Donning scuba gear and carrying a Red Epic camera equipped with a special filter, researchers recently dove into Scripps Canyon off the coast of San Diego to study how catsharks see the world.
On their own, catsharks are pretty bland species. The small, roughly 3-foot sharks spend most of their life at depths of around 2,000 feet, where only wavelengths of blue light penetrate. But researchers have known for awhile that these homely sharks are biofluorescent. This doesn't mean that the sharks generate light, but rather they absorb and then re-emit light at a different wavelength or color.
Over the last decade, David Gruber, a researcher at Baruch College, has discovered dozens of bioflouorecent fish and sharks around the world. But now he's interested in why the creatures have such bright patterns and, since they are not visible to the human eye, how the animals actually see them. So the team developed a special filter to get a shark’s-eye view of the ocean, focusing on two species of catsharks: the swell shark, Cephaloscyllium ventriosum, and the chain catshark, cyliorhinus rotifer.
The team first examined the eyes of catsharks and discovered long rods that allow the animals to see in extremely low light, writes Brian Clark Howard at National Geographic. They also found that the sharks have pigments that only allow them to see in the blue and green spectrum, as opposed to human eyes which have cones that pick up red, green, and blue light.
“Some sharks’ eyes are 100 times better than ours in low-light conditions,” Gruber says in a press release. “They swim many meters below the surface, in areas that are incredibly difficult for a human to see anything. But that’s where they’ve been living for 400 million years, so their eyes have adapted well to that dim, pure-blue environment.”
In addition to the dim light, the skin of the animals contains a little-understood pigment that absorbs the blue light and emits a fluorescent green. Using that information, Gruber and his team created their “shark’s eye” camera that simulates what sharks see and went on several night dives to record the animals. Though they were only able to film sharks in the shallower regions of the canyon, they were still impressed with the view.
“Imagine being at a disco party with only blue lighting, so everything looks blue,” Gruber tells Howard. “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.”
Through the filters, swell sharks were covered in bright green spots and females also had a “face mask” of glowing spots. The chain catsharks were covered in alternating light and dark areas, while the males’ pelvic claspers, used in reproduction, also glowed. The team recently published their results in the journal Scientific Reports.
According to Elizabeth Preston at The Atlantic, Gruber has found more than 180 fluorescing fish and at least one Day-Glo sea-turtle species in the last five years. He says he thinks the patterns make the animals more visible to each other in the deep ocean, and may be involved in methods of communication we haven’t yet discovered. “It makes perfect sense if you think about life in the blue ocean,” he tells Preston. “Why wouldn’t they come up with a way to make their world richer in texture?”
Beyond shark vision, Gruber hopes to create more cameras that simulate what other ocean animals see. “This work forces us to take a step out of the human perspective and start imagining the world through a shark's perspective,” Gruber tells Howard. “Hopefully it will also inspire us to protect them better.”