Twinkle, Twinkle, Little Shark

When we imagine a shark, many people picture a large, grey predator that lives in a tropical ocean, maybe around a reef or other shallow, coastal areas. However, there are many species of sharks that live in very deep oceans. As a result, they have evolved to look and behave completely differently to other sharks. Many species have developed one particularly spectacular trick - they can glow in the dark! How do they do this? And why is it glowing an advantage to these little sharks?

The American pocket shark expels bioluminescence liquid as a way to escape from predators (Claes 2020)

Shining Sharks

Being able to produce light is actually quite common for animals living in the deep ocean. This is known as "bioluminescence". Whilst it is not common in sharks (only 6% of deep-sea species can glow), there are at least 50 species that are be able to produce light (Claes & Mallefet, 2009).

Many sharks in the family Entmopteridae and Dalatidae are able to glow in the dark (Claes & Mallefet 2021)

Bioluminescence has many different functions depending on the species - how they live, how they behave, what they eat all affect how their fluorescence has evolved (Claes & Mallefet, 2009, Gruber et al, 2016, Park et al, 2019).

The ability to glow has developed several time within the shark evolutionary line. There are some species of wobbegongs (family Orectolobidae) and some catsharks (family Scyliorhinidae) and several different species of "Squaliformes" (dogfish), grouped into three families, known as Dalatiidae (kitefin sharks), Etmopteridae (lantern sharks) and Somniosidae (sleeper sharks) that can bioluminesce (Claes & Mallefet, 2009, Gruber et al, 2016, Park et al, 2019).

Swell shark fluorescence matches the pigment patterns that are visible under ambient light (Gruber et al, 2016).

Biofluorescence is completely unique to each species. Catsharks glow with intricate networks of patterns matching the markings of pigmentation in their skin. The kitefin sharks only glow on their belly (aka their "ventral side"), whereas the aptly-named lantern sharks glow in large patches all over their bodies (Claes & Mallefet, 2009, Gruber et al, 2016, Park et al, 2019).

These sharks are all relatively small. Only one species is larger than a metre long - the seal shark aka kitefin shark (Dalatias licha). They also all live at great depths, around 200 metres underwater in the "mesopelagic zone", where sunlight is either incredibly weak or completely absent (Claes & Mallefet, 2009, Mallafet, 2021).

Different species of sharks have different glowing patterns, but within the different families (Etmopteridae (lantern sharks) and Dalatiidae (kitefin sharks)) the patterns are similar between closely related species (Mallefet, 2021)

How do Sharks Glow in the Dark?

Some marine animals create their glow thanks to "symbiosis" with another species. This means that a larger animal, like a squid, is host to a bacteria that is able to create light (Claes & Mallefet, 2009).

Unlike squids which use light-emitting bacteria to glow, sharks like this blackbellied lantern shark, make their own bioluminescence thanks to specialised cells in their skin (Mallefet, 2021)

Sharks are different. They are actually able to create their own light thanks to thousands of specialised organs in their skin called "photophores". These cup-shaped structures contain pigmented cells (known as "photocytes") which are capable of biochemical reactions that produce blue-green light (with a wavelength of 455 and 486 nm). In some species specialised "lens cells" or reflector molecules sit on top on the whole structure, to focus and enhance this light (Mallefet 2021).

Sharks are actually also some of the only animals that can control their bioluminescence via hormones! The production of hormones, such as melatonin (amongst others), allow the bioluminescence to be switched on and off when it is needed (Mallefet 2021).

Chain catsharks have very detailed patterns of fluorescence that allows them to recognise her members of their species in the dark (Gruber, 2016)

Light to Talk

One of the first theories about why some sharks glow is the idea that light can be used for communication. In a very dark environment, where it is difficult to see much, the ability to glow would allow animals to recognise other members of their species (Park et al, 2019).

Swell sharks have sexual dimorphism in their fluorescence - this female has markings on her face that allow the male to recognise her as a potential mate (Gruber et al, 2016).

This would explain why bioluminescent sharks glow in very specific, detailed patterns. As each species is unique, the glowing markings allow the sharks to recognise their own kind. This would be vital for sharks that form schools or when they are looking for a mate to breed with (Park et al, 2019, Mallefet 2021).

Some species even show "sexual dimorphism" in their bioluminescence patterns. This means that the males and females have different markings, which can be used to tell the sexes apart. Swell sharks (Cephaloscyllium ventriosum), for example, use sexual dimorphism of their bioluminescence when searching for a mate (Mallefet 2021).

