Bioluminescence: How Glowing Sharks Light up the Deep
SHOW NOTES
Professor Jérôme Mallefet, a Belgian marine biologist, has spent his career studying living lights — animals that produce their own glow through a natural chemical reaction known as bioluminescence. It’s a fascination that began with a love for all things fish [06.27]. “Initially I wanted to study everything about fish,” he recalls, “but it’s impossible. There are 29,000 species … so it was impossible to know everything.”
Bioluminescence first entered his life in University, during a chance conversation with one of his lecturers who mentioned they worked with ‘luminous fish’ [08.33]. “I said, ‘Ooh, luminous fish.’” Jérôme laughs. “And then he gave me a paper, and I searched a little bit in the books and I saw a picture of a glowing viperfish — and I was so impressed.” From that first glowing fish, Jérôme was hooked — literally, as he likes to joke. His PhD work on luminous fishes opened a window into one of the ocean’s most mysterious phenomena.
The definition of bioluminescence is the emission of visible light by living organisms [09.58]. It is the result of a chemical reaction which occurs inside specialised cells called photocytes. Luminous animals often have light-emitting organs that house these cells, called photophores. Inside each cell, a substrate called luciferin reacts with oxygen, catalyzed by an enzyme named luciferase. The reaction releases energy not as heat — but as light. “That is the magic aspect of bioluminescence,” says Jérôme. “You are releasing light instead of releasing heat. That’s why bioluminescence is also called cold light.” It’s the same concept as a glowstick.
As similar as they sound, bioluminescence is not to be confused with biofluorescence – although the two often are [11.18]. Where, like in a glow stick, bioluminescence is the production of light via a chemical reaction, biofluorescence works instead like those glow-in-the-dark stars you would have on your bedroom ceiling as a child. You have to shine light on the object (or animal) first to get it to glow. “While bioluminescence is producing its own light system,” explains Jérôme. “It’s not fluorescence because I don’t have to shine light on the sharks to see it glowing. It’s glowing by itself.”
If glowing fish seem rare to us, that’s only because we live at the surface. ‘Considering the vastness of the deep sea … it is now more and more obvious that producing light at depth must play an important role in structuring the biggest ecosystem on our planet,’ Mallefet wrote in a scientific paper published in 2021. He expands on that thought [17.26]: “Planet Earth — we call it the blue planet — because 70 percent is ocean, with an average depth of 4,000 metres. That makes 98 percent of the living area ocean.” When he and colleagues finally compiled a global list of luminous marine species, the scale astonished even them. “There are more than 2,900 species … It’s huge. In the water column, 70 percent of the species are glowing in the dark.”
And included in this list are a surprising number of glowing sharks [24.06]. Out of the 561 species of sharks, an astonishing 11 percent are ‘luminous’ – that’s one shark out of every 10 that is able to produce light. Most live below 200 metres, where there is too little light for photosynthesis to be possible. Some light does penetrate at that depth, but it is limited, as Jérôme explains: “When you enter the twilight zone, from 200 to 1,000 metres, the light is blue — deep blue — and strangely enough, the sharks are producing blue light from their belly.”
The first shark he ever worked with was the velvetbelly lantern shark, caught in the fjords of Norway [27.04]. “It was fascinating because we saw the shark swimming in a tank, and they were glowing … It was just like, I don’t know, a living Christmas tree in the aquarium,” he recalls. When they measured the light, they found it perfectly matched the dim blue hue of the ocean around it — about 480 nanometres. “That was the moment we realized,” he says, “that these sharks are producing exactly the right color, the right distribution… They glow from their belly just to disappear.”
This disappearing act is known as counter-illumination — a kind of natural camouflage [29.10]. Imagine you are swimming in the ocean, and you look above you. Because of the direction of the light shining down from above, everything above you will appear as a silhouette. So, if an animal wants to appear ‘invisible’, they would need to produce light from their ventral side – their belly – to match the light coming from above. This is why many of the luminous sharks, like the velvetbelly, have photophores located on their underside.
But that’s not the only luminous tricks velvetbellies have up their sleeve [31.23]. “They also have marks on the sides — flank marks — and each flank mark is different from species to species … They can recognize: that’s my species, that’s not my species,” explains Jérôme. This avoids any awkward social interactions, or allows you to find a mate in the darkness. On that note, these little sharks also have another handy feature: light-up genitals! “So they can recognize [their species] by the marks on their side. And then if you go below and up, you can see, ‘hey, I am a man. Hey, you are a female. It’s time to make our business’” Jérôme says.
