There are many species of sharks which have beautiful patterns on their skin: the whale sharks (Rhincodon typus) with their blue and white spots and stripes, the wobbegongs (Family Orectolobidae) with their bands and whorls, and the zebra aka leopard shark (Stegostoma tigrinum), which changes from black and white stripes to yellow and black spots during maturity. Some sharks which live deep underwater have even developed more incredible colouration... with fluorescence. This allows them to glow in the dark! So why has this evolved? How does it benefit these little sharks?
Meet the Mesmerising Chain Catshark
The chain catshark (Scyliorhinus retifer) is a small shark (reaching a maximum size of 50 cm), which lives throughout the northwest Atlantic, into the Gulf of Mexico and the Caribbean. Found up to depths of 750 metres on the continental shelf and slope, these deepwater sharks spend a lot of their time on the "substrate", often burrowing amongst boulders and rubble. They are harmless to humans and feed predominantly on small fish, squid and crustaceans (Compagno, 1984).
These sharks have a beautiful marble patten of pigmentation in their skin, which inspired their name. To the human eye, they appear yellowish brown, with bold, black markings. However, in their natural habitat at great depths, they look quite different to other sharks! They glow in the dark! This is known as "biofluorescence". The glow is possible because they express a green fluorescent protein in their skin, which absorbs high energy blue light (that is common in deep water) and reemits it at longer, lower energy wavelengths, which look bright green when viewed with fluorescence imaging equipment (Gruber et al, 2016; Park et al, 2019).
I See You
Other sharks are able to detect these wavelengths of light through specialised cells in their eyes known as "rod cells". So when we look at them using imaging machines which can detect fluorescence, we can see how they might look to other sharks in the depths of the oceans (Gruber et al, 2016; Park et al, 2019).
We can then see that in the chain catshark, different areas of their skin have different amounts of green fluorescent protein, and therefore emit a brighter or dimmer glow ("regional biofluorescence"). In the yellowish beige regions, the sharks have a high concentration of green fluorescent protein, but have very little amongst the bold, black chains (Gruber et al, 2016; Park et al, 2019).
Not only do sharks express their green fluorescent proteins in the different colour patches in their skin, but even the structure of their skin plays a part in creating the light show. Shark skin is made up of reticulating scales known as "dermal denticles". This reduces their drag and makes them very efficient swimmers. (To learn more, you can check out Sand Paper Shark Skin). When we look at the skin of the chain catshark under a microscope, you can see that the green fluorescent protein is not distributed evenly, but rather in very specific dermal denticles (Park et al, 2019).
There are two very different types of dermal denticles in the skin of the catshark. The first, channel the fluorescence along their entire length, amplifying the light waves. Whereas, the second, the larger of the two, are non-light-guiding, so appear darker when viewed under a microscope (marked with * on the images) (Park et al, 2019).
Keeping it in the Family
Biofluorescence is present in several different species of sharks and their relatives, including the American round stingrays (Family Urotrygonidae) and wobbegongs (Family Orectolobidae). Within the close relatives on the chain catshark, the swell shark (Cephaloscyllium ventriosum) and lesser spotted catshark (Scyliorhinus canicula) are also able to fluoresce. This means that biofluorescence has evolved three separate times in the different lineages (Gruber et al, 2016).
Let There Be Light
Scientists think that biofluorescence evolved repeatedly because it allows these sharks to recognise other individuals of their own species.
As they live as such great depths, there is very little natural light for chain catsharks to be able to see each other. However, these sharks are able to see the patterns of regional biofluorescence and pigmentation very clearly, which allows them to identify their own kind, even in very low light levels. Therefore, this is a type of "visual communication" (Gruber et al, 2016; Park et al, 2019).
For example, when you compare the regional biofluorescence and pigmentation patterns of chain catsharks against their close relative the swell shark, it becomes very obvious that although they seem quite similar to our eyes, the two species would actually look very different. If you were a shark, swimming in the relative dark of the deep ocean, it would be quite plain whether an individual was from your own species or not (Park et al, 2019).
Biofluorescence is be a great advantage to chain catsharks in their habitat, as it helps them to find a mate to breed with. As the patterns of skin pigmentation and bioluminescence also differ between the sexes (known as "sexual dimorphism"), it would also be quite clear to these sharks whether an individual they spotted through the gloom was a male or a female (Gruber et al, 2016; Park et al, 2019).
We are only just beginning to understand biofluorescence in sharks, as these deepwater species are so challenging for scientists to observe. There may well be many more species which glow in the dark! This quirky little shark is a reminder that the oceans are so vast and so deep, that there are still incalculable wonders yet to be discovered!
Bioluminescence of the swell shark (Cephaloscyllium ventriosum) (Park et al, 2019)
Compagno LJV (1984). Sharks of the world: An annotated and illustrated catalogue of shark species known to date. Vol.4:Part 2. Carcharhiniformes. FAO Species Catalogue, Rome. Access online.
Gruber DF, Loew ER, Deheyn DD, Akkaynak D, Gaffney JP, Smith WL, Davis MP, Stern JH, Pieribone VA & Sparks JS (2016). Biofluorescence in catsharks (Scyliorhinidae): Fundamental description and relevance for elasmobranch visual ecology. Scientific Reports, 6:24751. Access online.
Park HB, Lam YC, Gaffney JP, Weave JC, Krivoshik SR, Hamchand R, Pieribone V, Gruber DF & Crawford JM (2019). Bright green biofluorescence in sharks derives from bromo-kynurenine metabolism. iScience, 19, 1277–1286. Access online.