The deep sea

Our deep oceans are on of the most unexplored and least known about areas of our planet as well as being one of the harshest environments on the planet. it has pressures reaching up to 1000 times our atmosphere and averaging 600 times, waters reaching as low as 2°C, low oxygen levels and little to no light. To top all of this off food is also incredible scares as the only primary production that can be found is in the form of chemo-synthesis at hydro-thermal vents. Due to this most organisms found down there are predators or scavengers.  Even with all of these harsh conditions a wealth of life can be found with over 17,000 species being discovered, many of them being completely alien to anything else found in the world.

Black Dragonfish

Top picture shows a female Black Dragonfish, underneath shows a male and the bottom drawing shows the larval stage of this species taken from:


One species that is so alien that it was the inspiration behind the xenomorph from the alien films, the Black Dragonfish (Idiacanthus atlanticus) are long slender fish that display sexual dimorphism, the females grow to 40 cm in length, they are completely black, have a chin barble and a mouth filled with fang like teeth. Males are considerably smaller and get to a maximum size of 5 cm, are a dark brown colour, have no chin barble, teeth or pelvic fin and have a non-functional gut. Both sexes have tiny photophores all over their body and two rows of larger photophores along the side of their body, the females also have them in their barbel to create a slender luminous tip that is thought to be used to attract and catch prey. Larval Black Dragonfishes are most unusual. They are long, slender, transparent fishes that have their eyes at the ends of long stalks which can be up to half the length of the body, this trait is called the Stylophthalmine trait.

map showing the global sightings of Black Dragonfish taken from:

They are found in the subtropical and temperate waters of the southern hemisphere, females live below 500m and up to 2000m during the day but perform vertical migration at night to shallower waters where as males always stay below 1000m. Due to this sightings of this species are rare with them first being discovered in 1906 by August Bernhard Brauer and only 184 specimens being caught since.

life of the Black Dragonfish

Black Dragonfish feed predominantly on other fish species and may even show cannibalistic traits, they are also known to eat deep sea crustaceans. There are extremely well adapted to hunting in the pitch black environment of the deep sea,  the photophores on its body produce light of such a wave length that it is nearly infrared and invisible to the human eye, this paired with the fact that their eye is adapted to see a wider wavelength rather than being

A Black Dragonfishes bio luminescence taken from:
A Black Dragonfishes bio luminescence taken from:

adapted to being big and absorbing more light like many deep sea species are. This gives them an advantage as if prey is close enough to be illuminated by the light given off by the photophores then the Dragonfish will be able to see the prey but they prey won’t be able to see them. They also have the luminous barble attached to the underside of their lower jaw; this is used to lure prey in by looking like a smaller organism that can be preyed upon. When the smaller predator gets close enough the dragonfish will strike its prey, after it has it in its jaws it is very unlikely the prey would be able to escape because of the dragonfishes fang like teeth that curve inwards. The sexual dimorphism of the species is also thought to be related to feeding ecology, this is because as the males are smaller and have no teeth or functioning gut it means that they feed on different food sources to the females, this reduces competition within the species whilst allowing the genetic diversity of sexual reproduction.


Human interaction 

Due to the rarity of this species we know very little about whether they undergo any migration or not, we also know nothing about their reproduction other than the fact that the males of the species live only to mate. The sacristy of sightings of Black Dragonfish has also meant that population numbers are impossible to predict, due to this as well as the fact that they aren’t targeted by commercial fisheries they have been given an IUCN rating of  ‘least concern‘. This may change if exploitation of the deep sea becomes a common place thing. All of these things may become known to us in the near future with deep sea exploration becoming a more common place thing as well as the now growing interest in the deep sea.

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