Glow In The Dark Sharks

Are you afraid of the dark? Are you afraid of sharks? If, so you might think that the cold, dark depths of the deep-sea would be a nightmare. With predators such as the goblin shark, the knifetooth dogfish and the viper dogfish lurking in the shadows, you might be right. It gets creepier still. Some of the sharks that live down here have come up with an incredible way to go about their pitch black business. Some of the sharks down here can….glow in the dark.

By Christopher Bird (@SharkDevocean)

The deep-sea becomes a fireworks display with a diverse range of bioluminescent creatures.
The deep-sea becomes a fireworks display of light with a diverse range of bioluminescent creatures.

Some estimates suggest that 80-90% of all deep-sea creatures can produce their own light, in a process called bioluminescence. If you thought the deep-sea was a dark, lifeless expanse, think again. The diverse range of crabs, squid, jellyfish and fish emitting light must make the deep-sea look more like a scene from Star Wars than anything earth bound. Most of these creatures use light in one of three ways:

  1. To avoid being eaten.
  2. To acquire food through attracting prey.
  3. To communicate with members of same species.
Figure from Claes & Mallefet 2010 highlighting photophore excitement in the velvet belly lantern shark
Photophore excitement in the velvet belly lantern shark. Figure from Claes & Mallefet 2010

How it works:

Light organs, called photophores (in latin; light carrier), can work in one of two ways. Some fish harness the light produced by symbiotic bacterium that they engulf (passive), while others are able to produce their own light through internal chemical reactions. The deep-sea sharks, however, use a different method all together.

The black regions on this velvet belly lantern shark are covered in light emitting organs called photophores
The black regions on this velvet belly lantern shark are covered in light emitting organs called photophores.

Shark System:

Deep-sea sharks use a combination of hormones and neurotransmitters. Hormones melatonin and prolactin are responsible for short-term glows (20-60 mins) and long term glowing (several hours), respectively. Some species of lantern shark even go as far as to have bizarre hair-like skin denticles to maximise light emittance (It gives them an almost furry feel, nawhhhhh).

The denticles of many lantern sharks are hair-like to allow maximum light release.
The denticles of many lantern sharks are hair-like to allow maximum light release.

The Sharks:

There are only two families of deep-sea sharks that are able to glow in the dark. They are the Kitefin sharks (Dalatiidae) and (as the name suggests) the Lantern sharks (Etmopteridae). These two groups account for about 12% of all described shark species (>50 species). These small deep-sea sharks are incredibly diverse and can be found in all of the world’s oceans, apart from polar seas (a little too cold for these guys it would seem).

Pygmy shark (top) and cookie cutter shark (bottom) belong to the family of Kitefin sharks, Dalatiidae. Image: Symbz
Pygmy shark (top) and cookie cutter shark (bottom) belong to the family of Kitefin sharks, Dalatiidae. Image: Symbz

Camouflage: 

The majority of photophores are concentrated on the underside of the shark and it is now understood that they are used for camouflage. Now you may be thinking it seems counterintuitive to use light in the darkness of the deep-sea for camouflage; surely any light omitted would be like a flashing light for a free dinner. The opposite is true however.

Counter-illumination obliterates the silhouette of sharks outline from downwelling light from surface. Image: Rudolf Svensen
Counter-illumination obliterates the silhouette of sharks outline from downwelling light from surface. Image: Rudolf Svensen

Whilst the deep sea is relatively dark, residual light can still be seen penetrating from above. Using a process called “counter-illumination”, sharks are able to obliterate their silhouette from this residual down-welling light, thus making them invisible to predators from below (Its almost like an invisibility cloak for sharks).

Seen from below, counter-illumination would make glowing sharks invisible against downwelling light. Image: Jerome Mallefet
Seen from below, counter-illumination would make glowing sharks invisible against downwelling light. Image: Jerome Mallefet.

