How living creatures turn chemistry into glowing light

HostIt's such a strange thing to see a firefly in the woods or a glowing wave at the beach. You expect light to come from a fire or a bulb, but seeing it come from a tiny bug or a fish feels almost like magic.

HostHow do these creatures actually pull this off without a plug or a flame?

GuestIt's basically a very clever trick with chemistry. You can think of it like a glow stick you crack at a party. Inside that stick, you have two different liquids kept apart by a thin glass wall. When you bend the plastic, the glass breaks, the liquids mix, and they start to glow. Living things do something very similar, but they use their own bodies to mix the parts together. There are two main players in this game. One is a kind of fuel, and the other is a helper that gets things moving. When the helper meets the fuel and there's a bit of oxygen around, they react. But instead of that reaction making a lot of heat, like a fire would, it releases almost all its energy as pure light.

HostBut wait, if I touch a light bulb, it burns my hand. If these bugs are making light with chemicals, shouldn't they be cooking themselves from the inside out?

GuestThat's the wildest part about it. Most of our lights are really wasteful. A regular old bulb turns most of its energy into heat, and only a tiny bit becomes light. These creatures are the most efficient light makers on earth. Nearly all the energy they use goes straight into the glow. We call it cold light because it produces almost zero heat. A firefly can glow all night and stay perfectly cool to the touch. They have spent millions of years perfecting this. If they were as messy with their energy as our light bulbs are, they would fry.

HostSo they have this amazing power, but what's the point? It seems like a great way to get eaten if you're shining like a neon sign in a dark forest.

GuestWell, for some, it's about finding a date. Fireflies use their flashes like a secret code to find partners. Each kind of firefly has its own pattern, like a unique pulse of light, so they can find their own kind in the dark. But in the deep ocean, where it's pitch black, the rules change. Down there, light is a tool for hunting. Think of the anglerfish. It has a little glowing bulb hanging right in front of its mouth. Small fish see that light and swim toward it, thinking they found a snack or a friend, only to become a meal.

HostI always thought every glowing fish was just born with that ability built into its body. Is it always something they make themselves?

GuestNot always. There's a little squid called the Hawaiian bobtail squid, and it has a very strange deal with the world. It's not actually born with the power to glow. Instead, it has a special pouch that it keeps empty until it can scoop up a very specific kind of glowing bacteria from the water. It gives those tiny germs a safe place to live and plenty of food, and in return, the germs provide the light. The squid even has a way to dim the light or make it brighter, almost like it has a built in dimmer switch for its bacterial friends.

HostThat seems like a lot of work just to have a flashlight. Why go through the trouble of feeding bacteria?

GuestIt's more like a cloak of invisibility. The squid uses that light to blend in. If you're a hungry fish swimming below the squid and you look up, you would usually see a dark shape against the moonlight coming from the surface. But the squid turns on its belly lights to match the glow from above. It disappears. It's a brilliant way to hide in plain sight.

HostSo it's not just about seeing or being seen. It's about tricking the eyes of anything watching. Are there any other ways they use it to defend themselves?

GuestOh, some of the tricks are much more active. There's a type of deep sea shrimp that doesn't just glow on its own. When a predator comes near, it spits out a cloud of glowing blue goo. It's like a underwater smoke screen, but instead of hiding in the dark, it blinds the attacker with a bright flash. While the predator is confused by this glowing cloud, the shrimp swims away into the darkness. There's also the burglar alarm trick. Some tiny plants in the ocean glow when something bumps into them. If a small shrimp starts eating these plants, they light up immediately. This tells any bigger fish nearby that there's a shrimp right there. The big fish comes over to eat the shrimp, and the tiny plant is saved because its enemy got eaten.

HostIt sounds like the deeper you go, the more the world is run by these little lights.

GuestIt really is. In the deep sea, where the sun never reaches, most of the animals we find have some way to make light. We used to think of the deep ocean as a silent, dark desert, but it's actually full of these chemical signals. We're just starting to map out how they all talk to each other. Some fish even have lights under their eyes that act like red headlamps. Most fish can't see red light, so these hunters can shine their red beam on a target and see it perfectly, while the target has no idea it's being watched.

HostThe ocean is basically a giant game of laser tag where the stakes are life and death.

GuestThat's a great way to put it. And we're still finding new versions of this every year. Scientists recently found that even some sharks have glowing bellies. We're realizing that for most of the life on our planet, light isn't something that comes from the sky, it's something you carry with you.

HostFireflies in our backyards are really just the tiny, visible part of a massive world of glowing life hidden in the deep.

GuestMarine biologists estimate that about three quarters of all the creatures in the deep ocean make their own light, which means this is actually one of the most common ways to communicate on our entire planet.

HostThe tiny flash of a bug in the summer air is a small reminder that the dark is never as empty as it looks.