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Glow-in-the-Dark Sharks & Other Fascinating Bioluminescent Fish

bioluminescent fish
Sparks, J. S.; Schelly, R. C.; Smith, W. L.; Davis, M. P.; Tchernov, D.; Pieribone, V. A.; Gruber, D. F., CC BY 4.0, via Wikimedia Commons

You know those little glow-in-the-dark stars that dot the ceilings of children’s rooms? Yeah, I didn’t have stars. Instead, I had little glowing dolphins, starfish, and sharks “swimming” on my ceiling at night. As it turns out, my room wasn’t the only place where some of these fish glow in the dark.


biofluorescent coral
Biofluorescent coral photographed off Little Cayman Island. Note the eel (lower right). Gruber D, Gaffney J, Mehr S, DeSalle R, Sparks J, Platisa J, Pieribone V, CC BY 4.0, via Wikimedia Commons

A team of researchers, including ichthyologist John Sparks and marine biologist David Gruber, were studying biofluorescent coral when they made a peculiar discovery. Under a certain light, much like the coral, some marine fish in the area also exhibited biofluorescence.

In Their Glowing Era

warty frogfish biofluorescence
Biofluorescence of a warty frogfish (Antennarius maculatus). Sparks, J. S.; Schelly, R. C.; Smith, W. L.; Davis, M. P.; Tchernov, D.; Pieribone, V. A.; Gruber, D. F., CC BY 4.0, via Wikimedia Commons

The team soon found that over 200 species of marine fish, including sharks, found all over the world emitted vibrant biofluorescent hues, a stark contrast to the dark ocean depths.

Devoid of Bright Color

color spectrum
Overview of the type and color of the different wavelengths. Tom Gaimann, CC BY-SA 4.0, via Wikimedia Commons

The ocean functions like a filter, only allowing light at shorter wavelengths to reach greater depths. At the surface of the ocean, the visible light spectrum ranges from 400–700 nanometers (nm). The deeper below the surface, the quicker red-shifted light (closer to 700 nm) starts to fade and appear grey. 

Observing Fluorescence

color spectrum underwater
Approximation of how deep certain colors of light penetrate underwater. Tomemorris, CC BY-SA 4.0, via Wikimedia Commons

Red fluorescence, however, still appears reddish and bright in deep water, often in strong contrast to other colors. Researchers can use filters, which are placed over masks or underwater cameras, to better observe red fluorescence. These filters function by blocking blue-green light.

What Exactly Is Fluorescence?

fluorescent minerals
Collection of various fluorescent minerals under ultraviolet UV-A, UV-B, and UV-C light. (Hgrobe 06:16, 26 April 2006 (UTC)) – credit: Hannes Grobe/AWI, CC BY-SA 2.5, via Wikimedia Commons

Fluorescence refers to the ability of a substance to absorb light at one wavelength and emit light at a longer wavelength, which ends up producing a visible glow. Light at shorter wavelengths are considered “blue-shifted” and longer wavelengths “red-shifted.” So, if an object absorbs green light, but emits yellow light, it would be considered “fluorescent.” That is because, relative to one another, green is blue-shifted and yellow red-shifted. 

Are Humans Fluorescent?

melanin skin
Illustration from Anatomy & Physiology, Connexions Website. OpenStax College, CC BY 3.0, via Wikimedia Commons

You and I are not fluorescent. Our skin simply absorbs and emits more or less light of the same wavelength. People who have skin with more melanin will absorb more light than what is reflected, and those with less melanin reflect more light. This is why we have different skin tones. 

So… Biofluorescence? 

goby fish biofluorescence
Diversity in red fluorescence in 14 goby species. Nico K Michiels, Nils Anthes, Nathan S Hart, Jürgen Herler, Alfred J Meixner, Frank Schleifenbaum, Gregor Schulte, Ulrike E Siebeck, Dennis Sprenger and Matthias F Wucherer, CC BY 2.0, via Wikimedia Commons

Biofluorescence specifically refers to fluorescence in living organisms, such as coral and, as the research team discovered, many different types of fish. These organisms have “fluorescent proteins” that can absorb light of a shorter wavelength (blue-shifted) and emit light of a longer wavelength (red-shifted).

What Does This Have To Do With Sharks?

spiny dogfish
Rarely seen deep-sea spiny dogfish shark. National Marine Sanctuaries, Public domain, via Wikimedia Commons

So, like I said a little earlier, only light at shorter wavelengths can extend deeply into the ocean. This is why the ocean appears blue. So, many marine organisms, including some sharks, have fluorescent proteins that allow them to appear a more contrasting color than their surroundings. 

