92-425: Hypsibius is a genus of tardigrades in the class Eutardigrada . The genome of Hypsibius dujardini has been sequenced. Hypsibius dujardini has a compact genome and a generation time of about two weeks. It can be cultured indefinitely and cryopreserved. This sequenced strain has been redesignated Hypsibius exemplaris since 2018. The genus includes the following species: This tardigrade -related article
184-481: A compact genome of 100 megabase pairs and a generation time of about two weeks; it can be cultured indefinitely and cryopreserved. The genome of Ramazzottius varieornatus , one of the most stress-tolerant species of tardigrades, was sequenced by a team of researchers from the University of Tokyo in 2015. While previous research had claimed that around one-sixth of the genome had been acquired from other organisms, it
276-650: A dehydrated state up to five years, or longer in exceptional cases. Depending on the environment, they may enter this state via anhydrobiosis , allowing tardigrades, along with some other micro-metazoans (such as worms, rotifers, and crustaceans), protozoans and plants, the ability to survive in habitats that might otherwise be fatal. In addition to offering protection from desiccation and freezing under normal circumstances, anhydrobiosis also permits resistance to unnatural abiotic extremes such as subzero temperatures, cryobiosis , osmobiosis , or anoxybiosis . Their ability to remain desiccated for such long periods of time
368-555: A dune forms, plant succession occurs. The conditions on an embryo dune are harsh, with salt spray from the sea carried on strong winds. The dune is well drained and often dry, and composed of calcium carbonate from seashells. Rotting seaweed , brought in by storm waves adds nutrients to allow pioneer species to colonize the dune. For example, in the United Kingdom these pioneer species are often marram grass , sea wort grass and other sea grasses. These plants are well adapted to
460-513: A few rare specimens from Cretaceous and Neogene amber . The Siberian tardigrade fossils differ from living tardigrades in several ways. They have three pairs of legs rather than four, they have a simplified head morphology, and they have no posterior head appendages, but they share with modern tardigrades their columnar cuticle construction. Scientists think they represent a stem group of living tardigrades. There are multiple lines of evidence that tardigrades are secondarily miniaturized from
552-590: A few tens of metres except at their nose, where vegetation stops or slows the advance of accumulating sand. Simple parabolic dunes have only one set of arms that trail upwind, behind the leading nose. Compound parabolic dunes are coalesced features with several sets of trailing arms. Complex parabolic dunes include subsidiary superposed or coalesced forms, usually of barchanoid or linear shapes. Parabolic dunes, like crescent dunes, occur in areas where very strong winds are mostly unidirectional. Although these dunes are found in areas now characterized by variable wind speeds,
644-407: A larger ancestor, probably a lobopodian and perhaps resembling Aysheaia , which many analyses place close to the divergence of the tardigrade lineage. An alternative hypothesis derives tactopoda from a clade encompassing dinocaridids and Opabinia . A 2023 analysis concluded, on the basis of numerous morphological similarities, that luolishaniids , a group of Cambrian lobopodians, might be
736-407: A low-lying pan within the basin where the water evaporates, depositing the gypsum and forming crystals known as selenite . The crystals left behind by this process are eroded by the wind and deposited as vast white dune fields that resemble snow-covered landscapes. These types of dune are rare, and only form in closed arid basins that retain the highly soluble gypsum that would otherwise be washed into
828-523: A lower egg laying rate. In May 2011, Italian scientists sent tardigrades on board the International Space Station along with extremophiles on STS-134 , the final flight of Space Shuttle Endeavour . Their conclusion was that microgravity and cosmic radiation "did not significantly affect survival of tardigrades in flight, and stated that tardigrades represent a useful animal for space research." In November 2011, they were among
920-412: A significant role in minimizing wave energy as it moves onshore. As a result, coastal dunes, especially those in the foredune area affected by a storm surge , will retreat or erode. To counteract the damage from tropical activity on coastal dunes, short term post-storm efforts can be made by individual agencies through fencing to help with sand accumulation. How much a dune erodes during any storm surge
