“A biofluorescent swellshark. Preliminary analysis suggests that male and female sharks may have different patterns of biofluorescence, opening up the possibility that these patterns are used for signaling or identification.”
Scientists have developed Shark Vision to learn how predators see each other
by Elizabeth Preston / April 25, 2016
“David Gruber sees glowing life forms everywhere he looks. He’s found dozens of fluorescent corals in the Great Barrier Reef. In 2014, he reported on more than 180 fish species that fluoresce. Last year, he even stumbled across fluorescent sea turtles. Now Gruber, a biologist at the City University of New York’s Baruch College, wants to know why all these species are glowing. He and his colleagues built a “shark-eye” camera to simulate how fluorescent sharks appear to each other, in part so that humans view these creatures a little more kindly.
Animals like fish and turtles don’t generate their own light, as a firefly does. Being biofluorescent means molecules in their skin absorb light of a certain wavelength, and bounce it back at a different wavelength. In the ocean, that usually means they absorb blue light and transform it into green, red, or orange. It’s hard to notice with human eyes in the dim ocean, though a person might detect a greenish cast to a shark’s skin, for example.
Finding biofluorescence in so many sea animals led Gruber to wonder what advantage it conferred upon a species. He and his co-authors have begun to answer that question for two biofluorescent sharks, the Atlantic-dwelling chain catshark and the Pacific-living swell shark. They have done so by looking deep into their eyes—not in the romantic sense, but in the dissection sense. They found that although these species seem to have excellent low-light vision, they’re monochromats.
That means unlike humans, who build color vision using three types of pigment molecules in our eyes, these sharks have just one pigment. It detects blue-green light. That makes sense, Gruber says. “The ocean is this huge blue filter, and it becomes more perfectly blue as you go deeper.” If there aren’t any other colors of light to see, why bother?
Next the team asked what parts of a shark’s body fluoresce. Both species have mottled patterns, which in an aquarium’s light would appear light beige-and-dark gray, or light-beige-and-black. (The chain catshark’s pattern looks almost like a giraffe’s.) The researchers studied sharkskin in the lab, and used a special camera setup to vividly capture the sharks’ fluorescence in the wild. They dove at night, shining blue light onto the animals. Then they used a camera with blue light filtered out of it to capture only the green fluorescence that shone back at them.
The fluorescence mostly came from the sharks’ beige patches. But the swell shark also revealed “these twinkling, very bright spots all over,” Gruber says. Additionally, the researchers saw fluorescence shining eerily from the sharks’ eyes. Finally, the team used what they’d learned about vision in the two species to create the shark-eye camera. It’s “a very high-resolution movie camera,” Gruber says, with filters added to simulate what the monochromatic animals would see.
“The view from the shark-eye camera”
The result doesn’t look too splashy. But the real question is what difference it makes to a shark. Does the extra green light make a shark’s patterns easier to see against its ocean backdrop? In a model, the researchers found that as sharks swim deeper in the ocean, their fluorescent patterns should stand out more strongly to other sharks’ eyes. They published their results in Scientific Reports.
Not everyone is convinced by the team’s model. Nathan Hart, a biologist at Macquarie University in New South Wales, Australia, who studies shark vision, wonders whether blue light in the deep ocean is really bright enough to make the sharks’ fluorescence stand out. Christine Bedore, of Georgia Southern University, adds that she’s “pretty doubtful that the fluorescence has any ecological relevance.” Gruber stresses that the study is only a first pass at figuring out how sharks see their own glow. And biofluorescence seems to have evolved many times in fish—a clue that it has a purpose. “It makes perfect sense if you think about life in the blue ocean,” Gruber says. “Why wouldn’t they come up with a way to make their world richer in texture?”
If fluorescence does help sharks see other members of their species, it could help them find each other for mating or socialization. But biofluorescing might also make the sharks more obvious to predators. Gruber says it’s not clear what animals eat these species—maybe other, bigger sharks—or what their own visual abilities might be. Very few shark species have been “brought to the eye doctor,” he says. “This study really opened my eyes up,” Gruber adds (no pun intended), “to how little we know about shark vision.”
“A well-disguised biofluorescent fish hiding amongst coral by Gruber and his team [PLOS]”
One of his next steps will be to create cameras representing other animal eyes, thanks to a new technology called a hyperspectral camera. This kind of camera could let researchers record footage underwater, then write algorithms back in the lab to transform the footage into the viewpoint of various species. Ultimately, Gruber hopes seeing the world through other animals’ eyes will have practical benefits. It’s hard to convince people about the importance of protecting the ocean, he says, when they can’t relate to the animals that live there. People may think of marine creatures as mysterious, or scary, or simply food. But if we put ourselves into their perspective, Gruber believes, “It could draw us closer to these species.
Marine biologist David Gruber and his team dive with a “shark-eye camera” to get a sense of how catsharks see the world. Biofluorescence is widespread in fish and has even been seen in sea turtles, Gruber and his colleagues reported in 2014. The phenomenon occurs in the low-light conditions above 3,280 feet (1,000 meters) in the ocean, Gruber said in a TED Talk on the subject. Below that level, animals often create their own light using bioluminescence.”
