ABYSSAL ZONE: CHARACTERISTICS, FLORA AND FAUNA - BIOLOGY - 2025

The abyssal zone is one of the regions in which the sea is divided according to its bathymetry. Some authors place it between 2000 and 6000 meters deep, although others point out that it begins at 3000 or 4000 meters.

The abyssal zone is a zone of perpetual darkness (aphotic) because the rays of the sun cannot penetrate it. The waters of this area are cold, with a temperature that generally oscillates between 0 and 1 ºC.

A. Sub-littoral zone or internal continental shelf (0-90 m). B1. Circalitoral area (90-200 m). B2. Bathyal or continental slope zone (200-3,000 m). C. Abyssal zone (3,000-6,000 m). D. Hadal zone (6,000- + 10,000 m).

This area is devoid of plants due to the permanent absence of light and the fauna has had to suffer strong adaptations to the extreme conditions of absence of light, low oxygen concentrations, high pressures and low temperatures.

characteristics

This zone is located between 2000 and 6000 meters deep, just below the bathyal zone and above the hadal zone.

The pressure is very high because in the marine environment it increases at a rate of 1 atmosphere every 10 meters, which means that the pressure in the abyssal zone is in the range of 200 to 600 atmospheres.

Sunlight does not reach this area, so there are no photosynthesizing organisms in it. Primary productivity in this zone is carried out by bacteria and other chemosynthesizing organisms.

The waters are generally rich in nutrients because there are no autotrophic organisms that take advantage of them, so they are concentrated. This is what allows those areas where deep water outcrop occurs, to be highly productive places.

The partial pressure of oxygen in this area is very low because there are no photosynthesizing organisms that release this compound in the environment.

The salinity of the deep waters is also quite uniform.

Abyssal Zone Flora

There is no flora in the deep seabed, due to this, the primary production is carried out by chemosynthetic bacteria that grow associated with the ocean floor. These bacteria develop mainly in places such as the bones of large dead animals, logs and other plant debris of terrigenous origin, hydrothermal volcanoes and cold seeps.

Fauna of the abyssal zone

Abyssal fauna. Taken and edited from: Hemmans. The abyssal fauna can be divided into two large groups: the abyssopelagic and the abysobentonic fauna.

Abysobentonic fauna

It lives associated with the seabed, either fixed to it, buried or simply living on top of it. Among this type of fauna are hedgehogs, stars, holothurians, polychaetes, crabs, shrimp, isopods, pycnogonids as well as sponges and sea squirts, among others.

These species can suffer a phenomenon called gigantism because they reach extremely large sizes compared to their shallower water pairs. For example, deep-sea isopods can reach 40 cm in length, while shallow-water species rarely exceed 2 cm.

Most abysobentonic species feed on food particles that fall from the upper waters. While some species feed on these particles that are still suspended in the water, others feed on the particles that have already settled in the sediment.

Predators can also be found among the fauna of the abyssal zone, but they seem to be rare, and are represented, for example, by pycnogonids, starfish, ophiuros and crabs.

Some species of fish are also benthic, among which we can mention tripod fish, grenadiers, witches, brotulids and some species of eels.

The hydrothermal windows, cold seeps and the carcasses of large animals are a kind of oasis in the abyssal bottoms, which support a high diversity of species. Recent works place the number of species that inhabit these environments at 400.

Abyssopelagic fauna

It is the fauna located directly in the water column of the abyssal zone. It is comprised of some invertebrates such as mollusks, jellyfish, ctenophores, polychaetes and fish.

Some species are completely blind, others have disproportionately large eyes to take advantage of the little light from bioluminescence. Many species use bioluminescence both to attract congeners for reproductive purposes and to attract potential prey.

Due to the limited amount of food available, the different species are not very abundant, that is why fish have adopted hermaphroditism as a strategy to guarantee their reproduction. However, this has not happened with invertebrates, in which hermaphroditism is rare.

All abyssal fish lack a swim bladder, which is probably due to the energy cost of filling this bladder being too high due to the high pressures they must withstand.

Some species of fish have adopted the strategy of male parasitism, which consists in that when the male reaches sexual maturity and gets a female of his species, he clings to her and parasitizes her, in that way, he will always be available to fertilize the female in reproductive period.

Among the physiological adaptations that both fish and abyssal invertebrates have undergone is the development of a slower metabolism, thus requiring much less oxygen and food than the species in the upper bathymetric zones.

Representative species

Bathynomus

Giant isopod Bathynomus giganteus. Taken and edited from: Yale Peabody Museum of Natural History Organisms of this species are known as giant isopods. They live in deep waters of the Atlantic Ocean. The species was discovered in 1879 and described by the French zoologist Alphonse Milne-Edwards, based on a juvenile male.

It can be up to 50 cm long, has a segmented body and resembles the scale of moisture or pellet bugs that commonly live under rocks and flowerpots in gardens.

These organisms have a very extensible stomach which indicates that their food is probably scarce and you should make the most of it when you can find it. No known predator to date.

Bathypterois grallator

Known as a tripod fish for presenting projections of its pelvic and tail fins that allow it to lean on the ocean floor as if they were stilts. This organism has an average height of 30 cm, but it can measure up to 43 cm and its fins can measure more than one meter.

This fish has been found from 878 m to 4720 m deep, and it is cosmopolitan, since it inhabits both the Atlantic Ocean and the Pacific and Indian Oceans.

Cryptopsaras couesi

The female of this species of fisher fish can reach 30 cm, while the male only reaches between 1 and 3 cm and parasitizes the female. This species is cosmopolitan and is found in all the great oceans of the world at depths ranging from 75 to 4000 meters.

References

1. R. Barnes, D. Cushing, H. Elderfield, A. Fleet, B. Funnell, D. Grahams, P. Liss, I. McCave, J. Pearce, P. Smith, S. Smith & C. Vicent (1978). Oceanography. Biological Enviroment. Unit 9 The pelagic system; Unit 10 The Benthic system. The Open University.
2. G. Cognetti, M. Sará & G, Magazzú (2001). Marine biology. Editorial Ariel.
3. G. Huber (2007). Marine Biology. 6 ^th edition. The McGraw-Hill Companies, Inc.
4. Abyssal zone. On Wikipedia. Recovered from: en.wikipedia.org.
5. D. Rodríguez. Abyssal plain: characteristics, elements, flora, fauna. Recovered from: lifeder.com.
6. Abyssal fauna. On Wikipedia. Recovered from: es.wikipedia.org.
7. C. Lyre. Hadal zone: characteristics, flora and fauna. Recovered from: lifeder.com.