Abyssal creatures: what secrets do the ocean depths hold?
After centuries of research, the species that populate the Earth are well known, at least those that are within our reach. Because there are others, present in remote and practically inaccessible places, which are largely unknown. These include those found in the deep sea. Below, we take a closer look at the abyssal creatures that inhabits the seas and oceans of our planet.
Several hundred metres below sea level, where light cannot penetrate, there are thousands of species that have made the deep sea their habitat. If we talk about the viperfish, the telescope octopus or vampire squid, they probably don't even ring a bell — even if their name makes you think of monstrous creatures and they may indeed look monstrous — but these are only three of the thousands of species that, according to the Census of Marine LifeExternal link, opens in new window., live in the deep sea.
These thousands of species, many of them discovered during the last decades thanks to the Ocean Biodiversity Information System (OBIS)External link, opens in new window., make up the abyssal creatures or, in other words, those animals that live beyond the epipelagic zone, i.e. 200 metres below the surface. Although there are many, scientists agree that more than 80 % of the ocean floor has not yet been surveyed with modern technology, so the secrets of the deep ocean have only just begun to be revealed.
WHAT IS THE ABYSSAL ZONE
The abyssal zone, also known as the abyssopelagic zone, is one of the levels into which the oceans are divided and it is found between 3,000 and 6,000 metres below the surface. In fact, the word abyssal comes from the Latin abyssalis, meaning abyss, something bottomless or extremely deep. However, abyssal fauna expands beyond this area. In fact, from a zoological point of view, abyssal creatures is considered to be all the fauna that lives below a depth of 200 metres, as few species are capable of living beyond 1,000 metres.
Characteristics of the abyssal zone
The abyssal zone, due to its depth, is an extremely demanding environment for living beings: it is an aphotic region, i.e. it lacks light; the temperature ranges between 0 ºC and 3 ºC; there is a shortage of nutrients, which makes it difficult for the species that inhabit it to feed and grow; and the hydrostatic pressure increases with depth, for example, in the Challenger Deep, the deepest point of the ocean at almost 11,000 metres, the pressure is a thousand times higher than at sea level.
What is the abyssal plain
The abyssal plain is a practically flat strip of land typical of the abyssopelagic zone that extends beyond the continental shelf, the continental barrier and the continental rise. It makes up about 50 % of the ocean floor and below it we would only find the oceanic trenches, that can reach a depth of 11,000 metres — such as the Challenger Deep, mentioned above, which is located at the southern end of the Mariana Trench. Light does not reach the plain, so it harbours little life, mainly chemosynthetic bacteria, some invertebrates (worms) and some vertebrates (fish).
Ever since the British government launched the Challenger expedition in 1872 to map the deep sea, discovering forty new species, there have been numerous scientific voyages that have gradually unravelled a small percentage of its mysteries. The expeditions by the research teams of the US National Oceanic and Atmospheric Administration (NOAA) or those by the aforementioned OBIS are a good example of this.
In 2018, an Australian expedition discovered more than 100 species of abyssal fish at a depth of 4,800 metres, some of them faceless, i.e. without visible eyes or mouths, others terrifying, such as the lizardfish with a huge mouth, photos of which went viral. Their appearance is justified by their harsh living conditions, as they have had to adapt to the environment in order to survive, and this gives them a very special set of attributes: soft bodies, transparent skins, sharp teeth, underdeveloped eyes, extendable stomachs, etc.
If there is one feature that stands out above all others, it is bioluminescence. This phenomenon, which occurs in 90 % of the animals living in the deep sea, allows them to create light through a chemical reaction, which they use for defence, to locate food or as a lure for reproduction. Bioluminescence can be diffuse, localised or appear in specialised organs.
Here are some examples of the amazing abyssal creatures:
- Caulophrynidae: are characterised by a peduncle above the mouth with a luminous organ at the tip, which they use for hunting. Living on the abyssal bottom, they find it difficult to find a mate, so the larger female hosts the male.
- Chauliodus danae: the viperfish has disproportionately large and very sharp teeth that allow it to impale its prey. Males can grow up to 15 centimetres long. The chauliodus sloani, in the same family and larger in size, inhabits waters up to 4,000 metres deep.
- Saccopharynx: similar to eels, they have extensible stomachs, large jaws — hence their nickname of gulper eels — and a bulb-shaped luminous organ in the tail. They live at depths of around 2,000 metres and can reach up to two metres in length.
Abyssal crustaceans and abyssal molluscs
- Colossendeis: this genus of marine spiders, some of which are bioluminescent, lives in deep water and is notable for the length of their limbs, which can reach 40-50 centimetres, in contrast to their small bodies.
- Vampyroteuthis infernalis: known as the vampire squid, it is capable of living in the aphotic depths and lacks an ink reservoir, instead releasing a sticky cloud of bioluminescent mucus to scare off predators.
- Amphitretus pelagicus: native to the abyssal zone, the telescope octopus inhabits the tropical and subtropical waters of the Indian and Pacific Oceans, and is distinguished from other octopus species by its translucent, tubular eyes.