Transforming the ocean’s blue light into a bright green color is a known ability by certain shark species. But the change of color, only seen by them, couldn’t be clearly explained by science yet. New research has brought new insight into the reasons for the phenomenon.

A group of researchers has identified that the shark’s bright green hue is caused by a previously unknown family of small-molecule metabolites. The mechanism is different from how most marine creatures glow and could also have useful roles for sharks, including helping them identify each other in the ocean and fight against microbial infections.

“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, co-corresponding author of the study. “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.”

Working with Jason Crawford, a professor at Yale University, Gruber focused on two species of sharks, the swell and the chain. They noticed that their skin had two tones, light and dark, and extracted chemicals from the two skin types. They found a type of fluorescent molecule that was only present in the light 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,” Gruber said.

 These types of small-molecule metabolites are known to be fluorescent and activate pathways similar to those that, in other vertebrates, play a role in the central nervous system and immune system. But in the sharks, the molecule variants account for the biophysical and spectral properties of their lighter skin.

“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 that their eyes can detect,” Crawford says. “Imagine if I were bright green, but only you could see me as being bright green, but others could not.”

The molecules also serve multiple other purposes, including to help the sharks identify each other in the ocean and potentially provide protection against microbial infections, the researchers found.

While the study focused on two biofluorescent shark species, Gruber and Crawford hope to more broadly explore the bioluminescent and biofluorescent properties of marine animals, which can ultimately lead to the development of new imaging techniques.

“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. Imaging is an incredibly important biomedical objective that these types of systems could help to propel into the future,” Crawford said.