These Sharks Glow Green in the Dark and Scientists Now Know Why

Scientists have identified a substance that enables some sharks to give off a bright green glow, which they believe only other sharks can see.

The authors of the paper, published in the journal iScience, previously showed that swell sharks—who live in California's Monterey Bay to southern Mexico and the coast of Chile—are biofluorescent. That means they soak up light—in this case ambient blue ocean light—and use it to give off light in a different color, usually green, red or orange.

In their latest study, researchers wanted to learn more about what being biofluorescent means for these creatures. They looked at the swell shark, as well as the chain catshark that inhabits the western Atlantic ocean.

Past studies have shown the lighter beige parts of the species have a higher intensity of green fluorescence, compared with the darker areas.

Now, by testing the dark and light patches, the team found a group of small molecule metabolites in the lighter skin. What are known as brominated tryptophan-kynurenine small-molecule metabolites are different to the green fluorescent proteins used by biofluorescent creatures like jellyfish and corals.

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A glowing chain catshark: one of the species the scientists studied. iScience

Study co-author David Gruber, professor at City University of New York, told Newsweek: "I was surprised that the biofluorescence in sharks was so chemically different from the other forms previously discovered."

As well as giving sharks their glow, the metabolites also appear to have other jobs, like fighting infection, and finding one another in the ocean.

Jason Crawford, a professor at Yale University who co-authored the work, explained in a statement: "Imagine if I were bright green, but only you could see me as being bright green, but others could not."

He explained: "It's a completely different system for them to see each other that other animals cannot necessarily tap into. They have a completely different view of the world that they're in because of these biofluorescent properties that their skin exhibits and their eyes can detect."

"This study opens new questions related to potential function of biofluorescence in central nervous system signaling, resilience to microbial infections and photoprotection," said Gruber.

The team hopes the findings could be used to create new imaging techniques, which could be used in scientific research or medicine.

Gruber told Newsweek: "To me, this study highlights how much more we still have to learn about sharks and how many mysteries they still hold. Sharks have been around for over 400 million years and have survived multiple extinction events, but humans are currently killing them at a rate of 40 million to 100 million a year. This study focuses on two shy and reclusive species of shark that have been relatively under-examined in the past."

Crawford told Newsweek: "The ocean is an alien world right here on the planet Earth! We propose to explore other marine animals, elucidate the molecular origins of their light phenotypes, and repurpose them in biomedical imaging applications"

Matthew Davis, an associate professor of biology at St. Cloud State University who did not work on the study, told Newsweek: "Previous studies have highlighted the importance of proteins such as green-fluorescent protein as a mechanism of fluorescence in various animal lineages. This study highlights how little we still know about how bioflourescence functions across the tree of life by describing how metabolites are involved in shark fluorescence."

"This work further confirms that biofluorescence has repeatedly evolved across the animal tree of life and that the underlying mechanisms for fluorescent glowing can be strikingly different among animals and that much remains to be discovered, particularly regarding how vertebrates fluoresce," he said.