The hadal zone, also known as the hadopelagic zone, is the deepest region of the ocean, lying within oceanic trenches. The hadal zone ranges from around below sea level, and exists in long, narrow, topographic V-shaped depressions. The cumulative area occupied by the 46 individual hadal

habitats In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...

worldwide is less than 0.25% of the world's seafloor, yet trenches account for over 40% of the ocean's depth range. Most hadal habitat is found in the Pacific Ocean.

Terminology and definition

Historically, the hadal zone was not recognized as distinct from the abyssal zone, although the deepest sections were sometimes called "ultra-abyssal". During the early 1950s, the Danish '' Galathea II'' and Soviet ''Vitjaz'' expeditions separately discovered a distinct shift in the life at depths of not recognized by the broad definition of the abyssal zone. The term "hadal" was first proposed in 1956 by Anton Frederik Bruun to describe the parts of the ocean deeper than , leaving abyssal for the parts at . The name refers to

Hades Hades (; grc-gre, ᾍδης, Háidēs; ), in the ancient Greek religion and myth, is the god of the dead and the king of the underworld, with which his name became synonymous. Hades was the eldest son of Cronus and Rhea, although this also ...

, the ancient Greek god of the underworld. Depths in excess of are generally in ocean trenches, but there are also trenches at shallower depths. These shallower trenches lack the distinct shift in lifeforms and are therefore not hadal. Although the hadal zone has gained widespread recognition and many continue to use the first proposed limit of , it has been observed that represents a gradual transition between the abyssal and hadal zones, leading to the suggestion of placing the limit in the middle, at . Among others, this intermediate limit has been adopted by UNESCO. Similar to other depth ranges, the fauna of the hadal zone can be broadly placed into two groups: the hadobenthic species (compare

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

) living on or at the seabottom/sides of trenches and the hadopelagic species (compare pelagic) living in the open water.

Ecology

The deepest ocean trenches are considered the least explored and most extreme marine ecosystems. They are characterized by complete lack of sunlight, low temperatures, nutrient scarcity, and extremely high hydrostatic pressures. The major sources of nutrients and carbon are fallout from upper layers, drifts of fine sediment, and landslides. Most organisms are scavengers and

detrivore Detritivores (also known as detrivores, detritophages, detritus feeders, or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...

s. Over 400 species are currently known from hadal ecosystems, many of which possess physiological adaptations to the extreme environmental conditions. There are high levels of endemism, and noteworthy examples of

gigantism Gigantism ( el, γίγας, ''gígas'', "giant", plural γίγαντες, ''gígantes''), also known as giantism, is a condition characterized by excessive growth and height significantly above average. In humans, this condition is caused by ove ...

in amphipods,

mysids Mysida is an order of small, shrimp-like crustaceans in the malacostracan superorder Peracarida. Their common name opossum shrimps stems from the presence of a brood pouch or "marsupium" in females. The fact that the larvae are reared in this ...

, and isopods and

dwarfism Dwarfism is a condition wherein an organism is exceptionally small, and mostly occurs in the animal kingdom. In humans, it is sometimes defined as an adult height of less than , regardless of sex; the average adult height among people with dw ...

in nematodes,

copepods Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have p ...

, and kinorhynchs. Alicella gigantea Japan Trench 2022

Marine life decreases with depth, both in

abundance Abundance may refer to: In science and technology * Abundance (economics), the opposite of scarcities * Abundance (ecology), the relative representation of a species in a community * Abundance (programming language), a Forth-like computer prog ...

and

biomass Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...

, but there is a wide range of metazoan organisms in the hadal zone, mostly

benthos Benthos (), also known as benthon, is the community of organisms that live on, in, or near the bottom of a sea, river, lake, or stream, also known as the benthic zone.fish,

sea cucumber Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuria ...

bristle worms Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made ...

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

, isopods,

sea anemone Sea anemones are a group of predation, predatory marine invertebrates of the order (biology), order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrestrial flowering plant. Sea anemones are classifi ...

s, amphipods,

copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...

s, decapod crustaceans and gastropods. Most of these trench communities probably originated from the abyssal plains. Although they have evolved adaptations to high pressure and low temperatures such as lower metabolism, intra-cellular protein-stabilising osmolytes, and unsaturated fatty acids in cell membrane

phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...

s, there is no consistent relationship between pressure and metabolic rate in these communities. Increased pressure can instead constrain the

ontogenic Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...

or larval stages of organisms. Pressure increases ten-fold as an organism moves from sea level to a depth of , whilst pressure only doubles as an organism moves from . Over a

geological time scale The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronol ...

