A research team led by scientists from the American Museum of Natural History has determined the existence of a correlation between the visual capability of catsharks exhibiting biofluorescence and the animal’s emission of fluorescence.
Organisms that exhibit biofluorescence are able to absorb light and re-emit it as a different color. This newly-emitted light can only seen by humans while the organism is being illuminated by an external light source that mimics the ocean’s light, including certain types of ultraviolet light.
David Gruber is an associate professor of biology and environmental science at Baruch College. The paper that he authored to describe this discovery was recently published in Scientific Reports In an interview, he explained the recently discovered purpose of biofluorescence.
“It [biofluorescence] is a brand new phenomenon that our research team has recently found to be widespread in the ocean,” Gruber said. “The purpose is still not totally known, but we are finding that some animals have the potential to use it for communication.”
Gruber’s paper, entitled Biofluorescence in Catsharks (Scyliorhinidae): Fundamental Description and Relevance for Elasmobranch Visual Ecology, explains that as the depth of the ocean increases, the spectral quality of sunlight permeating the ocean becomes more and more restricted to a set range of wavelengths of blue light. Meanwhile, the intensity of the light decreases exponentially.
In order to determine why certain marine animals exhibit biofluorescence and might use biofluorescence to communicate with one another at this depth, the team developed a special camera filter that mimicked the eye of a catshark.
“First I took one of these sharks to an eye doctor at Cornell University, because we did not know anything about the eyes,” Gruber said. “When we first noticed that this shark was fluorescent, we looked through the science literature and there was nothing published on the kinds of pigments in a shark’s eyes.”
“Once we took it to the eye doctor and we were able to determine the kinds of pigments in the eye, we were then able to design the camera around their vision.”
After the team developed the filter and used it on the lens of a Red Epic camera, they proceeded to go on several night dives into Scripps Canyon, a mile-long narrow, underwater gorge off the coast of San Diego. Through the filter, they discovered that swell sharks, which are a species of catsharks, were covered in bright green dots. Chain catsharks, on the other hand, were covered in alternating dark and light areas.
The gender of the shark also mattered, as female swell sharks exhibited unique light spots on the center of each side of their faces and more dense ventral spots that extended further than their male counterparts. The pelvic claspers of chain catsharks, used during mating to channel semen, were found to be glowing.
Sharks live 1,500 feet under the ocean. Since human visibility is limited at that depth, it is referred to as “twilight zone.” Sharks, however, have adapted to this environment.
“[Sharks] can see quite well there and the colors are different there, so the ocean is the color blue because at that depth, the only color there is one color, just blue,” Gruber said.
Although biofluorescence has been found to be widespread throughout marine animals, studies relating to fluorescent properties of marine organisms, such as the one done by Gruber’s team, are necessary to understand the evolution of biofluorescence and its impact on the ecosystems of these organisms.
“If you think about things like bats, it has not been that long since we noticed that bats can communicate in these very high frequency sounds that us humans did not hear.” Gruber said. “We have to understand other life forms beyond what our human senses can see and it encourages me to be more empathetic with animals to try to understand them more and to understand their senses and other ways they are communicating.”
Other marine animals, such as certain species of fish, have also been proven to have the capability to see biofluorescence. According to the paper, Olindias formosa, also known as the “flower hat jellyfish,” uses fluorescence at the tips of its tentacles to lure juvenile rockfishes. Cirrhilabrus solorensis, a species of wrasse native to Indonesia, responds to red biofluorescence, while fluorescence in Lysiosquillina glabriuscula, also known as the “mantis shrimp,” has been noted to enhance signaling.
In the future, Gruber plans on creating a camera filter that mimics the eyes of turtles, which are even more complex than human eyes. Adding to this complexity is the fact that turtles can see both above and below water.
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