Biofluorescence in Sharks: Light Emission and Possible Functions
What Is Biofluorescence?
Light production by living things is an interesting and often beautiful phenomenon. Some animals in the ocean are able to produce colored light by fluorescence. During this process, an animal absorbs light with one color and then emits light with a different color. Marine animals that are fluorescing generally look green, red, or orange to us. Some produce a different color from different parts of their bodies. Researchers suspect that the light has important functions.
The list of marine animals that produce light by biofluorescence (fluorescence by living things) is already long. It’s getting even longer as scientists make more discoveries. Currently, certain species of fish, squid, shrimp, coral, jellyfish, and siphonophores are known to fluoresce. Siphonophores are colonial organisms that look somewhat like jellyfish. An example is the Portuguese man-of-war. In this article I’ll focus on biofluorescence in two species of sharks—the swell shark and the chain catshark.
The numbers in the spectrum above represent the wavelength in nm (nanometers). A nanometer is a billionth of a meter.
Wavelength and Color Perception
In order to understand how fluorescence works and becomes visible to us, it's helpful to know some facts about light and color perception.
- "White" light is actually a mixture of different wavelengths of electromagnetic radiation, each of which is perceived as a different color when viewed individually and interpreted by our brain.
- The shortest wavelength of visible light appears blue to us, as shown in the spectrum above. It has the highest energy.
- The longest wavelength appears red to us. It has the lowest energy.
- The brain uses wavelengths that are reflected or transmitted by objects and received by our eyes to create the colors that we see. Wavelengths that are absorbed by objects don't reach our eyes and can't be seen.
- Color filters are made of a semi-transparent material that absorbs or reflects some wavelengths and transmits others. They can be used to block certain colors from our eyes.
- A filter that is yellow in color blocks blue light but transmits green and red light, which reach our eyes.
Swell shark photo by Aquaimages, CC BY-SA 2.5 license
Chain catshark photo by Cliff, CC BY 2.0 License
Detecting Fluorescence in the Ocean
The light in water that is deep but still illuminated is predominantly blue. Other colors are filtered out by the water above. To the unaided eye, all of the creatures in the deep water appear to be a shade of blue. In very deep water the light may be so weak that the creatures are hard to see. In order to see fluorescence in these conditions, we need to follow specific procedures.
Illumination by Blue Light to Trigger or Enhance Fluorescence
Some illumination must be present in order for fluorescence to occur. If the environment is too dark, researchers may illuminate the area with blue light to enhance the natural light that is present.
When a fluorescent organism absorbs the blue light, it's triggered to emit light with a longer wavelength and less energy (and therefore a different color). The fluorescence is often relatively weak and masked by the blue light that the organism reflects, however. As a result, we can’t see it unless the reflected light is filtered out. Once this is done, the green or red light emitted by the organism can be seen.
Blockage of Reflected Blue Light by a Yellow Filter
The blue light that is reflected by the organism is blocked by a yellow filter. Scuba divers or people in underwater vehicles known as submersibles wear glasses made from a yellow filter in order to see fluorescence. The filter blocks the transmission of blue light and allows the green or red light emitted by the organism to pass through. A yellow filter on a camera does the same thing, so explorers can make a visual record of the biofluorescence that they discover.
Some animals emit light by luminescence instead of fluorescence. Luminescence doesn't require incident light and can be seen in a completely dark environment. It's generated by a chemical reaction in a light organ and is produced by the animal itself or by bacteria living in its body. Luminescent fish generally live in deeper water than fluorescent ones.
Two Fluorescent Sharks in California
More than 200 species of fish are currently thought to be biofluorescent. The first fluorescent vertebrate that was discovered was an eel. The discovery was accidental. The researchers were filming biofluorescent coral and were "photobombed" by a glowing green eel that swam into view.
Since the eel discovery, scientists have discovered that two species of sharks in the catshark family are fluorescent—the swell shark (Cephaloscyllium ventriosum) and the chain catshark (Scyliorhinus rotifer). Both live in the relatively deep water of Scripps Canyon off the coast of California and both produce beautiful patterns of green light. Their fluorescence was discovered by a team led by David Gruber.