Why Is Contrast In The Ocean Important? 

false stonefish
Biofluorescence of the false stonefish (Scorpaenopsis diabolus). This fish appears a stone-white color when observed without fluorescence filters. Sparks, J. S.; Schelly, R. C.; Smith, W. L.; Davis, M. P.; Tchernov, D.; Pieribone, V. A.; Gruber, D. F., CC BY 4.0, via Wikimedia Commons

Biofluorescence can be used by fish for a variety of functions, including communication, such as species recognition to attract a mate, camouflage to blend in with their surroundings, such as among coral, or for predation. Notably, “biofluorescence” isn’t like bioluminescence (what fireflies can do) and cannot be turned on and off. It simply refers to how they reflect light, and can often only be seen by humans using filters. 

What Fish Are Biofluorescent? 

fluorescent fish
Diversity of fluorescent patterns and colors in marine fish. Sparks, J. S.; Schelly, R. C.; Smith, W. L.; Davis, M. P.; Tchernov, D.; Pieribone, V. A.; Gruber, D. F., CC BY 4.0, via Wikimedia Commons

Biofluorescence is actually pretty common across many different marine fish, particularly those deep underwater. Pictured are: A) swell shark; B) ray; C) sole; D) flathead; E) lizardfish; F) frogfish; G) false stonefish; H) false moray eel; I) false moray eel; J) pipefish; K) sand stargazer; L) goby; M) goby; N) surgeonfish (larval); and O) threadfin bream.

Applications in Biomedical Science

fluorescent cells
Fluorescence microscopy of induced human pluripotent stem cells (iPSCs) from a patient with familial Alzheimer’s disease. Morozless, CC BY-SA 4.0, via Wikimedia Commons

Fluorescent proteins can be used as biomarkers to illuminate cells in tissues and organs of animals, including humans, allowing scientists to physically see cells and neuronal pathways. Notably, fluorescent proteins are used in cancer patients to better visualize the spread and aggressiveness of cancer cells.

Green Fluorescent Protein

Aequorea victoria jellyfish
Hydrozoan jellyfish known as “crystal jelly” (Aequorea victoria). Sierra Blakely, Attribution, via Wikimedia Commons

Currently, the most studied and developed fluorescent protein is known as green fluorescent protein (GFP), which is derived from a hydrozoan jellyfish (Aequorea victoria). GFP emits a green color under UV light (UV is blue-shifted) and is used to study cellular processes, gene expression, and disease development and progression at a molecular level. 

Let’s Go Back To Sharks

biofluorescent swell shark
Biofluorescence of a swell shark (Cephaloscyllium ventriosum). Sparks, J. S.; Schelly, R. C.; Smith, W. L.; Davis, M. P.; Tchernov, D.; Pieribone, V. A.; Gruber, D. F., CC BY 4.0, via Wikimedia Commons

You clicked on this article for sharks, and I’ve gone and rambled about light and proteins. However, I do want to speak about sharks in particular. Sharks are fascinating because the first few sharks David Gruber and his team investigated did not exhibit biofluorescence. It wasn’t until the team were diving in a deep canyon off the coast of California that they found the first “glow-in-the-dark” shark – a swell shark. 

A Surprising Finding

swell shark
Swell shark (Cephaloscyllium ventriosum) under normal lighting. Ripley’s Aquarium, Canada. City.and.Color, CC BY 2.0, via Wikimedia Commons

The team took a small skin sample from a living swell shark and extracted the compounds to get a better understanding of biofluorescence in sharks. Firstly, they found that the biofluorescence was caused by a unique form of an amino acid, tryptophan, which is different to the protein that causes it in jellyfish. Interestingly, they also found that the biofluorescent molecule might be used for more than just communication, but that it also provides resilience against microbial infections. 

Wrapping Up

coral reef fish
Coral Reef. Image by joakant via Pixabay

Important to note, the team of scientists conducted their investigations with care and compassion for marine life. It is a great example of how we can learn from animals and natural phenomena while still being gentle. Discoveries like the ones described in this article can only be made if these ecosystems are preserved.


underwater photography
Diver exploring kaleidoscopic-colored coral reefs with an abundance of diverse marine life. Image by NEOM via Unsplash

Thanks for reading! To watch David Gruber’s fascinating talk on the study, click here.

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Diana Venter

Wednesday 10th of July 2024

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