In previous postings to this blog I’ve talked about the wide variety of marine fishes and other animals, including sharks, that produce a colored fluorescence, usually green but in some cases orange or even red. This phenomenon is common in catsharks, a family of mostly smallish, bottom-dwelling sharks that inhabit shallow to very deep waters around the world.

How do these animals produce their glow….and why? Some fluorescent animals, like the jellyfish Aequorea victoria, use a protein aptly called GFP, green fluorescent protein. This protein has been widely adapted for use in biomedicine, as a way to mark individual cells in culture or even in living animals with an easily visible color. Few fields of biomedical research have not been advanced by the use of fluorescent cell tracking. A few additional fluorescent molecules are known from other marine species, but much of this field remains unexplored. Biofluorescence in marine creatures is thought to function as a mechanism of communication, and may play a role in mating, as the sexes of some species show different patterns of fluorescence.

A new paper from Park et al has now analyzed chemically how certain sharks emit their eerily cool green light. Studying two species of catsharks, the swell shark (Cephaloscyllium ventriosum) and the chain catshark (Scyliorhinus retifer), these authors discovered an entirely new group of fluorescent compounds. Both sharks have skin patterning with light and dark areas, and it was already known that the fluorescence was stronger in lighter colored areas of the skin. This gives the striking patterns these sharks display (see photo). By comparing cells from light and dark areas of the skin, and looking for chemical differences between otherwise similar cells, the authors identified an entirely new group of fluorescent molecules unrelated to GFP-type proteins. These chemicals have the clunky name brominated tryptophan-kyneurines, which makes them metabolites (essentially break-down products) of the amino acid tryptophan. The authors also found that in S. retifer (but not in C. ventriosum), there are specialized denticles on the sharks’ skin that actually amplify and focus the fluorescent light.

The pathway these sharks use to produce fluorescent compounds from tryptophan is an ancient one conserved from bacteria to humans. In humans it is involved in regulation of the immune system, and is implicated in a diverse set of human diseases. Testing the shark compounds against two types of marine bacteria that bottom-dwelling sharks are exposed to, the authors found several fluorescent compounds were able to slow the growth of bacteria. This suggests that in addition to known functions of deep-water fluorescence, it may also help protect the animals against bacterial infection. Sharks may once again be able to shed some light on human biology and medicine.

The article is: Park HB, Lam YC, Gaffney JP, Weaver JC, Krivoshik SR, Hamchand R, Pieribone V, Gruber DF and Crawford JM. Bright green biofluorescence in sharks derives from bromo-kynurenine metabolism. (2019) iScience, 19:1291-1336.

Catsharks Find a New Way to Glow