, so the ostracod lives to glow another day.

Another blue-glower, sea sparkle, isn't even an animal, it's a type of creature called a protist. It floats through the ocean as plankton, not doing very much, but when it's disturbed by predators or even just choppy surf, it activates its own version of luciferase. Why sea sparkle does this isn't entirely clear but it might be similar to the reason the ostracods do it. When sea sparkle is glowing, predators tend to avoid eating it. Whatever the reason, when it blooms in places like Hong Kong and the Maldives, sea sparkle can turn the water by the shore a brilliant blue. Best of all, unlike some of its close relatives, sea sparkle isn't toxic to humans. If you're lucky enough to be around one of these blooms, it's safe to swim and surf and then the blue light will follow you around like a personal glowing trail.

Most marine life like the ostracod and the sea sparkle produce blue light because it travels best through water. But there is an interesting and scientifically important exception - one species of jellyfish, the crystal jelly, glows green. To do that it uses two proteins. The first one, aequorin, actually does make blue light. The second protein called GFP is fluorescent, meaning that when it's hit by one color of light, in this case the blue from the aequorin, it absorbs some of its energy and then releases a lower energy version of that light, green. Why the jellyfish shifts its glow from blue to green, or why it glows in the first place, no one knows. It's tough to see the crystal jelly's bioluminescence at all unless researchers disturb it so we can't tell what it uses it for when no one's watching.

Now we may not know what the jellyfish uses its light for but scientists use it for a lot of things like making cellular structures visible under a microscope. So in 2008, GFP's discoverers shared the Nobel prize in chemistry for finding a way to make things light up in the lab. Luciferase, especially in fireflies, has its uses too. Mostly in helping us discover what condition you need to turn on genes. You just replace the gene with luciferase and you see it light up when those conditions are met.

Not only have these creatures evolved ways to make themselves seen, their glowing proteins let us see the life that we study too. So we can thank fireflies for both blinking lights on warm summer evenings and for advances in biology and medicine. So thank you, fireflies, and thanks for watching, though I don't really know why fireflies watch YouTube videos. It's another unexplained mystery of science.

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