1012-419: A single duct opening either just above the anus or directly into the rectum, which forms a cloaca . Tardigrades are oviparous , and fertilization is usually external. Mating occurs during the molt with the eggs being laid inside the shed cuticle of the female and then covered with sperm. A few species have internal fertilization, with mating occurring before the female fully sheds her cuticle. In most cases,
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#17328907849251104-462: A star dune superimposed on its crest is the most common complex dune. Simple dunes represent a wind regime that has not changed in intensity or direction since the formation of the dune, while compound and complex dunes suggest that the intensity and direction of the wind has changed. The sand mass of dunes can move either windward or leeward, depending on if the wind is making contact with the dune from below or above its apogee. If wind hits from above,
1196-413: A triradiate, muscular, sucking pharynx . The stylets are lost when the animal molts , and a new pair is secreted from a pair of glands that lie on either side of the mouth. The pharynx connects to a short esophagus , and then to an intestine that occupies much of the length of the body, which is the main site of digestion. The intestine opens, via a short rectum, to an anus located at the terminal end of
1288-804: A unidirectional wind. In the sheltered troughs between highly developed seif dunes, barchans may be formed, because the wind is constrained to be unidirectional by the dunes. Seif dunes are common in the Sahara. They range up to 300 m (980 ft) in height and 300 km (190 mi) in length. In the southern third of the Arabian Peninsula, a vast erg , called the Rub' al Khali or Empty Quarter, contains seif dunes that stretch for almost 200 km (120 mi) and reach heights of over 300 m (980 ft). Linear loess hills known as pahas are superficially similar. These hills appear to have been formed during
1380-485: Is a stub . You can help Misplaced Pages by expanding it . Tardigrade Tardigrades ( / ˈ t ɑːr d ɪ ɡ r eɪ d z / ), known colloquially as water bears or moss piglets , are a phylum of eight-legged segmented micro-animals . They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbär ' little water bear ' . In 1776,
1472-477: Is a type of sandstone that is formed when a marine or aeolian sand dune becomes compacted and hardened. Once in this form, water passing through the rock can carry and deposit minerals, which can alter the colour of the rock. Cross-bedded layers of stacks of lithified dunes can produce the cross-hatching patterns, such as those seen in Zion National Park in the western United States. A slang term, used in
1564-442: Is a very large aeolian landform, with a length of several kilometers and a height of tens to hundreds of meters, and which may have superimposed dunes. Dunes are made of sand-sized particles, and may consist of quartz, calcium carbonate, snow, gypsum, or other materials. The upwind/upstream/upcurrent side of the dune is called the stoss side; the downflow side is called the lee side. Sand is pushed (creep) or bounces ( saltation ) up
1656-449: Is an early step in mating and was first observed in tardigrades in 1895. Research shows that up to nine males aggregate around a female to mate. Most tardigrades are phytophagous (plant eaters) or bacteriophagous (bacteria eaters), but some are carnivorous to the extent that they eat smaller species of tardigrades (for example, Milnesium tardigradum ). In addition, a few extant species, such as Tetrakentron synaptae , alongside
1748-526: Is attached to a large ganglion below the esophagus, from which a double ventral nerve cord runs the length of the body. The cord possesses one ganglion per segment, each of which produces lateral nerve fibres that run into the limbs. Many species possess a pair of rhabdomeric pigment-cup eyes, and numerous sensory bristles are on the head and body. Tardigrades all possess a buccopharyngeal apparatus (swallowing device made of muscles and spines that activates an inner jaw and begins digestion and movement along
1840-410: Is blown along the beach. Dunes form where the beach is wide enough to allow for the accumulation of wind-blown sand, and where prevailing onshore winds tend to blow sand inland. The three key ingredients for coastal dune formation are a large sand supply, winds to move said sand supply, and a place for the sand supply to accumulate. Obstacles—for example, vegetation, pebbles and so on—tend to slow down
1932-445: Is by saltation , where sand particles skip along the ground like a bouncing ball . When these skipping particles land, they may knock into other particles and cause them to move as well, in a process known as creep . With slightly stronger winds, particles collide in mid-air, causing sheet flows . In a major dust storm , dunes may move tens of metres through such sheet flows. Also as in the case of snow, sand avalanches , falling down
