Deep beneath the waves, in the absence of light, rendezvousing with your pals can be a little bit tricky. But for certain species of shark, there is a simple solution – you glow in the dark.
Now, research published in the journal iScience has uncovered how certain species produce their impressive green glow using a type of marine biofluorescence never found before in sharks. The bright-green light, which only other sharks can see, may have numerous roles, such as fighting infection and signaling to fellow sharks.
Studying the skin of swell sharks and chain catsharks, the team managed to identify the molecule responsible for producing their other-worldly fluorescence – a previously unknown family of small-molecule metabolites.
"Studying biofluorescence in the ocean is like a constantly evolving mystery novel, with new clues being provided as we move the research forward," said David Gruber, a professor at the City University of New York and co-corresponding author of the study, in a statement. "After we first reported that swell sharks were biofluorescent, my collaborators and I decided to dive deeper into this topic. We wanted to learn more about what their biofluorescence might mean to them."
The researchers noticed that the sharks had two different skin tones – light and dark. Analyzing chemicals within the skin, they discovered a new kind of fluorescent molecule that was only present in the lighter skin.
"The exciting part of this study is the description of an entirely new form of marine biofluorescence from sharks – one that is based on brominated tryptophan-kynurenine small-molecule metabolites," said Gruber. Amazingly, these molecules may also have antimicrobial properties, the researchers say, perhaps explaining why the sharks don’t find themselves covered in tiny marine organisms despite living on the ocean floor.
Biofluorescence, not to be confused with bioluminescence, is when a creature absorbs blue light (like that found throughout the ocean) and emits it as a different color. Gruber has previously found biofluorescence in other sea creatures, from corals to hawksbill turtles to eels.
But how can the researchers see the sharks’ fluorescence if only other sharks can see it? Back in 2016, the team examined the sharks’ eyes to determine how they see color. Equipped with this knowledge, they were able to create a special camera that acted like a shark’s eye. Through this “shark-eye” lens, the sharks glowed bright green, despite being difficult to spot in low light with a regular human eye.
"If you can harness the abilities that marine animals have to make light, you can generate molecular systems for imaging in the lab or in medicine,” said Jason Crawford, a professor at Yale University and co-corresponding author of the new study. “Imaging is an incredibly important biomedical objective that these types of systems could help to propel into the future."
Sharks across the globe are facing extinction thanks to overfishing, mainly as a result of Asia’s demand for shark fin soup, which is considered a delicacy. A study released this week found that sharks are smaller and much less abundant near areas of human activity. Learning more about elusive shark species and how they behave and interact could help us protect them more effectively.
"Sharks are wonderful animals that have been around for over 400 million years. Sharks continually fascinate humans, and they hold so many mysteries and superpowers," Gruber added. "This study highlights yet another mystery of sharks, and it is my hope that this inspires us to learn more about their secrets and work to better protect them."
