Hadal Zone
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The hadal zone, also known as the hadopelagic zone, is the deepest region of the
ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...
, lying within
oceanic trench Oceanic trenches are prominent, long, narrow topography, topographic depression (geology), depressions of the seabed, ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers ...
es. The hadal zone ranges from around below
sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...
, and exists in long, narrow, topographic V-shaped depressions. The cumulative area occupied by the 46 individual hadal
habitats In ecology, habitat refers to the array of resources, 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 manifestation of its ...
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 The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...
, the deepest of the conventional oceanic divisions.


Terminology and definition

Historically, the hadal zone was not recognized as distinct from the
abyssal zone The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. The word ''abyss'' comes from the Greek word (), meaning "bottomless". At depths of , this zone remains in perpetual darkness. It covers 83% of the total area ...
, although the deepest sections were sometimes called "ultra-abyssal". During the early 1950s, the Danish '' Galathea II'' and Soviet '' Vityaz'' 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 (; , , later ), in the ancient Greek religion and Greek mythology, mythology, is the god of the dead and the king of the Greek underworld, underworld, with which his name became synonymous. Hades was the eldest son of Cronus and Rhea ...
, the ancient Greek god of the
underworld The underworld, also known as the netherworld or hell, is the supernatural world of the dead in various religious traditions and myths, located below the world of the living. Chthonic is the technical adjective for things of the underworld. ...
. About 94% of the hadal zone is found in
subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...
trenches. Depths in excess of are generally in
ocean trench Oceanic trenches are prominent, long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic t ...
es, 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 The United Nations Educational, Scientific and Cultural Organization (UNESCO ) is a List of specialized agencies of the United Nations, specialized agency of the United Nations (UN) with the aim of promoting world peace and International secur ...
. Similar to other depth ranges, the
fauna Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...
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 the Ancient Greek word (), meaning "the depths". ...
) living on or at the seabottom/sides of trenches, and the hadopelagic species (compare
pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...
) living in the open water.


Ecology

The deepest ocean trenches are considered the least explored and most extreme marine
ecosystems An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...
. 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. As of 2020, over 400
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
are known from hadal ecosystems, many of which possess physiological adaptations to the extreme environmental conditions. There are high levels of
endemism Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...
, and noteworthy examples of
gigantism Gigantism (, ''gígas'', "wiktionary:giant, giant", plural γίγαντες, ''gígantes''), also known as giantism, is a condition characterized by excessive growth and height significantly above average height, average. In humans, this conditi ...
in
amphipods Amphipoda () is an order (biology), order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly detritivores or scavengers. There are more than 10,700 amphip ...
, mysids, and
isopods Isopoda is an Order (biology), order of crustaceans. Members of this group are called isopods and include both Aquatic animal, aquatic species and Terrestrial animal, terrestrial species such as woodlice. All have rigid, segmented exoskeletons ...
and
dwarfism Dwarfism is a condition of people and animals marked by unusually small size or short stature. In humans, it is sometimes defined as an adult height of less than , regardless of sex; the average adult height among people with dwarfism is . '' ...
in
nematodes The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (he ...
, copepods, and
kinorhynchs Kinorhyncha (, ' "snout") is a phylum (biology), phylum of small marine invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are commonly called mud dragons. Modern species are or less, but Cambrian ...
. Marine life decreases with depth, both in abundance and
biomass Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...
, but there is a wide range of
metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia (). With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a hol ...
n 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 A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
,
sea cucumber Sea cucumbers are echinoderms from the class (biology), class Holothuroidea ( ). They are benthic marine animals found on the sea floor worldwide, and the number of known holothuroid species worldwide is about 1,786, with the greatest number be ...
, bristle worms,
bivalves Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...
,
isopods Isopoda is an Order (biology), order of crustaceans. Members of this group are called isopods and include both Aquatic animal, aquatic species and Terrestrial animal, terrestrial species such as woodlice. All have rigid, segmented exoskeletons ...
,
sea anemone Sea anemones ( ) are a group of predation, predatory marine invertebrates constituting the order (biology), order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrestrial flowering plant. Sea anemone ...
s,
amphipods Amphipoda () is an order (biology), order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly detritivores or scavengers. There are more than 10,700 amphip ...
,
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 (living in the water column), some are benthos, benthic (living on the sedimen ...
s,
decapod The Decapoda or decapods, from Ancient Greek δεκάς (''dekás''), meaning "ten", and πούς (''poús''), meaning "foot", is a large order of crustaceans within the class Malacostraca, and includes crabs, lobsters, crayfish, shrimp, and p ...
crustaceans and
gastropods Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...
. Most of these trench communities probably originated from the
abyssal plain An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth's surface. They ...
s. 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 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 geochronolo ...
, 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 historically an assault-specialist soldier who threw hand grenades in siege operation battles. The distinct combat function of the grenadier was established in the mid-17th century, when ...
and natantian prawns. Trench communities do, nevertheless, display a contrasting degree of intra-trench
endemism Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...
and inter-trench similarities at a higher taxonomic 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 Benthos, bottom-dwelling fish, ...
s, pearlfish, cusk-eels, snailfish and eelpouts. Due to the extreme pressure, the theoretical maximum depth for vertebral fish may be about , below which
teleost Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...
s would be hyperosmotic, assuming trimethylamine N-oxide requirements follow the observed approximate linear relationship with depth. Some invertebrates do occur deeper, such as bigfin squid, certain polynoid worms, myriotrochid sea cucumbers, turrid snails and pardaliscid amphipods in excess of . In addition, giant
protist A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...
s known as Xenophyophora (
foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...
) live at these depths.


