Why Do Animals Glow? The Science of Bioluminescence

From the depths of the ocean to the forest floor at night, some animals can do something that seems almost magical: they make their own light.

This phenomenon is known as bioluminescence. But glowing isn’t just for show. For many species, producing light is a powerful survival strategy.

In this episode of Big Ideas, we dive into the chemistry that makes living light possible.

It’s a hot summer night and you’re standing at the beach, and you notice that with each crashing wave, the ocean begins to glow with ethereal blue light.

Fish are leaving trails of light behind them, like underwater shooting stars.

It might look magical, but in actuality, it’s just your friendly neighborhood dinoflagellates—single-celled organisms that glow when disturbed.

That light you’re seeing is called bioluminescence.

Some cultures once believed that these lights were doorways to a mythical realm or the spirits of those who passed away.

Today we know it’s actually organisms creating their own light.

But how does this actually happen, and why would these creatures even do it in the first place?

Hi, I’m Niba.

Today we’re shining a light on bioluminescence, diving deep into how it works—and even making a light show of our own.

Welcome to Big Ideas, a new show from the team behind Deep Look.

While Deep Look zooms in on one small animal, Big Ideas zooms out, answering the big questions about how animals survive.

Bioluminescence can be found all over our planet, from marine plankton to fungi and even deep-sea creatures.

According to researchers, three-quarters of deep-sea animals make their own light.

Like this female anglerfish. In the pitch-black depths of the ocean, she dangles a glowing lure.

Inside are bioluminescent bacteria that entice unsuspecting prey before she swallows them whole.

Now, bioluminescence isn’t as widespread on land as it is underwater, but it does exist.

You’ll find it in some species of mushrooms and in the mycelia, or root structure, of certain fungi.

And of course, one of the most famous glowing creatures is the beloved firefly, also known as lightning bugs.

But they’re actually neither flies nor bugs—they’re beetles.

Out of about 400,000 beetle species, only half of 1% can actually glow.

So yes, you are special, little firefly—and so are all 2,000 beetle species in the Lampyridae family.

Anyways, while most glowing beetles thrive in tropical humidity, fireflies are very adaptable.

These remarkable insects have spread to every continent except Antarctica. I guess even fireflies draw the line somewhere.

So what exactly makes these beetles—not flies or bugs—flash when the sun goes down? What’s actually going on when they emit light?

Basically, bioluminescence is light produced inside an organism through a chemical reaction.

These insects produce light in a special organ in their abdomen called a photophore. We sometimes refer to it as a lantern, for obvious reasons.

Scientists discovered that for these beetles to create their special glow, four chemicals need to work together: oxygen, an enzyme called luciferase, the light-producing compound luciferin, and the energy molecule ATP—adenosine triphosphate.

So when the beetles want to light up, they redirect oxygen into their lantern through structures called peroxisomes. That’s where you find the enzyme luciferase.

Luciferase has these pockets, or cubby holes, that hold the light-producing compound luciferin right next to the energy currency of ATP. And now the stage is set for the beetle’s light show.

When oxygen is introduced, it excites the luciferin and the ATP, causing the duo to release a burst of energy—which is how these tiny insects create that big yellow-green glow.

We don’t have any fireflies in our studio, but we can create a similar chemical reaction using different substances.

We have luminol, sodium hydroxide, bleach, and the MVP ingredient: oxygen. This will create a chemiluminescent reaction similar to what’s happening in the firefly. Safety first, though.

Okay, let’s see what happens when I introduce the luminol solution to the bleach.

What? Yo, that was so cool!

The chemical reactions taking place in bioluminescence and in fire are quite similar. Both involve oxygen to create light—but there’s a key difference.

Fire produces both light and heat—this is called “hot light.” But bioluminescence produces only light, which is why it’s called “cold light.”

The chemical reaction behind bioluminescence is amazing because it converts nearly all its energy into light rather than releasing it as heat.

Imagine if this beloved glowing beetle emitted hot light inside its lantern—I don’t think it would survive that experience.

This is a really cool evolutionary marvel: producing light without heat. It must serve some kind of purpose.

Some species of glowing beetles start using light as a warning before they’re even born—while still in their eggs.

As larvae, they light up to tell predators like frogs or toads, “Hey, don’t eat me. I taste awful and might even be toxic.”

But as adults, these glowing beetles use their flashing lanterns for the ultimate goal: attracting a mate. Like nature’s version of a dating app.

The males are the flashy ones, flying around and showing off their light patterns like, “Hey ladies, check me out.”

Meanwhile, the females hang out in the grass or on plants. When a female sees a male whose flashing pattern catches her eye, she flashes back—that’s her way of saying she’s interested.

But it’s not all romantic fairytales.

In the beetle world, some sneaky female fireflies—called “femme fatales” in the Photuris genus—are masters of deception.

They copy the flash patterns of females from other species to trick males into thinking they’ve found a mate.

But when the poor guy flies down to meet her… surprise. He becomes dinner instead.

These glowing beetles have been around since the age of dinosaurs. The oldest known fossilized firefly is about 99 million years old.

And while it took generations of scientists to figure out how fireflies create their glow, today researchers are putting that knowledge to work in some pretty mind-blowing ways.

It’s even being used in cancer research.

Scientists can genetically modify cancer cells to glow, then inject them into mice.

Using specialized cameras, they can track tumor growth and spread in real time, helping them better understand how cancer progresses—and how it responds to treatment.

So bioluminescence is way more than just a fun light show.

It’s a survival tactic, a mating signal, and maybe one day a key part of new scientific discoveries.

It’s pretty amazing that these tiny glowing beetles are helping us better understand the world we live in.