Tiny Fish Use Bacteria to Glow in the Dark | National Geographic

(Calming music) - I was in the Solomon Islands on a National Geographic expedition. We were working in a shallow reef, and we had a big blue light that we were filming fluorescent corals. One of the safety divers, Brendan Phillips, came up to me and just started tugging on my camera and basically just gave me the message, you know, follow me.

So I turn off my lights, I followed him for several hundred meters in the dark. Suddenly, I see why he pulled me there. There are literally thousands of blue, blinking bioluminescent lights. And they were coming together, and they were joining, and there would be circles of them, and it was almost like a blue, bioluminescent brick road just descending down the reef, making all these shapes. It's the closest thing I've ever had to an Avatar moment.

This is the largest aggregation of Flashlight Fish that I believe humans have ever come across. These animals, they don't even come out when the moon is out. They're so sensitive to light because they're so easily gobbled up by a bigger predator.

So it has this subocular bioluminescent organ under its eye, and it grows, like a garden, these bioluminescent bacteria. And it grows them in these tubes, and it even projects the light outward. It's even grown this vasculature to feed, to pump oxygen, to keep these bioluminescent bacteria glowing bright.

One thing that they do is when they're actually eating, they will keep their light on so they can see the food. So they're very visual creatures. And they're using their light to feed. But when they're not feeding, they're using their light to be able to move in a school.

A quarter of all fish species, sometime in their life, they school. And there's all kinds of benefits to schooling. There's safety in numbers, and it makes it harder for a predator to really zone in on one specific fish.

What's unique about these animals is the relationship they have with this bioluminescent bacteria that they harvest in their eye. Only nine species have this ability. We do know that they do something called a blink and run. When they want to evade a predator, they will start swimming in one direction, blink, and then immediately turn in the other direction.

So a predator trying to follow in the dark will lose it. Recording this and proving this opens up the possibility that the deep sea is filled with billions of bioluminescent schooling fish, and us humans have just not seen this yet because we're not in the deep sea with all our lights off.