Swell sharks generally keep a low profile, squeezing between rocky crevices to keep out of the way of predators.

Living up to 500m (1,640ft) beneath the waves, they are easily camouflaged in the darkness and often missed by divers.

But scientists have discovered the shark (Cephaloscyllium ventriosum) actually glows bright green thanks to fluorescent proteins inside its skin which are activated by blue light – the wavelength of visible light that is least absorbed as it travels through water.

This phenomenon, called biofluorescence, is thought to be a form of communication to other swell sharks that – unlike humans – can see the extraordinary light display.

Dr David Gruber, an associate professor of biology at City University of New York, studies the sharks off the coast of California, US.

“On land we have the whole range of colours in the [visible] spectrum, as soon as you drop beneath the sea you quickly lose the reds and the violets and it becomes a monochromatic blue environment,” he explains.

“What the swell sharks are doing is using the blue light to create other colours of light to make their world richer in colour.”

To be able to see the shark in all of its luminous glory Dr Gruber had to use cameras with yellow filters, which block out the natural blue light in the same way that shark eyes do.

The results – filmed off the coast of Santa Barbara, US, as part of a new BBC / Discovery coproduction television series – were the first time Dr Gruber had seen the sharks using biofluorescence in the wild.

He was part of the team which first discovered biofluorescence in more than 180 species of fish and suggested that the animals may be using it as camouflage and to find mates.

“It’s almost like there’s been this disco party going on underwater and it’s possibly been going on for millions of years and we’re just beginning to tune into it,” he says.

Biofluorescence had previously been reported in coral and jellyfish and through the development of fluorescent tags, which allow researchers to visually track how cells work, has led to medical advances in the study of conditions such as AIDS, cancer and Alzheimer's disease.

Dr Gruber is about to publish his latest research into a new family of fluorescent proteins taken from eels, which he hopes scientists can use to further our understanding of the human body.  

Finding biofluorescence in marine animals which can see and are known to have filters in their eyes which may make the luminous colours stand out, led Dr Gruber to investigate why they might have evolved the phenomenon.

When you see all these little bright spots and patterns it’s like flowers and butterflies

After swimming with the swell sharks in their natural habitat he noticed that they were the only things that glowed green, so he decided to find out if they could actually see that colour themselves.

“Surprisingly with sharks we know very little about their visual apparatus and what we know just comes from a few species, so we didn’t know anything about the visual apparatus of the swell shark,” he explains.

Dr Gruber had the shark’s vision analysed by experts at Cornell University and discovered that it can only see blue/green hues which he says is a perfect adaption for the environment it lives in.

“By creating more green [through biofluorescence] in an environment where it’s just blue they’re creating much more contrast and when you see all these little bright spots and patterns it’s like flowers and butterflies.

"Why do they make patterns? It’s to attract each other, it’s to recognise each other,” Dr Gruber says.

He is now looking at whether male and female swell sharks have different fluorescent patterns or if these markings are specific to individual sharks, which will help uncover what biofluorescent signalling is used for – finding mates or identification.

“It is almost like a hidden mode of communication, like a covert form of communication just among themselves or just among animals with similar kinds of vision,” he says.

“It’s really my greater hope that by showing off this diversity of biofluorescence in the ocean and the possible intelligence in communicative patterns of these creatures, we’ll better understand the animals and will want to protect them more."

In the UK you can watch the concluding episode of 'Shark' on Thursday 21st May at 21:00 BST, on BBC One. The series will be broadcast at a later date in other countries.

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