“For the first time, scientists have studied the eyes and skin of a group of shy sharks that live deep in the water, in a dark blue realm of low light. The team discovered that the secretive, little-known animals use biofluorescence, or glowing, to become more visible to each other, presumably so they can mate.
The team also found new evidence of the evolutionary history of biofluorescence in fishes, suggesting that the phenomenon is more widespread and more important than previously believed. In fact, biofluorescence in fishes was only discovered a few years ago, and scientists are only starting to figure out how it works. It is thought to be used in more than 200 species of sharks and bony fish, as well as marine turtles.
New research published in the Nature journal Scientific Reports described biofluorescence in two species of catsharks, the chain catshark (Scyliorhinus rotifer) and the swell shark (Cephaloscyllium ventriosum). These small sharks grow no more than three feet (one meter) long and spend much of their time on the bottom, to a depth around 1,600 to 2,000 feet (500 or 600 meters). They are shy and nocturnal and often hide in crevices.
“The cool thing about this research is it literally shines a light on animals that are often overlooked,” says David Gruber, the study’s lead author and a National Geographic Emerging Explorer, who is also a researcher at Baruch College, City University of New York and the American Museum of Natural History. These two species are often caught by fishermen as bycatch and we studied them not far from San Diego’s best surfing beaches, but no one had looked at them,” says Gruber. (See pictures of other animals that glow.)
The catsharks generally live deep enough that they are bathed only in blue light, since the rest of the wavelengths of light are blocked by the water above. The sharks have a special, as-yet-unidentified pigment in their skin that absorbs blue light and re-emits it as the color green, in a process called biofluorescence. This is different from bioluminescence, where animals either produce their own light through a series of chemical reactions, or host other organisms that give off light.
To better understand how the biofluorescence works, Gruber and team examined the sharks’ eyes. They found really long rods, which help the animals see in low light. They also found one visual pigment for color detection, which lets them see in the blue and green spectrum. Humans, in contrast, have three color pigments—red, green, and blue—allowing us to see a wider range of colors. At the high end, mantis shrimp have 12 pigments and can see an even wider array of colors.
Once the scientists worked out how the sharks likely see, they created a “shark-eye” camera to approximate the vision. They did this by adding filters in front of a lens of a Red Epic camera to restrict the wavelengths of light passing through, mimicking the shark’s eye. To enhance the effect of fluorescence, they also sometimes shined blue lights. The team donned scuba gear and swam into Scripps Canyon, off San Diego, to find catsharks. Under the natural, dim lighting, the sharks were hardly visible against the walls of the canyon to the human eye.
But through the shark-eye camera, they appeared to glow a bright green, thanks to their biofluorescence. Their glowing patterns really “popped” against the background, thanks to the high contrast, Gruber notes. “Imagine being at a disco party with only blue lighting, so everything looks blue,” says Gruber. Some things will look lighter blue and others will look darker blue. “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.”
The two shark species showed distinct glowing patterns and differences between the sexes. The swell shark is covered with small, bright green fluorescent spots over much of its body (those spots appear light beige under white light). But the females have a unique “face mask” of glowing spots and more dense ventral spotting that extends further back than males. The chain catshark has an alternating light and dark fluorescent pattern but no spots. In females the reticulated pattern is even more pronounced. In males the pelvic claspers, used in mating, glow. The sharks are thought to be primarily nocturnal and solitary, although not much is known about their behavior or lifecycles. Biofluorescence is likely to be important to their survival, Gruber says, but right now scientists can only guess at its function. The most likely explanation is so potential mates can find each other.
“But they might also be using biofluorescence to communicate in a way we haven’t thought of,” says Gruber. “It reminds me of when researchers first tuned in to the high frequency of bat sounds, and they discovered all this hidden chatter. They had to then figure out what it meant.” It’s also unclear whether the animals’ predators—larger sharks—or their prey—smaller fish and invertebrates—can see the glowing. Gruber’s team took their work a step further to review everything that’s been published on glowing fish. They found that the ability seems to have evolved at least three times among the sharks and rays, in the distantly related families Urotrygonidae (American round stingrays), Orectolobidae (wobbegongs), and Scyliorhinidae (catsharks). That’s significant because it further suggests biofluorescence plays an important role in the animals’ lives, and it suggests there are different ways it is done. Further, more glowing organisms are also likely waiting to be discovered. Gruber and team’s work is “exciting and at the frontier of this science,” says Victoria Elena Vasquez, a shark researcher at Cal State’s Pacific Shark Research Center. “Their work delves into the next steps of biofluorescent research in marine species.”
Previously, scientists had thought sharks mostly relied on smell, hearing, and the sensing of electrical signals to find their way around their environment, but this recent research suggests vision may be more important than experts previously realized. Vasquez adds that she is particularly intrigued by Gruber’s finding that biofluorescence in the sharks appears to allow their contrasting coloration to be more discernible at greater depths. The fact that this might help the sharks find mates seems plausible, she says, although more research is needed. “This work forces us to take a step out of the human perspective and start imagining the world through a shark’s perspective,” says Gruber. “Hopefully it will also inspire us to protect them better,” he says, noting that an estimated 100 million sharks are killed by people each year.”
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