Scientists have figured out why certain species of shark can absorb blue light in the ocean and essentially turn the light green, making them appear to glow. It's due to a newly discovered family of small-molecule metabolites in the lighter parts of the sharks' skin, according to a new paper in the journal iScience.

The phenomenon is known as biofluorescence, not to be confused with a related phenomenon, bioluminescence. These are not "glow in the dark" sharks. Fluorescence is a phenomenon where light is absorbed and emitted at a longer wavelength. "There are some bioluminescent sharks, and some animals have both properties, making it even more confusing," said co-author Dave Gruber of the City University of New York. "The simplest way to think about it is that some animals make their own light [bioluminescence] and some transform light [biofluorescence]."

Most bioluminescent species thrive deep in the ocean, below the so-called "photic zone," where no photons from the sun can reach, so the animals must make their own light. "Biofluorescence is more of a shallow phenomenon, because that's where the light is," said Gruber.

Gruber became interested in studying sharks several years ago, when he discovered that biofluorescence was surprisingly common in more than 180 marine species, some of which were species of sharks and stingrays. Prior to that, most biofluorescence studies had focused on jellyfish and corals. "There has been a lot of debate about the function of fluorescence in corals, but sharks are animals with very strong visual senses," said Gruber—something the corals are definitely lacking. He and Crawford were working on another project when Gruber mentioned the existence of biofluorescent sharks, and it became a joint passion project, even though they didn't have funding at first.

Glowing green

The most famous mechanism for biofluorescence is green fluorescent protein (GFP). In 1961, Osamu Shimomura and Frank Johnson isolated the protein from jellyfish that glowed green in sunlight, yellow under a light bulb, and fluorescent green under UV light. (Shimomura shared the 2008 Nobel Prize in Chemistry for this work.)

GFP contains a special chromophore that absorbs and emits light. Shining UV or blue light on the chromophore causes it to absorb the energy, become excited, and then emit the excess energy as green light. GFP has since become a standard tagging tool for researchers all over the globe, enabling them to study biological processes previously invisible to the naked eye at the cellular level. Corals, too, can fluoresce very prettily in a wide range of hues, and the proteins that cause this are in the same family as GFP.

The new compounds found in the skin of swell sharks (Cephaloscyllium ventriosum) and chain cat sharks (Scyliorhinus retifer) are similar to GFP and its family of proteins, in that they emit a bright, green fluorescence when a certain wavelength of light is shown on them (typically UV light). "The difference is that our molecules are small metabolites, whereas GFP is a protein," said co-author Jason Crawford of Yale University. Metabolites are substances produced during metabolism (digestion or other bodily chemical processes) that are responsible for any number of bodily functions.

Shark skin is already pretty amazing stuff. Anyone who has touched a shark knows the skin feels smooth if you stroke from nose to tail. Reverse the direction, however, and it feels like sandpaper. That's because of tiny translucent scales, roughly 0.2 millimeters in size, called "denticles" (because they strongly resemble teeth) all over the shark's body, especially concentrated in the animal's flanks and fins. It's like a suit of armor for sharks and helps make the animals more hydrodynamic by reducing drag, much like the dimples on a golf ball.

The two shark species studied in this latest paper have two different tones to their skin: light and dark. It's the lighter portions of the skin that contain a special molecule responsible for the animals' biofluorescence. "They take light from the sun and, at varying depths of the ocean, they absorb certain wavelengths of that light, and then emit it at a longer wavelength so that they look bright green," said Crawford. At least in the case of the chain cat shark, the dark denticles serve as optical waveguides, channeling the fluorescence signal along their body length.

A shark's-eye view

The sharks can see that particular wavelength, too. Gruber conducted a 2015 study involving swimming with sharks outfitted with a "shark-eye" camera to study the visual pigments of these two species. "The first time I was absolutely terrified," Gruber admitted, given that media events like Shark Week inevitably focus on the rare shark species that are aggressive, and most people don't have any other exposure to the creatures. "Now that I've swum with sharks quite a bit, I see them from a different perspective. Plus they go back 400 million years. Swimming with them is like this portal into the past."

Sharks are monochromats, according to Gruber, meaning they only have one visual pigment (a single rod) sensitive to the blue-green interface. "Rods are usually more for light intensity, for seeing things in low light conditions," he said. "So the shark likely sees very well in low light conditions at the blue-green end of the spectrum." The ocean is predominantly a blue environment, and thus the ability to emit and detect this green fluorescence helps create more contrast in that environment.

"It's a completely different system for them to see each other, that other animals cannot necessarily tap into," said Crawford. "They have a completely different view of the world because of these biofluorescent properties that their skin exhibits and their eyes detect. Imagine if I were bright green, but only you could see me as being bright green, and others could not."

Once the team had isolated the compounds—tracking them using their spectral properties—they began testing them for other useful properties. They discovered that two of these new molecules also have antimicrobial properties. It's not an especially potent antimicrobial, and thus far they have only observed high concentrations in laboratory in vitro samples; the antimicrobial activity still needs to be tested in real-life conditions. But the fact that it's there at all intrigues Gruber.

"Whales actually slough their skin much faster than humans to stop barnacles and other things from growing on them," he said. "This is a species of shark that lies on the bottom at the sediment water interface where there are a lot of microbes. Yet they don't have algae growing all over them, or something of that sort."

DOI: iScience, 2019. 10.1016/j.isci.2019.07.019 (About DOIs).