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)
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:
- To avoid being eaten.
- To acquire food through attracting prey.
- To communicate with members of same species.
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.
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).
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).
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.
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).
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!!
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.
- 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.
- 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!
- 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.
- 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.
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.
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.
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!
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!
Twitter: @Shark Devocean
N.B. Any image not taken by myself has been credited and hyperlinked to source.