Conditions

The only known
primary producers An autotroph is an organism that can convert abiotic sources of energy into energy stored in organic compounds, which can be used by other organisms. Autotrophs produce complex organic compounds (such as carbohydrates, fats, and proteins) us ...
in the hadal zone are certain bacteria that are able to metabolize
hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
and
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
released by rock and seawater reactions (
serpentinization Serpentinization is a hydration and Metamorphic rock, metamorphic transformation of ferromagnesian minerals, such as olivine and pyroxene, in mafic and ultramafic rock to produce serpentinite. Minerals formed by serpentinization include the Serp ...
), or
hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...
released from
cold seep A cold seep (sometimes called a cold vent) is an area of the ocean floor where seepage of fluids rich in hydrogen sulfide, methane, and other hydrocarbons occurs, often in the form of a brine pool. ''Cold'' does not mean that the temperature ...
s. Some of these bacteria are
symbiotic Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...
, for example living inside the mantle of certain thyasirid and vesicomyid bivalves. Otherwise the first link in the hadal
food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Position in the food web, or trophic level, is used in ecology to broadly classify organisms as autotrophs or he ...
are
heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
organisms that feed on
marine snow In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to ...
, 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 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 an underwater vehicle which needs to be transported and supported by a larger ship, watercraft or dock, platform. This distinguishes submersibles from submarines, which are self-supporting and capable of prolonged independent ope ...
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 In developmental psychology and moral, political, and bioethical philosophy, autonomy is the capacity to make an informed, uncoerced decision. Autonomous organizations or institutions are independent or self-governing. Autonomy can also be defi ...
(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 Challenger Deep is the List of submarine topographical features#List of oceanic trenches, deepest known point of the seabed of Earth, located in the western Pacific Ocean at the southern end of the Mariana Trench, in the ocean territory o ...
, the deepest known part of the ocean located in the
Mariana Trench The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deep sea, deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maxi ...
, 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, who resides in New Zealand. He is a major figure in the post-New Hollywood era and often uses novel technologies with a Classical Hollywood cinema, classical filmmaking styl ...
also reached the bottom of Mariana Trench in March 2012 using the '' Deepsea Challenger''. 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 In Greek mythology, Nereus ( ; ) was the eldest son of Pontus (the Sea) and Gaia ( the Earth), with Pontus himself being a son of Gaia. Nereus and Doris became the parents of 50 daughters (the Nereids) and a son ( Nerites), with whom Nereus ...
'' (lost at sea in 2014), and the '' Haidou-1''.


See also

* * * * * ; includes shallow waters and coral reefs


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