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If we want to see more clearly in the dark, we shine a light - but we aren't the only species that does that. Some fish use active photolocation to shine their own light in the deep sea!

Hosted by: Michael Aranda

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[♪ INTRO].

When we want to see something more clearly in the dark, we shine a light—because light helps with seeing. But... we’re not the only species that does this.

Some fish have evolved a nifty trick called active photolocation, which is a fancy way of saying they can shine lights to see better! Now, most predators find it a whole lot easier to catch things when they can see them. But even with great eyesight, seeing isn’t always easy, especially when you live underwater.

Water is often murky, and more to the point, it filters out light. So, the deeper you go, the darker it gets. That may be why the only non-human animals we know of that can perform active photolocation come from the sea.

Take black-faced blennies, for instance. These little fish can’t make their own light, but they appear to manipulate the light from the Sun. Thanks to the spherical lens covering their eyes, some of the sunlight from above misses their pupils—so it doesn’t go into their eyes.

Instead, it bounces off mirror-like spots that sit just underneath them, generating a sideways beam of light known as an ocular spark. Studies suggest the fish can direct this spark to help them find prey, as many undersea creatures have eyes that reflect light, much like a cat’s. And they can probably turn this spark on or off or even change its color through small eye movements that slightly redirect the sunlight.

Some of the below-pupil spots contain a blue pigment. So, when sunlight hits there, only blue light is reflected outward. Others contain fluorescent compounds that absorb blue light and emit lower-energy red light instead.

So, when sunlight hits them, the fish emit a red spark. And studies have shown that the spark color changes depending on the background the fish are looking at! Which makes sense, since it’s kind of useless to look for blue light bouncing off things if everything looks blue.

That may actually be why they need the spark in general: because their favorite food does a great job blending in. They snack on tiny critters that are easily mistaken for algae. But algae don’t have reflective eyes, so shining lights at potential meals can help the fish separate yummies from seaweed.

These ocular sparks may also help them spot predators. When researchers gave fish little, light-blocking caps, they struggled to avoid predators lying in wait for a snack. Ocular sparks are so useful, in fact, that these blennies might not be the only fish to have them.

Researchers think the phenomenon may have been overlooked because it happens on a very small scale. And they only discovered it in the past few years! We know a lot more about really obvious shiners—like flashlight fish.

That name pretty much speaks for itself. They have a bean-shaped organ underneath each of their eyes that shines light. But, since they hunt at night, they get no help from the Sun.

Their light comes from symbiotic bioluminescent bacteria instead. These helpful microbes live in the fish’s under-eye organs, and they generate a blue-green light with a wavelength of about 500 nanometers. Which, conveniently, travels farther underwater than other visible wavelengths.

Each organ is essentially a pouch with a big mirror in the back that directs light outward. And it’s connected to the edge of the fish’s eye in such a way that they can turn the light on and off by blinking. These fish likely light up their prey’s eyes in order to find them, too.

Their flashlights come in handy for communicating with one other, and maybe even for evading predators! But they’re not the only group of fish with blue flashlights. The appropriately-named lanternfish also have them.

And so do dragonfish. As their name implies, they’re ferocious predators with loads of pointy teeth. And, they, too, sport blue-light producing organs, right behind each eye.

Even more impressively, they make that light themselves, without the help of bacteria. But just to make things a little creepier, at least nine deep sea species also have organs under their eyes that emit red light! This light starts out the same blue that shines from behind their eyes.

But then, it hits fluorescent pigments that absorb it and re-emits it as red light. Some species even have a filter on top of this to ensure that they shine out at a very specific wavelength. Why go through all of that extra effort?

Well, red light is the first color to disappear as you dive deeper underwater because it carries the least energy of all visible wavelengths. That means it never reaches the deep sea. So, most of the species there have ditched their ability to see reds over evolutionary time.

Not the dragonfish though! Some species have special light-sensitive proteins in their eyes that detect it. Others use a special pigment that acts like — and comes from — chlorophyll, the main pigment plants use to harvest light energy.

This modified pigment is able to absorb red light energy and transfer it to the fishes’ other light-sensitive proteins, broadening their visual range. And since, down in the deep, pretty much only other dragonfish can see red light, they can shine it at whatever they want without alerting prey or predators. They basically have built-in night-vision!

Studying them and all these other flashlight-wielding fish can teach us about the complexities of vision, especially underwater. And that, in turn, could help us and our machines see better —beneath the waves, and in general. Plus, the more we learn about fish eyes, the better we’ll understand fish behavior and ecology.

So, you could say all this research on active photolocation is really shining a light on evolution in the murky depths. I couldn’t resist. If all the talk about dragonfishes and their glowing red eye-lights wasn’t quite enough nightmare fuel for you,.

I’d recommend watching our episode about their teeth. And be sure to subscribe and ring the notification bell to keep learning about this bizarre universe with us! [♪ OUTRO].