, trenches can become accessible as previously stenobathic (limited to a narrow depth range) fauna evolve to become eurybathic (adapted to a wider range of depths), such as

grenadiers A grenadier ( , ; derived from the word ''grenade'') was originally a specialist soldier who threw hand grenades in battle. The distinct combat function of the grenadier was established in the mid-17th century, when grenadiers were recruited from ...

and natantian prawns. Trench communities do, nevertheless, display a contrasting degree of intra-trench endemism and inter-trench similarities at a higher

taxonomic Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

level. Only a relatively small number of fish species are known from the hadal zone, including certain grenadiers,

cutthroat eel Cutthroat eels are a family, Synaphobranchidae, of eels, the only members of the suborder Synaphobranchoidei. They are found worldwide in temperate and tropical seas. Cutthroat eels range from in length. They are bottom-dwelling fish, found in ...

s, pearlfish, cusk-eels,

snailfish The Liparidae, commonly known as snailfish or sea snails, are a Family (biology), family of Saltwater fish, marine Scorpaeniformes, scorpaeniform fishes. Widely distributed from the Arctic Ocean, Arctic to Antarctic Oceans, including the ocea ...

and eelpouts. Due to the extreme pressure, the theoretical maximum depth for vertebral fish may be about , below which teleosts would be hyperosmotic, assuming trimethylamine N-oxide requirements follow the observed approximate linear relationship with depth. Some invertebrates do occur deeper, such as certain polynoid worms, myriotrochid sea cucumbers,

turrid Turrid, plural turrids, is a common name for a very large group of predatory sea snails, marine gastropod mollusks which until recently were all classified in the family Turridae. However, recently the family was discovered to be polyphyletic ...

snails and pardaliscid amphipods in excess of . Also giant protists known as

Xenophyophora Xenophyophorea is a clade of foraminiferans. Members of this class are multinucleate unicellular organisms found on the ocean floor throughout the world's oceans, at depths of . They are a kind of foraminiferan that extract minerals from their s ...

( foraminifera) live at these depths.

Conditions

The only known primary producers in the hadal zone are certain bacteria that are able to metabolize hydrogen and methane released by rock and seawater reactions ( serpentinization), or

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...

released from cold seeps. Some of these bacteria are

symbiotic Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...

, for example living inside the mantle of certain thyasirid and vesicomyid bivalves. Otherwise the first link in the hadal food web are heterotroph organisms that feed on marine snow, both fine particles and the occasional carcass. The hadal zone can reach far below deep; the deepest known extends to . At such depths, the pressure in the hadal zone exceeds . Lack of light and extreme pressure makes this part of the ocean difficult to explore.

Exploration

The exploration of the hadal zone requires the use of instruments that are able to withstand pressures of several hundred up to a thousand or more atmospheres. A few haphazard and non-standard tools have been used to collect limited, but valuable, information about the basic biology of a few hadal organisms. Manned and unmanned

submersible A submersible is a small watercraft designed to operate underwater. The term "submersible" is often used to differentiate from other underwater vessels known as submarines, in that a submarine is a fully self-sufficient craft, capable of ind ...

s, however, can be used to study the depths in greater detail. Unmanned robotic submersibles may be remotely operated (connected to the research vessel by a cable) or autonomous (freely moving). Cameras and manipulators on submersibles allow researchers to observe and take samples of sediment and organisms. Failures of submersibles under the immense pressure at hadal zone depths have occurred. ''HROV Nereus'' is thought to have imploded at a depth of 9,990 meters while exploring the Kermadec Trench in 2014.

Notable missions

The first manned exploration to reach Challenger Deep, the deepest known part of the ocean located in the Mariana Trench, was accomplished in 1960 by Jacques Piccard and Don Walsh. They reached a maximum depth of in the bathyscaphe ''Trieste''.

James Cameron James Francis Cameron (born August 16, 1954) is a Canadian filmmaker. A major figure in the post-New Hollywood era, he is considered one of the industry's most innovative filmmakers, regularly pushing the boundaries of cinematic capability w ...

also reached the bottom of Mariana Trench in March 2012 using the ''

Deepsea Challenger ''Deepsea Challenger'' (DCV 1) is a deep-diving submersible designed to reach the bottom of Challenger Deep, the deepest-known point on Earth. On 26 March 2012, Canadian film director James Cameron piloted the craft to accomplish this goal in ...

''. The descent of the ''Deepsea Challenger'' reached a depth of , slightly less than the deepest dive record set by Piccard and Walsh. Cameron holds the record for the deepest solo dive. In June 2012, the Chinese manned submersible ''Jiaolong'' was able to reach deep in the Mariana Trench, making it the deepest diving manned research submersible. This range surpasses that of the previous record holder, the Japanese-made ''Shinkai'', whose maximum depth is . Few unmanned submersibles are capable of descending to maximum hadal depths. The deepest diving unmanned submersibles have included the '' Kaikō'' (lost at sea in 2003), the '' ABISMO'', the '' Nereus'' (lost at sea in 2014), and the ''Haidou-1''.

References

External links

Forscher filmen lebende Fische in Rekordtiefe (In German) from Spiegel 10/09/2008 about an expedition filming fish at a depth of more than 7,000 m
*{{cbignore Hydrology Oceanographical terminology Extreme points of Earth 1950s neologisms