Biofluorescent Sharks’ Glowing Messages Revealed By Shark-Vision Camera | @GrrlScientist
Scientists built a “shark vision camera” that simulates what deep-sea sharks see, and it revealed that sharks may use biofluorescence to communicate with each other.
Although scientists have long known that corals are biofluorescent, only recently did they learn that biofluorescence is relatively common in other marine organisms, too. This discovery was purely accidental: in 2014, marine biologist David Gruber, an Associate Professor of Biology and Environmental Science at Baruch College/CUNY and a research associate at the American Museum of Natural History, was filming biofluorescent corals at night when his team was “photobombed” by a green fluorescent eel (ref). This eel was the first biofluorescent vertebrate ever seen.
This discovery was so remarkable that Professor Gruber teamed up with marine fish biologist John Sparks, curator-in-charge and associate curator of Ichthyology at theAmerican Museum of Natural History, and an adjunct professor of Ecology, Evolution, and Environmental Biology at Columbia University. Together, they searched the world’s coral reefs for biofluorescent animals and found more than 180 species of biofluorescent bony fishes (ref) and other creatures. They also discovered a biofluorescent reptile, the hawksbill sea turtle, Eretmochelys imbricata (ref).
During one nighttime dive, the team filmed a biofluorescent sting-ray, which is a cartilaginous fish. Since sting-rays are more closely related to sharks than to the bony fishes that the researchers had been filming, the team then wondered whether some shark species might also be biofluorescent.
“It turns out that some of the smaller sharks, the catsharks, are brilliantly fluorescent”, said Professor Gruber. “[They’re] as bright as some of the brightest fish, as bright as the corals.”
Professor Gruber and his team report that they found two biofluorescent species of reclusive deep-sea catsharks lurking in Scripps Canyon, which lies underwater near San Diego. Both sexes of these sharks feature distinctive glowing patterns of bold neon-green on their bodies (ref).
Can these biofluorescent sharks see each other? Professor Gruber’s team found that both shark species possess a photoreceptor — just one — in their eyes that absorbs light in the blue-green portion of the spectrum, indicating that they can see each other as well as other biofluorescent animals in the darkness. In contrast, humans have three photoreceptors — red, green, and blue — but even still, we cannot see biofluorescence with our naked eyes.
Biofluorescence is not the same as bioluminescence. Bioluminescence is the production and emission of light by a living organism, whereas biofluorescence occurs when a living organism absorbs light of one wavelength, or color, and re-emits that light at a different wavelength, or color. Since short wavelength (blue) light is the only color that has enough energy to penetrate waters that are deeper than ten meters, some marine creatures are adapted to absorb this light and re-emit it at longer wavelengths, thereby creating green, orange, and red fluorescence at depths where these colors would not otherwise be visible. But fluorescence only occurs in the presence of light.
“Turn off the light source, and there’s no fluorescence”, said Professor Gruber.
Combining their knowledge of fluorescence with their findings of the properties of these two shark species’ photoreceptors, the team designed and built a special underwater lighting system that produces blue light and is attached to an underwater camera that only records longer wavelengths of light (ref).
The resulting underwater light show is invisible to the human eye. To record this activity, the researchers custom-built underwater “shark vision” cameras that replicate what the sharks see.
“In order to see the phenomenon of biofluorescence, we used scientific filters … a very tight-notch blue filter” to recreate this pure blue light, said Professor Gruber. “That is the light bathing these animals. They live in a blue world — their world is blue all the time.”
The camera lens had yellow filters to block out blue light, and the scientists wear yellow head visors that allow them to see the biofluorescent glow while swimming on the reef.
It took the researchers longer than two years to design and build their underwater “shark vision” camera and its powerful lighting system. But the results are nothing short of spectacular.
“It’s like an underwater light show”, said Professor Gruber in his recent Solomon Islands TED talk.
When the team looked at images and film of sharks that had been captured using their shark vision camera system, they could see biofluorescence “all over — it’s in their eyes, it’s on their skin”, said Professor Gruber. “This makes me think this is something important physiologically for this animal.”
Previous research indicates that animals use bioflourescence for a variety of purposes including communication, defense, and predation. Since these small sharks emit and see biofluorescence whilst other predators lack the same abilities suggests that they may use it for communication or for species recognition.
“At this moment, we do not know what the function is”, said Professor Gruber.
By reconstructing the evolutionary relationships for cartilaginous fishes and mapping biofluorescent species onto this tree, the authors of this paper found that biofluorescence, along with the visual ability to detect it, arose at least three separate times (Figure 13). This suggests that biofluorescence is potentially very important to the behavior and biology of these animals.
“What I found most surprising is just how widespread [biofluorescence] is across the Tree of Life for bony fishes and cartilaginous fishes as well”, said Professor Sparks.
“The cryptically patterned gobies, flatfishes, eels, and scorpionfishes — these are animals that you’d never normally see during a dive,” said Professor Sparks. “To our eyes, they blend right into their environment. But to a fish that has a yellow intraocular filter, they must stick out like a sore thumb.”
“The discovery of green fluorescent protein in a hydrozoan jellyfish in the 1960s has provided a revolutionary tool for modern biologists, transforming our study of everything from the AIDS virus to the workings of the brain,” said Professor Gruber in a statement.
These biofluorescent “jellyfish proteins” are now used widely in a variety of biochemical, molecular and cellular biology techniques — an innovation that was recognized with the 2008 Nobel Prize in Chemistry for Osamu Shimomura at the Marine Biological Laboratoryat Woods Hole, Martin Chalfie at Columbia University, and Roger Y. Tsien at the University of California, San Diego.
“This study suggests that fish biofluorescence might be another rich reservoir of new fluorescent proteins” that can be used in research, according to Professor Gruber.
“I just find this real serenity and beauty to being in the reef at night. And now when we add on this fluorescent layer, it’s like being on another planet.”
David F. Gruber, Ellis R. Loew, Dimitri D. Deheyn, Derya Akkaynak, Jean P. Gaffney, W. Leo Smith, Matthew P. Davis, Jennifer H. Stern, Vincent A. Pieribone & John S. Sparks (2016). Biofluorescence in Catsharks (Scyliorhinidae): Fundamental Description and Relevance for Elasmobranch Visual Ecology, Scientific Reports,6:24751 | doi:10.1038/srep24751
David F. Gruber, Jean P. Gaffney, Shaadi Mehr, Rob DeSalle, John S. Sparks, Jelena Platisa, & Vincent A. Pieribone (2015). Adaptive evolution of eel fluorescent proteins from fatty acid binding proteins produces bright fluorescence in the Marine Environment, PLoS ONE, 10(11):e0140972 | doi:10.1371/journal.pone.0140972
David F. Gruber & John S. Sparks (2015). First Observation of Fluorescence in Marine Turtles, American Museum Novitates, 3845:1–8 | doi:10.1206/3845.1
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Originally published at Forbes on 30 April 2016.