In 2014, marine biologist and National Geographic Emerging Explorer David Gruber was on a nighttime dive to observe biofluorescent corals when he noticed a bright green eel darting across. This was the first time biofluorescence had been witnessed in a vertebrate. Curious to see if there were more such creatures just waiting to be discovered, Gruber and his team began combing through coral reefs around the world. Sure enough, they found that 180 species of animals, including bony fishes, reptiles, and even the hawksbill sea turtle, have the capability to “glow” in the deep dark ocean waters.
But despite the astounding discoveries, the team did not even consider the possibility of sharks with this ability until they encountered a glowing stingray. Since the cartilaginous fish are closely related to sharks, the researchers decided to expand their research further. Sure enough, they found two species of catsharks: The chain catshark (Scyliorhinus retifer) and the swell shark (Cephaloscyllium ventriosum) that display what Gruber describes as “brilliant biofluorescence.”
The reclusive fish that grow up to three-feet long, generally reside 1,600 – 2,000 feet below the surface of the ocean. At these depths, the water appears blue, because the rest of the colors cannot penetrate that far down. Similar to other biofluorescent marine animals, the sharks have a unique pigment in their skin that absorbs the blue light and ejects it as another color. This is different from bioluminescent animals who produce their own light through a chain of chemical reactions or by hosting organisms that emit light.
To see how biofluorescence works in the fish, the researchers studied the sharks’ eyes. In addition to finding long rods that enable the fish to see in dim light, they also discovered that the sharks have a single visual pigment to detect color, which lets them see in the blue and green spectrum. In comparison, humans have three — green, blue, and red — allowing us to see a wider range of colors, while the tiny mantis shrimp has twelve!
While most researchers would have concluded their study on this high note, Gruber ,and his team were not satisfied. They were curious to see the biofluorescence through the shark’s eyes. So they built a “shark eye camera” by covering the camera lens with filters that allowed only green and blue to penetrate through.
Then donned in their scuba gear, the team went deep into the ocean off the coast of San Diego to look for the catsharks. In the natural light, the scientists could barely see the shy fish that tend to stay close to the canyon walls. But things changed drastically when they viewed the fish through the shark-eye camera.
As Gruber later said, "Imagine being at a disco party with only blue lighting, so everything looks 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."
What was even more interesting is that the female and male catsharks of both species displayed unique and distinct patterns. While the glowing sharks can certainly see each other, researchers are not sure if they are visible to other marine animals. Though the biofluorescence is most likely used for mating, the scientists who published their findings in the journal Scientific Reports on April 25, believe it could also be used for communication, defense, and predation. Before this discovery, scientists had thought that sharks relied on smell, hearing, and the sensing of electric signals, to make their way around. However, now it appears that they may be using their vision as well.
Gruber, who intends to continue his research on biofluorescent marine animals, hopes that seeing the world through the sharks’ “eyes” will make people more aware of the importance of protecting these magnificent creatures. With an estimated 100 million sharks killed by humans each year, we sure need to be careful that we don’t lose them altogether!
Resources: nationalgeographic.com, forbes.com