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 Why do elasmobranchs glow in the dark?

750Xsrep24751 f7Biofluorescent imaging camera and lighting system. Photo Kyle McBurnie

Research suggests sharks only see in one colour and use biofluoresence to identify each other. And by using a specially constructed 4k camera, which replicates how the sharks would see each other, researchers believe they are making significant steps in understanding why sharks emit fluorescent light. 

David Gruber,  a biologist at the City University of New York’s Baruch College, had previously discovered dozens of fluorescent corals in the Great Barrier Reef. In 2014, he reported on more than 180 fish species that fluoresce. Last year, he found fluorescent sea turtles.

Fluorescence is a relatively recently discovered phenomenon. It is thought that some fish species which fluoresce such as lizard fish and some gobies do so to help identify others of their species as they emit distinct fluorescent patterns.

Biofluorescence has also been shown to play a role in the behaviour of marine organisms: fluorescence at the tips of the tentacles in the hydromedusa Olindias formosa attracts juvenile rockfishes; the fairy wrasse, Cirrhilabrus solorensis, responds to red biofluorescence; and fluorescence in the mantis shrimp, Lysiosquillina glabriuscula, enhances its signalling.

In recent years, it has been discovered that large numbers of elasmobranchs also biofluoresce.

Gruber established that the Atlantic-dwelling chain catshark and the Pacific-living swell shark, while having excellent low-light vision, are monochromats. That means unlike humans, who build colour vision using three types of pigment molecules in our eyes, the sharks have just one pigment. It detects blue-green light.

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a–d) Fluorescent and white light pigmentation pattern of a female chain catshark; and (e–h) of a male. Males have pelvic claspers that fluoresce, whereas the females lack claspers, and the reticulated pigmentation pattern is more pronounced in females (thicker dark black/brown lines), particularly ventrally, under both fluorescent and white lighting.

He argues this makes a lot of sense - the ocean is a huge blue filter and it gets bluer the deeper you go. So why bother to be able detect a wide range of colour when there aren't any other colours to see.

In a paper published  in Scientific Reports he suggests that the sharks use fluorescence to help see other members of their species. 

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The fluorescence from a female swell shark (A) and seen under white light (B)

 

It says: 'The repeated evolution of biofluorescence in elasmobranchs, coupled with a visual adaptation to detect it; and evidence that biofluorescence creates greater luminosity contrast with the surrounding background, highlights the potential importance of biofluorescence in elasmobranch behaviour and biology.'

 

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