The Glow in the Dark Sharks:
How Do the Sharks Glow?
Organisms can appear to glow in the dark in a few different ways. One way is bioluminescence, and another is fluorescence, and there is even phosphorescence. Bioluminescence is the emission of light as a result of a biochemical reaction in which an enzyme breaks a substrate down and one of the products of this reaction is light. Fluorescence is a physical process by which light excites electrons in the fluorophore to a higher energy state, and when that electron falls back down to its ground state it emits a photon. In contrast to fluorescence and phosphorescence, bioluminescent reactions do not require the initial absorption of sunlight or other electromagnetic radiation by a molecule or pigment to emit light. To understand how sharks’ glow, lets understand these reactions on a deeper level and with some examples.
Bioluminescence: made by living creatures such as fireflies, glow-worms, and many marine deep-sea creatures. The animal itself is the light source. Our glow in the dark sharks are bioluminescent.
Fluorescence: Fluorescence and bioluminescence are similar on an atomic level. Light in general is a byproduct of the excitement and subsequent calming of electrons. The difference is in the trigger. The bioluminescent trigger is the luciferin/luciferase complex, fluorescence is triggered when a pigment absorbs light from an outside source. In the case of coral fluorescence, the external source is sunlight, or artificial light in an aquarium. As the light passes through the tissues, some of it is absorbed by fluorescing pigments. These pigments make electrons available for excitation, which in turn give off light as they return to their normal energy levels. Fluorescence accounts for more color varieties than either bioluminescence or phosphorescence, because the emitted color is dependent upon the fluorescent pigment which absorbs the incoming light. Marine aquarists know that blue (actinic) light typically yields the most intense fluorescence. Ultraviolet light will produce an even stronger effect. This intensity occurs because blue and indigo are the most energetic colors of visible light, and therefore provide the most energy to excite electrons.
Phosphorescence: Phosphorescence is what you see when a child has glow-in-the-dark stars glued to the ceiling. Instead of absorbing and releasing energy instantly, the electrons re-release the energy more slowly than it was absorbed.
Photoluminescence: made by shining light at “luminous” (phosphorescent) paints.
…there is also…
Chemoluminescence: This is made by a chemical reaction like glow sticks.
Electroluminescence: This is made by passing electricity through something like a gas. Neon signs.
Röntgenoluminescence: made by shining X-rays at things. (Wilhelm Röntgen (1845–1923), the discoverer of X-rays.)
Sonoluminescence: This is made by passing energetic sound waves through liquids.
Thermoluminescence: This is made when photons are emitted from hot materials.
Triboluminescence: This is made by rubbing, scratching, or physically deforming crystals.
Most deep-sea creatures are capable of bioluminescence and it is essential to their survival in their harsh environments. Most of our glow in the dark sharks like our Kitefin sharks and Lantern sharks are a result of bioluminescence.
Some of our glowing Kitefin sharks include:
and the Velvet Belly Lanternshark, Etmopterus spinax. Some Lantern sharks have numerous hair-like denticles that help exaggerate the luminous intensity. Lantern sharks display a more complex and diverse range of photophore zonation and pattering.
For more detailed information on the above sharks, and all of the glowing Lantern sharks, visit our academy.
Most fish bio-luminesce by using their photophores in one of two ways: harnessing the light produced by symbiotic bacteria or producing their own light through chemical reactions. However, shark bioluminescence works in a different way.
For example, the Velvet Belly Lanternshark was used as a model for experimental studies on shark bioluminescence. It emits visible light from thousands of tiny epidermal photogenic organs, or photophores, which are made of a cluster of photogenic cells called photocytes. Hormones and neurotransmitters can result in some photophores to glow for long periods or shorter periods. Therefore, not all of our sharks can glow for the same amount of time. For example, the Cookiecutter shark is known to glow for several hours outside of the water after it is caught. Scientists have observed this and also fisherman who may have caught one bycatch by mistake. Our Kifetin shark’s bio-luminesce in this way. One interesting fact about the Taillight shark, is it emits a blue glowing liquid cloud, much like an octopus would excrete ink.
Image source: Plik ściągnięto
Research suggests that the organization of these photophores are used for multiple and varying behaviors like camouflage, attracting prey and intraspecies communication. Camouflage in this way is called counter illumination. It is a method of active camouflage where the organism that is producing light, in an effort to match the organism’s backgrounds in both brightness and wavelength. To a predator, that organism looks like the rest of the environment. With our sharks that rely on counter illumination, the photophores are concentrated on the ventral or belly underside of the shark.
These sharks conduct a diel vertical migration, meaning they migrate to different depths throughout the day to match their own light with that of the environment. During the day, at high levels of light, these glow-in-the-dark sharks will have to be at deeper depths to remain camouflaged. They will then migrate to shallower depths at night.
Unlike other counter illuminating sharks, the Cookiecutter shark (and some other Cookiecutters in the family, however, one species of Cookiecutter has been documented numerous accounts without a collar) has a banded area near its neck that is devoid of light emitting cells. The dark area is often referred to as a dog-collar due to its appearance. Some scientists believe this area behaves as a prey attractant by splitting the glowing regions into two groups as a way to attract larger prey. However, recent studies are beginning to question this theory, and thus the collars function is still not confirmed.
As mentioned above, Lantern sharks are more complex when it comes to bioluminescence. To understand further, lets dissect the different glowing parts of some of these sharks. Some have glowing patterns and stripes all along the body. In some species of Lantern shark, the photophores associated with the genitals, help sharks identify members of the opposite sex. Some females possess a light strip on the pectoral fins maybe to help males bite down and latch on during copulation. Some even have light organs located on the dorsal fin behind transparent dorsal fin spines.
Two rare florescent sharks:
Discovering that a shark is florescent isn’t easy, because florescence isn’t always displayed with the naked eye. For example, you can’t see the coral in your fish tank brilliantly glow until you switch on the aquarium lights. And for this reason, to discover that some sharks possess this capability is captivating! The glowing shark known as the Swellshark, Cephaloscyllium ventriosum, is fluorescent thanks to a protein within its skin that glows bright green when activated by blue light. As mentioned, humans cannot see this, but other Swellsharks can. The eyes of a Swellshark have yellow filters that block out natural blue light in order to see these new, amazing colors. Swellsharks use moonlight to turn itself luminous green, allowing it to blend in and stand out at the same time. Swellsharks belong to the family Scyliorhinidae or common name Catsharks. It is found in the subtropical eastern Pacific Ocean between central California and to southern Mexico, with an additional population off the coast of Chile. As a defense, the Swellshark is able to expand to approximately double its regular size by swallowing water. Dr. Gruber has been studying these sharks for some time now with his invented shark-eye camera. Check out this article on BBC Earth.
Another shark capable of biofluorescence is the Chain Catshark or Chain Dogfish, Scyliorhinus retifer. It is small, and is common in the Northwest Atlantic, Gulf of Mexico and Caribbean. Check out this article.
And our honorable mention shark is the Velvet Dogfish Zameus squamulosus. A harmless Sleeper shark of the family Somniosidae, found around the world between latitudes 64°N and 48°S, from the surface to 4,757 feet. It hasn’t been confirmed, but scientists now believe that this shark in a completely different family has bioluminescent photophores, and thus opening up research among another family of sharks.
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