Now here is where, at least I think, it gets even more interesting. The light produced from the photophores is of a relatively constant intensity. But Chris, how do the sharks stay camouflaged when light levels change? Great question, you inquisitive reader; it has been hypothesised that these sharks will actually migrate to different depths throughout the day to match their own light with that of the environment. This means that 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 to remain cryptic…crafty little ninja sharks!!

Some species of lantern shark may vertical migrate ensuring that their light matches that of the environment throughout the day. Image: Rudolf Svensen
Some species of lantern shark may vertical migrate ensuring that their light matches that of the environment throughout the day. Image: Rudolf Svensen

Other uses:

For the majority of kitefin sharks, photophores only serves as camoflauge. There is however an exception to this case, but more on that later. The lantern sharks, however, display a more complex and diverse range of photophore zonation and pattering.

Figure from Claes & Mallefet 2014
Lantern sharks display additional complex patterning of bioluminescence. Figure from Claes et al 2014.
  1. ID lights: The presence and distribution of glowing cells on the side of the shark is unique to that species (“la” in image below). This has lead scientists to believe that these glowing strips are used to help sharks identify members of the same species.
  2. Glowing genitals: In some species of lantern shark, the photophores associated with the genitals, help sharks identify members of the opposite sex (“ie” in image below). Very useful when trying to have procreative sex in the dark!
  3. Love handles: A light strip on the pectoral fins (“Pe” in image below) of females helps males bite down and latch on during copulation. This minimises the amount of fumbling around.
  4. LIGHTSABERS!! Light organs are located on the dorsal fin (“SAPs” in image below) behind transparent dorsal fin spines. The now glowing lightsabers, I mean fin spines, act as a visual deterrent for any potential predation attempts.
Photophores on dorsal fins turn fin spines into lightsabers. Figure from Claes et al 2013
Photophores on dorsal fins turn fin spines into lightsabers. Figure from Claes et al 2013

 The mysterious dog collar

The cookie cutter shark (Isistius brasiliensis), like other members of the Dalatiidae family, only has photophores on its underside. Unlike other counter-illuminating sharks however, the cookiecutter has a banded area near it’s “neck” that is devoid of light emitting cells. The dark area is often referred to as a “dog-collar” due to its appearance. Now, for some time, this area has been speculated to act as a prey attractant. By splitting up it’s glowing regions into two groups, large predatory fish would be attracted to the shark. The cookie cutter could then bite onto the “predator” and leave it’s distinctive cookie-like hole, tagged on its side. This has recently been brought into question though.

Cookie cutter shark dark "dog collar" is an area void of photophores. Image: NOAA
Cookie cutter shark dark “dog collar” is an area void of photophores that is probably used in social interactions. Image: NOAA

The usual prey items of the cookie cutter are megafauna such as whales, dolphins, seals and tuna. These large animals are either filter-feeders, and would thus not be attracted to a smaller glowing “fish”, or are of a large enough size that breaking the glowing region up into two pieces would be unimportant. It is now thought that this “dog collar” may actually be used as a form of social recognition among cookie cutters.

Velvet belly lantern sharks are often caught as by-catch in many trawl and long-line fisheries
Velvet belly lantern sharks are often caught as by-catch in many trawl and long-line fisheries

So, just when you thought sharks couldn’t get more fascinating, the deep-sea glow-in-the-dark sharks show us how much more badass they can be. Deep-sea sharks generally get less attention than their larger, shallow cousins. Can a white shark glow in the dark? No! Does a tiger shark use lightsabers to fend off predators? No! And do hammerheads have glowing genitals? Errr No!

Don't forget about the glowing dogfish of the deep! Image: Hakon
BOOOOO!! Don’t forget about the glowing dogfish of the deep! Image: Hakon

So next time you think about sharks, of course think about the large pelagic/coastal species (they are amazing in their own right) but maybe give the fascinatingly diverse groups of glowing deep sea sharks a thought. With this new found love for glow in the dark sharks, I’m expecting big things from fancy dress and pumpkin carving this Halloween!

Christopher Bird

Twitter: @Shark Devocean

N.B. Any image not taken by myself has been credited and hyperlinked to source.

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