Shark glows in the the dark waters

See the ocean’s glow-in-the-dark world on a fluorescent night dive

These kaleidoscopic swims help us better understand and protect our seas. Here’s why—and where—to try it.

A chain catshark, one of the 200 species of marine animals to display biofluorescence, is seen glowing neon green on a deep sea fluorescent dive near California‘s coast. Surreal diving experiences like this help scientists better understand and protect marine life—and travelers can now try it too.

From paddle trips atop glowing waters to festivals for twinkling fireflies, nature enthusiasts circle the globe to experience bioluminescence. But this eye-catching phenomenon, most prevalent in summer, isn’t the animal kingdom’s only mesmerizing light show.

Some travelers are discovering the ocean’s vast, yet often secretive, world of biofluorescence—a seascape where fish glow in lime-green patterns, corals sway in neon hues, and shimmering viridescent seahorses could be mistaken for extraterrestrials.

The way in? Fluorescent diving, an activity once reserved for marine biologists. “It’s like you’re on a different planet,” says longtime diver Alisha Postma, co-founder of scuba blog Dive Buddies 4 Life.

Like many marine phenomena, biofluorescence might as well be something from a different planet. More than 80 percent of the ocean has never been explored, and according to NASA, we know more about the moon’s surface than we do our own sea floor. And we’re only just beginning to understand topics such as ocean biofluorescence.

Scientists have tuned into the phenomenon within the past 10 years, says marine biologist and National Geographic Explorer David Gruber. He has helmed several breakthroughs on the subject, including a 2019 study that found how animals create these glowing effects with molecules that were previously unknown to science. He worked on a 2014 study proving biofluorescence is widespread among more than 200 fish. His research has also shown that certain sharks and even reptiles, including sea turtles, display glow-in-the-dark powers—a revolutionary finding that was named one of National Geographic’s top 20 discoveries of the 2010s.

According to Gruber, fluorescent dives and snorkeling trips make even the most familiar reefs and dive spots look brand new. “People who think they’ve seen the ocean and know it well, suddenly they’re just blown away,” he says. He notes that this glimpse behind the curtain, while fantastical in color, is not just for our novelty. “These animals are sharing a secret with us,” he says, and that it’s our responsibility to use this knowledge to protect them.

What is biofluorescence?

The logistics of a night dive are pretty simple: “You put on a yellow mask then turn on your blue light, and suddenly, especially on a coral reef, it’ll just totally light up,” says Gruber. The “why” is a bit more involved.

Just as animals hear the world differently—consider bats, which communicate largely via frequencies beyond our hearing range—they see the world differently, too.

With biofluorescence, blue wavelengths of light hit the surface of an animal and are then emitted as a different color, usually vivid greens, oranges, and reds. This is different from bioluminescence, where animals such as jellyfish or fireflies generate their own light through a chemical reaction.

(See amazing ocean creatures that glow.)

The ocean adds a layer of complexity to the phenomenon. While humans can see light in shades of red, green, and blue, our vision falters underwater. As the ocean gets deeper, certain wavelengths of color in the visible light spectrum are filtered out. At around 20 feet, red is gone. After 100 feet, it’s virtually all blue and green until you hit the light-free midnight zone, roughly 3,000 feet below the surface. 

Many marine animals closer to the sea floor have evolved to have yellow eye filters that help them detect biofluorescence in other fish. Humans, however, need special equipment like yellow mask filters and blue dive lights to see these creatures glow in kaleidoscopic colors underwater.

See this secret world

Fluorescent diving may feel like an exotic expedition, but it’s increasingly available in dive centers around the globe, according to Eric Albinsson of the global dive authority Professional Association of Diving Instructors (PADI). He says anywhere with “tropical seas and healthy coral” is ideal. Unlike bioluminescence, which is most common in summer, travelers can see biofluorescence year-round.

The only fluorescent diving requirement is the standard PADI open-water certification, but the organization’s night-dive specialty course promises added safety in these dark conditions. At the very least, it’s wise to first experience a few night dives with the usual bright torch lights (both for safety and the protection of fragile species). “You don’t want to be bobbing up and down and disrupting the [ocean] bottom,” says Postma, who’s taken multiple fluorescent diving trips off Bonaire, an island in the Caribbean.

Bonaire, known for impressively healthy (but increasingly threatened) reefs, has made a name for itself as a fluorescent diving hot spot. It was one of the first destinations to offer the experience to hobbyist divers, according to PADI. Lars Bosman of Bonaire-based Buddy Dive Resort says the activity lets divers see creatures they might miss during the day, such as anemones or sand-diver fish that hide on the ocean floor. One study based in Australia shows the ocean’s well-camouflaged species, known as “cryptic fish,” are up to 70 times more likely to glow than more visible critters.

In recent years, destinations including Thailand and the Maldives have made this colorful experience available to non-divers through fluorescent snorkeling. “Some reefs in the Maldives are quite shallow, [so] fluorescent snorkeling can be just as enjoyable as fluorescent diving,” says Ahmed Mujthaba, owner of Mujavaz Scuba and Travels. Mujthaba notes that distance from light pollution makes the biofluorescent hues more vibrant.

Explore, then protect

Whether you’re a first-time fluorescent snorkeler or a seasoned marine biologist, you’re bound to leave with new questions about the ocean. “It’s like a mystery novel that keeps going deeper,” says Gruber.

Gruber first noticed biofluorescence in fish when a neon green eel was spotted in his photo research of biofluorescent coral. Over the past decade, scientists have found evidence for biofluorescence in more than 200 species of fish, including two species of catsharks, and marine turtles. But with so little research on how and why these marine animals glow, scientists have more questions than answers. Likely theories for biofluorescence uses include interspecies communication, finding mates, camouflaging from predators, and hunting prey.

(These sharks glow underwater—thanks to tiny “lightsabers.”)

To chip away at these mysteries and better understand how light affects them, Gruber and his team create cameras that mimic the eyes of biofluorescent marine animals to see the world from their point of view. When they discovered biofluorescence in catsharks, they created a camera equipped with special filters to simulate how light hits their eyes. It unveiled two important insights: catsharks see the green biofluorescence they emit themselves, and they can increase the contrast of their fluorescent patterns.

Their research isn’t just important for science; it’s paving the way for smarter conservation practices. After all, the more we know about marine life, the better we can protect it.

The National Geographic Society, committed to illuminating and protecting the wonders of our world, funded David Gruber’s work. Learn more about the Society’s support of ocean explorers.

Stephanie Vermillion is a travel and outdoors journalist, filmmaker, and photographer. Follow her adventures on Twitter and Instagram.

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