So, luminous sharks can recognise both friends and foes, and they can disappear. But they also have another use for bioluminescence: defense [33.20]. When Jérôme and his student were studying the velvetbellies, they discovered two large spines located just in front of the dorsal fin, that also appeared to be glowing. But, on closer inspection, there was no photophores or photocytes to speak of. For a moment, Jérôme and his student were slightly confused. “I thought, hmm, we didn’t drink, we didn’t smoke,” laughs Jérôme. “So strange. We definitely saw the spine glowing.” Next time, they also examined the dorsal fin – and there was the answer. “We realized the border of the fin is full of light organs shining into the spine…the spine was working like a light guide,” he says. But the next question was then: why? Why would velvetbellies need lit-up spines on their back?
More information came after filming wild sharks in Japan [34.47]. “We filmed a big shark measuring about one metre biting one of our small guys … The shark took it in his mouth, bit once, then opened the mouth and the small shark escaped.” Jérôme remembers. After seeing this footage, it all made sense. It was a warning system: “If you are in the dark and you light up your spine, you just say to your predator, ‘Don’t bite me, I will puncture you and you will be hurt and I will escape.’ This is called aposematism.”
Another species Jérôme has studied closely is the largest luminous vertebrate on earth: the kitefin shark (Dalatias licha), which grows up to 180cm in length [35.18]. He is the first scientist to document the kitefin’s luminescent capabilities, having searched for one for years. Jérôme joined a month-long survey on New Zealand’s Chatham Rise, ran by the National Institute of Water and Atmospheric Research (NIWA), who have their own trawler to conduct fisheries research. Jérôme had heard stories of “20, 30, even 40” kitefin sharks showing up in previous trawls, and thought it was his best chance of trying to study one. It was not easy: “thirty-two days at sea, not touching land … so good that I don’t suffer from seasickness!” They captured many Dalatias, which were, in fact, glowing. But with the movement of the boat and a wriggling shark, it was very hard to take adequate photographs. It wasn’t until the very last trawl, 28 days in, that Jérôme was able to capture a good enough picture to be included in a scientific paper.
What they found changed how scientists understood the evolution of bioluminescence in sharks [43.53]. “We discovered that they control the light emission … the same way as the lantern sharks,” he says. “That could tell us there is only one occurrence of bioluminescence in sharks — one evolutionary origin … because it seems to be controlled the same way.”
That control is unique to the animal kingdom, in that it is all done hormonally [45.45]. “Other fish use nerves. But the sharks — we discovered that putting melatonin on the tissue lights it up … And you know, for us, melatonin is the hormone of sleeping … These guys transform the control of the skin into the control of their belly, glowing belly, with melatonin.” The hormone alpha-MSH then dims the light, allowing sharks to modulate their glow. “Melatonin triggers the light … and when you put the alpha-MSH, then you close the pigment. So that’s very particular — for the moment, it’s the only hormonal-controlled bioluminescence known.”
To Jérôme, understanding these living lanterns has been a lifelong passion [52.02]. But he also worries for the future of the glowing world he studies. Deep-sea mining, he warns, threatens to smother the twilight zone in plumes of sediment. “Everything will be dead,” he says bluntly. “Below 100 metres, you are living in the dark. But why are animals using light? Because they can communicate … Once they will be in the smoke of deep-sea mining, everything will be dead.”
Still, Mallefet remains hopeful, and hopes that sharing his joy for the luminous species will bring more light to the deep. “They are brighter than me — they can glow…But I hope I light up the darkness surrounding bioluminescence.
Bioluminescence, which was considered as magical, doesn’t seem to be magical. It seems to be a really important aspect of marine life.”
ABOUT OUR GUEST
After his Ph.D. graduation in 1982, Jérôme Mallefet became a postdoctoral researcher FNRS and was awarded a Canadian government postdoctoral fellowship at the Michel Anctil lab (Biology department – Université de Montréal, Canada). Since 1990, he has a permanent position as Research Associate FNRS at UCLouvain and leads his own research group in Marine Biology. His primary research interest is in the field of bioluminescence: he investigates the basis of light emission in various taxa (physiology, behavioral ecology and molecular bases), in collaboration with leading experts. He has published 113 articles in peer-reviewed journals or books chapters including the flagship journals Scientific reports, Biology letters and Proceedings Royal Society and Journal of Experimental Biology. He frequently is a plenary speaker on international meetings on bioluminescence and echinoderms and is a scientific advisor for the International Symposium of Bioluminescence and Chemiluminescence (ISBC from 2002). He is also a member of the editorial board of the journal Luminescence.
Papers mentioned in this episode:
Claes, J.M. and Mallefet, J., 2008. Early development of bioluminescence suggests camouflage by counter‐illumination in the velvet belly lantern shark Etmopterus spinax (Squaloidea: Etmopteridae). Journal of Fish Biology, 73(6), pp.1337-1350.
Mallefet, J., Stevens, D.W. and Duchatelet, L., 2021. Bioluminescence of the largest luminous vertebrate, the kitefin shark, Dalatias licha: first insights and comparative aspects. Frontiers in Marine Science, 8, p.633582.
