How deep-sea creatures make their own light

HostThink about being miles under the waves. It's pitch black, colder than a fridge, and the weight of all that water is heavy enough to crush a car. You would think it's a dead, empty place, but then, a tiny spark flickers in the dark. How do these fish actually pull off making their own glow when there's no sun to give them any energy?

GuestIt's basically like carrying a tiny chemistry set inside your body. Think about those glow sticks people shake at a party. When you snap the stick, two different liquids mix together and start to shine. Deep sea creatures do almost the exact same thing. They have these special parts in their skin or deep inside their bodies where they keep two types of stuff. One is a kind of fuel and the other is a helper that makes that fuel burn with light. But it's not a hot light like the bulb in your kitchen. If their light got hot, it would cook the fish from the inside out. They make what we call cold light. It's a very clever trick that turns almost all the energy into a glow instead of heat.

HostSo they just have these little bags of fuel sitting in their skin that they can use whenever they want?

GuestSome of them do. They have little spots that work like flashlights. Some even have shutters, kind of like a tiny eyelid over the light, so they can blink at each other or hide the glow if a hunter comes near. But here is the really wild part. Not every fish makes the light by themselves. A lot of them are more like landlords. They have tiny glowing bacteria living inside them in a little pouch. The fish gives the bacteria a safe place to live and plenty of food, and in return, the bacteria provide the light. It's a win for everyone involved.

HostWait, so the fish is basically carrying around a bag of glowing germs? That seems like a lot of work just to see where you're going in the dark.

GuestWell, that's the thing. It's usually not about seeing the path ahead. In the deep ocean, you actually don't want to be seen. If you turn on a big bright light, every hungry thing in the area knows exactly where to find its next meal. So, most of the time, they use the light for trickery. You might know about the anglerfish with that little fishing pole on its head. That glowing tip is a lure. It looks like a tiny, tasty snack to other fish. They swim toward that little spark thinking they found lunch, and then, before they know what happened, they become the lunch. But light can also be used as a way to hide.

HostThat doesn't make sense. In a dark room, if I turn on a lamp, I'm easier to find, not harder. How can light be a cloak?

GuestIt feels backwards, but it works because the ocean isn't totally black everywhere. If you're a fish swimming in the middle depths, there's still a tiny bit of blue light drifting down from way up at the surface. If a shark is swimming below you and looks up, your body creates a dark shadow against that dim blue light. You're a clear target. To fix this, these fish have glowing bellies. They have rows of light spots that match the color and the brightness of the light coming from the surface. They turn themselves into a window so the shadow disappears. From below, they just look like more water.

HostSo they're hiding by turning into a light bulb.

GuestIn a way, yeah. They blend into the sky above them. But they also use light to talk. Since they live in such a huge, empty space, finding a mate can be nearly impossible. Some of these creatures have very specific patterns of lights along their sides, like the lit up windows on a plane. Only their own kind has that exact pattern. It's like a secret code in the dark. There's even a type of shrimp that spits out a cloud of glowing blue slime when it gets scared. It's like a smoke screen made of light that confuses a hunter while the shrimp swims away into the dark.

HostBut surely there's a price to pay for all this. You can't just keep glowing forever without running out of those chemicals or the energy to feed the bacteria.

GuestThey're very careful with it. Making light takes a lot of fuel, and food is very hard to find down there. They don't just leave the lights on like we might do in a hallway at home. They use it in very short bursts. And the colors they use are very specific too. Almost all the light you see down there's blue or green because those colors travel the best through water. But a few clever fish can make red light.

HostDoes the color really change things that much?

GuestIt's a massive edge. Almost nothing in the deep sea can see the color red. Their eyes are only tuned to see blues and greens. So these fish shine a red beam out of their eyes like a pair of secret night vision goggles. They can see a shrimp or a smaller fish clearly, but the prey has no idea it's being watched. It's like being the only person in a dark room with a flashlight that no one else can see. They can hunt in total secret.

HostSome fish even use light like a burglar alarm, where they glow as bright as they can when they're caught so a bigger predator will see the thing eating them and come to help.

GuestThose tiny flashes are the difference between life and death in a place where the sun never shines.

HostFish use these glowing bellies to vanish into the dim light from above, proving that in the deep, the best way to stay hidden is to stop being a shadow.