These sharks glow bright green in the dark

[JacksinPA]

These sharks glow bright green in the dark - CNN

Glowing green sharks swim in the dark depths of the ocean thanks to an entirely different kind of biofluorescence than other marine animals.

Previously, researchers knew that some shark species produce a green glow that is only visible to other sharks. Researchers discovered that previously unknown small-molecule metabolites are the cause of the green glow. The glow helps sharks identify each other and even fight against infection on a microbial level.
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I've overused my access to the Science online article abstract data base for this month so I don't know exactly what the CNN writer is talking about, either biofluorescence or bioluminescence, but these are different phenomena. But the chemistry involved is unique & involves an entirely new form of marine biofluorescence from sharks--one that is based on brominated tryptophan-kynurenine small-molecule metabolites, according to a Dr. David Gruber. Brominated tryptophan is an interesting molecule & is also found in both a unique marine toxin & in a 'sleep peptide' used by predatory cone snails. The bromine atom, found in sea water, is added to the tryptrophan in what is called a 'post translational modification.'

 

[JacksinPA]

From the New York Times article: 'shark skin interacts with light, bringing in blue photons, and sending out green.' So the phenomenon is biofluorescence, which involves the absorption of light at one wavelength (blue) & re-emitting it it at a different, lower energy wavelength (green). This is apparently sex-specific in these sharks & serves as a 'come hither' signal to the opposite sex.

5-Bromotryptophan is a component of a unique toxin, bunodosine 391, found in a sea anemone. The addition of halogens such as bromine to marine products usually serves to make them more toxic. In the case of the sharks, the compounds found that incorporate this molecule have antibiotic activity, giving further evolutionary advantage to the sharks.

 

[JacksinPA]

Here is the link to the article: https://static1.squarespace.com/sta.../1565278366193/2019_Biofluorescent+sharks.pdf

The green fluorescent compound is, as I suspected, a derivative of 6-bromotryptophan. Chemically it is 6-bromokynurenine. See Kynurenine pathway - Wikipedia The antibacterial derivative is 9R,9S-9-Bromotryptophan diketopiperazine.