A team of U.S. researchers has discovered a previously undescribed group of small molecule metabolites responsible for the green biofluorescence in two species of sharks. Not only is the newly-discovered chemical mechanism different from how most marine creatures glow, but it may also play other useful roles for the sharks, including helping them identify each other in the ocean and fight against microbial infections.

Biofluorescence is a widespread phenomenon in the marine environment, which results from the absorbance of the ambient blue ocean light and its re-emittance at longer, lower-energy wavelengths, visually resulting in green, orange, and red fluorescence.

“Studying biofluorescence in the ocean is like a constantly evolving mystery novel, with new clues being provided as we move the research forward,” said City University of New York’s Professor David Gruber.

“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.”

Professor Gruber and colleagues focused on two species of sharks, both in the family Scyliorhinidae: the swell shark (Cephaloscyllium ventriosum) from the eastern Pacific and the chain catshark (Scyliorhinus retifer) from the western Atlantic.

The scientists noticed that the sharks’ skin had two tones — light and dark — and extracted chemicals from the two skin types.

What they found was 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,” Professor 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 novel small-molecule fluorescent variants account for the biophysical and spectral properties of their lighter skin.

“This mechanism is different from animals in the upper ocean, such as jellyfish, that commonly use green fluorescent proteins as mechanisms to transform blue light into other colors,” Professor Gruber said.

“It’s a completely different system for them to see each other that other animals cannot necessarily tap into,” said Yale University’s Professor Jason Crawford.

“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.”

“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,” he added.

“It is also interesting that these biofluorescent molecules display antimicrobial properties. These catsharks live on the ocean bottom, yet we don’t see any biofouling or growth, so this could help explain yet another amazing feature of shark skin.”

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

The findings were published in the journal iScience.

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Hyun Bong Park et al. Bright Green Biofluorescence in Sharks Derives from Bromo-Kynurenine Metabolism. iScience, published online August 8, 2019; doi: 10.1016/j.isci.2019.07.019