Glowing bright green may not seem like the most effective disguise, but some sharks take on the colour to make sure only their own kind can identify them.
And while a green hue may be a sign of illness in humans, it could help the sharks fight off infections, scientists say.
In the depths of the sea, certain species transform the ocean's blue light into a bright green colour that only other sharks can see.
Researchers have now identified what lies behind this transformation – a family of small-molecule metabolites.
They say this method of biofluorescence is different from how most marine creatures glow.
David Gruber, a professor at City University of New York and co-corresponding author of the study, said: "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."
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.
Corresponding author, Jason Crawford, said: "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.
"Imagine if I were bright green, but only you could see me as being bright green, but others could not."
Prof Gruber 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 signalling, resilience to microbial infections, and photoprotection."
The study, published in the iScience journal, focused on two species of sharks – the swell shark and the chain catshark.
Researchers noticed that the sharks' skin had two light and dark tones – light and dark – and extracted chemicals from the two skin types to analyse.
They had previously reported that swell sharks were biofluorescent, but decided to dive deeper into the issue and learn what their biofluorescence might mean to the sharks.
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