When they're not inflating themselves to ward off would-be predators, swell sharks aren't particularly eye-catching. Add some blue light, however, and the unassuming fish pull off a spectacular light show. 


We've known about this green glow for a long time, but now, a team of researchers has used special filters to create a "shark's-eye camera" – one that allows us to see these fluorescent animals, for the first time, as they might see each other.

Fluorescence in fish – and even sharks – isn't a new phenomenon. In fact, it was discovered back in 2014 when scientist David Gruber, a researcher at City University of New York and the American Museum of Natural History, was conducting a search for glowing corals in the Cayman Islands.

"The first fish that we found to be biofluorescent was as a small eel," he told National Geographic after the discovery was made. "We looked at it for a long time, you know, trying to figure out if there was a problem with the camera." 

Since the eel was found, Gruber and his colleagues have discovered over 180 other species of florescent fish, including seahorses, stingrays and, of course, sharks. But until now, we could only speculate about whether these animals could see the vibrant hues themselves. Without that information, it would be impossible to solve the mystery of why they glow in the first place.  

Unlike bioluminescent animals, which produce light through chemical reactions within their bodies, biofluorescent animals don't create light. Instead, they absorb, transform and eject (or re-emit) light as a different colour by harnessing the power of special pigment.

"It's all over [the sharks]," Gruber said. "It's in their eyes, it's on their skin. This made us think that fluorescence is something that's important physiologically for these animals. The cool thing about this research is it literally shines a light on animals that are often overlooked." 

Image: National Geographic

The team looked at both swell sharks (Cephaloscyllium ventriosum) and chain catsharks (Scyliorhinus retifer), two species which belong to the catshark family. Named for the almond shape of their eyes, catsharks pose no danger to humans. The fish grow to just one to three feet (less than a metre) long, and feed on crustaceans and small fish. Being nocturnal, they spend their days hidden away in rocky crevices or lolly-gagging on the seabed.

High-energy blue light is strong enough to penetrate down to the sharks' deep dwellings, and the fluorescent pigments in their bodies are tuned to absorb it. This is the same process by which the strawberry squid comes to have a single, glowing green eye

Squid gif 2-2015-6-25
Image: MBARI

In the case of the squid, a fluorescent eye helps it pick out prey against the blackness of the deep sea. But in sharks, the prevalence of these pigments elsewhere on the body led the researchers to suspect a different function: advertising.

If catsharks use their rave-ready appearance to advertise their whereabouts –perhaps during courtship or battles over territory – we'd expect them to be able to detect the glow of their sharky brethren. And during a recent study, Gruber and his colleagues discovered that they can. Catshark eyes, they found, function well in dim light thanks to long rod cells, and they also possess a visual pigment for colour detection in the blue-green spectrum.

Using highly tuned filters, the team then created a camera system that mimics shark vision, and sent it fathoms below for a second look. To the human eye, the catsharks were hardly visible against the dark waters and rocky canyons.  But through the shark-eye camera, the green fluorescence began to pop.

"Imagine being at a disco party with only blue lighting, so everything looks blue," Gruber explained during a follow-up interview. "Suddenly, someone jumps onto the dance floor with an outfit covered in patterned fluorescent paint that converts blue light into green. They would stand out like a sore thumb. That's what these sharks are doing."

What's more, different species of catshark were found to posses unique patterns, a trait that might help the animals recognise one of their own. In the swellsharks, males and females were distinguishable.

While this certainly supports the "shark advertising" hypothesis, the team notes that there is still a lot to learn. It's very possible that the fish are using fluorescence in ways we have yet to understand.

"This work forces us to take a step out of the human perspective and start imagining the world through a shark's perspective," added Gruber. "It reminds me of when researchers first tuned in to the high frequency of bat sounds, and they discovered all this hidden chatter."

The next step will be to send the shark's-eye cam deeper, to find out if enough blue light reaches the lowest regions of the sharks' habitat to allow detectable fluorescence. Gruber also plans to make cameras that mimic other animal eyes, like those of catsharks' known predators. Does glowing make these fish more vulnerable? Does it help them hunt? These are the kinds of questions the team hopes to answer in the coming months. 


Top header image: National Geographic/YouTube

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