The areas on a shark's body that respond to incident light and emit new light contain fluorescent pigments. These appear to be proteins. The researchers have discovered that the two sharks can very likely see the fluorescence created by their neighbors. The opening screen in the video above shows the chain catshark when it's emitting fluorescence and the one in the video below shows the swell shark.
The Eyes of Catsharks
The scientists have examined the eyes of the catsharks in their study and have made some interesting discoveries. One is that the animals have much longer rods than us. Rods are cells that provide good vision in dim light but don't respond to color. A second discovery is that the eyes contain a visual pigment that responds to blue-green light, which is the color range that is found in the shark's environment and in their fluorescence. This is the only visual pigment that the animals possess. In contrast, humans have three visual pigments—red, green, and blue—and can see a wide range of colors.
It certainly seems that the sharks' eyes are adapted to see fluorescence. We can't tell exactly what color the emitted light looks like to them, however, or how bright it appears to be under natural conditions. We also don't know if the light is visible to sharks at all the depths in the water in which they are found. In addition, researchers don't yet know whether the shark's predators or prey can see the fluorescence. Although it might seem logical that they don't, we shouldn't assume that this is the case.
The Swell Shark
The body of an adult swell shark is generally a little under three feet long. It's typically yellow-brown in color under white light. The surface of the animal is covered with a mix of light and dark bands, blotches, and spots. The shark is found at depths of 16 to 1500 feet but is most common between 16 and 120 feet. It's a nocturnal animal that hides in caves and crevices during the day and hunts on the ocean bottom at night. It feeds on small fish, crustaceans, and molluscs.
The swell shark got its name from an unusual behavior. When it's in danger of being attacked, it grabs hold of its tail to form a U shape and quickly fills its stomach with water or air. This causes its body to swell up and look threatening. If the animal is hiding in a rock crevice, its swollen body may lock it in place and prevent or discourage a predator from attacking. When the danger is past, the shark lets go of its tail and expels the water or air from its stomach with a barking sound.
The Chain Catshark
The chain catshark gets its name from the dark, interlocking lines on its body, which produce a pattern that looks like the links of a chain. The rest of the body is cream to brown in color. Chain catsharks have horizontally oval eyes which are green in color. Their pupils are elongated and are reminiscent of those of cats. Adults are about eighteen inches long. The animal is also known as the chain dogfish.
Chain catsharks are found at depths of around 240 to 1800 feet. Stomach analysis shows that the sharks eat fish, squid, marine worms, and crustaceans (crabs, lobsters, and shrimp). The animal is benthic, or bottom dwelling. It often rests on the ocean bottom when it's not hunting.
The color pattern on the surface of the swell shark and chain catshark helps to camouflage them against their background. Interestingly, in the first video in this article the narrator says that his team tends to find fluorescence in animals with cryptic coloring which helps to hide them from predators and prey. The camouflage may hide them from their own species as well, which could be a problem in some situations.
Function of the Fluorescent Light Patterns
Though the function (or functions) of shark fluorescence aren't known, scientists suspect that the feature must be important since it's widespread and noticeable. The light is thought to play a role in mating. The pattern produced by the fluorescence is different in the males and females of a species, at least in the two catsharks. Interestingly, the claspers of the male chain catshark glow green. Claspers are used to insert sperm into the female's body and are attached to the pelvic fins of the male. Researchers suspect that the light is important in non-mating communication as well.
The catshark family are not the only biofluorescent sharks. Wobbegongs also fluoresce. They are bottom-dwelling fish that are often camouflaged to match the ocean floor. They typically have flaps of skin on their head or around their mouth that resemble tassles.
The Puzzle of Biofluorescence
Biofluorescence has developed in many species of fish. The light is impressive and often gorgeous as viewed by humans. It likely has important functions, since the ability to fluoresce is so common. What these functions are is still mysterious, however. The results of future research may be illuminating.
Exploring biofluorescence from National Geographic
Swell shark information from the Aquarium of the Pacific
More facts about swell sharks from the ReefQuest Centre for Shark Research
Chain catshark facts from the ReefQuest Centre for Shark Research
Information about the chain dogfish from the Florida Museum of Natural History
Biofluorescence in catsharks from National Geographic
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© 2017 Linda Crampton