How fish started glowing over 100 million years ago

Biofluorescent fish have been glowing across the ocean for at least 112 million years. When blue light hits their skin, these animals absorb it and re-emit it as green, yellow, orange, or red.

Fish biofluorescence is invisible to us in daylight, but under the right light it turns reefs into underwater neon signs. It is a natural light show that has evolved silently beneath the waves, largely unnoticed by human eyes until recently.

Tracing the origins of glowing fish

Two new studies by scientists at the American Museum of Natural History’s Richard Gilder Graduate School trace how and when this glow took hold.

The research maps biofluorescence across 459 species and shows just how varied the colors have become.

The team combed through every known case of glowing bony fish, then added 48 previously unreported species. The analysis suggests that biofluorescence first showed up in ancient eels and has evolved on its own more than 100 times since.

“Researchers have known for a while that biofluorescence is quite widespread in marine animals, from sea turtles to corals, and especially among fishes,” said Emily Carr, lead author of both studies.

“But to really get to the root of why and how these species use this unique adaptation – whether for camouflage, predation, or reproduction – we need to understand the underlying evolutionary story as well as the scope of biofluorescence as it currently exists.”

Coral reefs speed up the glow

Life on a coral reef seems to speed up this evolutionary trick. Reef-dwelling fish acquire biofluorescence about ten times faster than species that stay in open water.

After the mass extinction 66 million years ago, when dinosaurs vanished and reefs rebounded, glowing species multiplied quickly, taking advantage of newly available habitats and ecological niches.

“This trend coincides with the rise of modern coral-dominated reefs and the rapid colonization of reefs by fishes, which occurred following a significant loss of coral diversity in the K-Pg extinction,” Carr said.

“These correlations suggest that the emergence of modern coral reefs could have facilitated the diversification of fluorescence in reef-associated teleost fishes.”

The color spectrum widens

In a second set of experiments, the team photographed museum specimens under special lights. Some families flashed at least six distinct peaks of fluorescence – far more variety than scientists had documented.

“The remarkable variation we observed across a wide array of these fluorescent fishes could mean that these animals use incredibly diverse and elaborate signaling systems based on species-specific fluorescent emission patterns,” said study co-author John Sparks.

“As these studies show, biofluorescence is both pervasive and incredibly phenotypically variable among marine fishes. What we would really like to understand better is how fluorescence functions in these highly variable marine lineages, as well as its role in diversification.”

How glowing fish could help medicine

Fluorescent proteins borrowed from jellyfish transformed molecular biology. The many new hues found in fish could lead to the next generation of imaging tools, helping doctors spot tumors or guide surgery with light.

For now, the ocean keeps many of its secrets hidden in plain sight. But every flash of color brings us closer to understanding how fish communicate, hide, hunt, and find mates – turning a midnight reef into a silent conversation of light.

Future research on fish biofluorescence

While these findings shed light on the deep history and diversity of glowing fish, there is still a lot more to uncover.

Scientists are now working to understand how different species actually see and use these colors in their everyday lives. Do they glow to blend in, to attract mates, or to send messages we can’t decode yet?

The answers could not only explain how these fish thrive in their underwater environments but also help us develop new tools in optics, communication, and even biotechnology.

Each discovery adds a small piece to a much bigger puzzle – one where light doesn’t just brighten the ocean, but also points the way to future innovation.

The full study was published in the journal PLOS One.

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