Chain catsharks (A) and swell sharks (B) have very different patterns of bioluminescence, so scientists think this helps the sharks recognise their own kind (Park et al, 2019)

Light to Hide

Further research has also shown that bioluminescence might also be used for camouflage and to avoid predators. This sounds counterintuitive doesn't it! How could emitting bright light in a dark environment help you hide!? Wouldn't that be a neon sign advertising your presence!?

Illustration showing how counterillumination and strategically placed black markings on the belly help to hide the silhouette of the cookie cutter shark (Widder 1998)

In fact, it depends on your perspective.

The glowing tummies of kitefin sharks allow them to become invisible in the water! Many predators in the deep, dark oceans search for prey by looking upwards. This is because in very low light it is easiest to spot the dark sillhoette of prey contrasting against the light coming down from above. The kitefin sharks have evolved to be more difficult to see because their glowing belly matches the light background. This is known as "counter illumination" (Widder 1998).

One species of sharks uses its bioluminescence particularly spectacularly. The American pocket shark (Mollisquama mississippiensis) doesn't only have gleaming skin, but it also has specialised glands which allow it to squirt luminescent liquid into the water to confuse predators (Claes 2020). To learn more you can check out Glowing in the Deep.

Artist's impression of the American pocket shark squirting it's bioluminescent liquid into the water to confuse predators (Image Credit: Mark Grace, Source: www.sharkreferences.com)

Light to Hunt

Scientists have also learned that bioluminescence can be used for hunting in some species of sharks. Take for example the cookiecutter shark (Isistius brasiliensis). These little sharks rely on counter illumination for camouflage, but their light is also thought to help them when they are hunting their prey (Widder 1998).

The largest species of bioluminescent shark - the seal shark - can grow to over a metre in length [Scale bar: 10 cm] (Mallefet 2021)

The only part of the cookiecutter's belly that does not emit light is a dark collar around the throat. Scientists wonder if this shape and colour of this marking has evolved because it acts as a kind of lure, to draw the shark's prey in close (Widder 1998).

The largest glowing shark (in fact the largest glowing vertebrate in the world!), the seal shark is also thought to use its luminescence to hunt. This shark has few predators thanks to its size, so it does not need counterillumination for concealment. Instead, scientists think it might use its own glow to as a flashlight to search for prey on the sea floor (Mallefet 2021).

All sharks exploit their bioluminescence in different ways and being able to glow probably serves multiple functions for each species. As these sharks live in such deep habitats, where they are hard for us to observe, we are still learning about them. Who knows what new discoveries are still out there waiting to be found and how many other species might glow-in-the-dark...

Cookiecutter sharks use bioluminescence both for hiding from predators and for hunting their own prey [Scale bar 5 cm] (Claes & Mallefet 2021)

References Claes JM & Mallefet J (2009). Bioluminescence of sharks: First synthesis. Bioluminescence in Focus - A Collection of Illuminating Essays, 51-65. Access online.

Claes JM, Delroisse J, Grace MA, Doosey MH, Duchatelet L & Mallefet J (2020). Histological evidence for secretory bioluminescence from pectoral pockets of the American Pocket Shark (Mollisquama mississippiensis). Scientific Reports, 10:18762. Access online.

Gruber DF, Loew ER, Deheyn DD, Akkaynak D, Gaffney JP, Smith WL, Davis MP, Stern JH, Pieribone VA & Sparks JS (2016). Biofluorescence in catsharks (Scyliorhinidae): Fundamental description and relevance for elasmobranch visual ecology. Scientific Reports, 6:24751. Access online.

Mallefet J, Stevens DW & Duchatelet L (2021). Bioluminescence of the largest luminous vertebrate, the kitefin shark, Dalatias licha: First insights and comparative aspects. Frontiers in Marine Science. Access online.

Park HB, Lam YC, Gaffney JP, Weave JC, Krivoshik SR, Hamchand R, Pieribone V, Gruber DF & Crawford JM (2019). Bright green biofluorescence in sharks derives from bromo-kynurenine metabolism. iScience, 19, 1277–1286. Access online.

Widder EA (1998). A predatory use of counterillumination by the squaloid shark, Isistius brasiliensis. Environmental Biology of Fishes, SQ:267-273. Access online.

By Sophie A. Maycock for SharkSpeak