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#17328907849252024-428: Is due to a number of pressures related to their proximity to the ocean and confinement to growth on sandy substrates. These include: Plants have evolved many adaptations to cope with these pressures: In deserts where large amounts of limestone mountains surround a closed basin , such as at White Sands National Park in south-central New Mexico , occasional storm runoff transports dissolved limestone and gypsum into
2116-400: Is now known that less than 1.2% of its genes were the result of horizontal gene transfer . They also found evidence of a loss of gene pathways that are known to promote damage due to stress. This study also found a high expression of novel tardigrade-unique proteins, including Damage suppressor (Dsup) , which was shown to protect against DNA damage from X-ray radiation. The same team applied
2208-404: Is related to its location on the coastal shoreline and the profile of the beach during a particular season. In those areas with harsher winter weather, during the summer a beach tends to take on more of a convex appearance due to gentler waves, while the same beach in the winter may take on more of a concave appearance. As a result, coastal dunes can get eroded much more quickly in the winter than in
2300-582: The Cambrian more than 500 million years ago. Tardigrades are usually about 0.5 mm (0.020 in) long when fully grown. They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or suction disks. Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a low-power microscope , making them accessible to students and amateur scientists. Johann August Ephraim Goeze originally named
2392-530: The FOTON-M3 mission carrying the BIOPAN astrobiology payload. For 10 days, groups of tardigrades, some of them previously dehydrated, some of them not, were exposed to the hard vacuum of outer space, or vacuum and solar UV radiation. Back on Earth, more than 68% of the subjects protected from solar UV radiation were reanimated within 30 minutes following rehydration; although subsequent mortality
2484-1026: The Himalayas (6,000 m; 20,000 ft, above sea level), and in the deep sea (−4,000 m; −13,000 ft); from the polar regions to the equator , under layers of solid ice , and in ocean sediments. Many species can be found in milder environments such as lakes, ponds, and meadows , while others can be found in stone walls and roofs. Tardigrades are most common in moist environments but can stay active wherever they can retain at least some moisture. Tardigrades are thought to be able to survive even complete global mass extinction events caused by astrophysical events , such as gamma-ray bursts , or large meteorite impacts . Some of them can withstand extremely cold temperatures down to 0.01 K (−460 °F; −273 °C) (close to absolute zero ), while others can withstand extremely hot temperatures up to 420 K (300 °F; 150 °C) for several minutes, pressures about six times greater than those found in
2576-512: The Western Desert of Egypt . The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than three kilometres, are in China's Taklamakan Desert . Abundant barchan dunes may merge into barchanoid ridges, which then grade into linear (or slightly sinuous) transverse dunes, so called because they lie transverse, or across, the wind direction, with the wind blowing perpendicular to
2668-407: The phospholipids bilayers , avoiding structural damage upon rehydration. Also, TDPs, being highly hydrophilic, are thought to be involved in a vitrification mechanism, where a glass-like matrix forms within cells to protect the cellular contents upon desiccation. Their DNA is further protected from radiation by a protein called " dsup " (short for damage suppressor ). In this cryptobiotic state,
2760-495: The phylum Tardigrada, a part of the superphylum Ecdysozoa consisting of animals that grow by ecdysis (shedding an exoskeleton ) such as arthropods and nematodes . The earliest known true members of the group are known from Cretaceous (145 to 66 million years ago) amber , found in North America, but are essentially modern forms. Their origin is therefore likely much earlier, as they diverged from their closest relatives in
2852-454: The southwest US , for consolidated and hardened sand dunes is "slickrock", a name that was introduced by pioneers of the Old West because their steel-rimmed wagon wheels could not gain traction on the rock. Sand dunes can have a negative impact on humans when they encroach on human habitats. Sand dunes move via a few different means, all of them helped along by wind. One way that dunes can move
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2944-659: The stoss (upflow) side, where the sand is pushed up the dune, and have a shorter slip face in the lee side. The valley or trough between dunes is called a dune slack . Dunes are most common in desert environments, where the lack of moisture hinders the growth of vegetation that would otherwise interfere with the development of dunes. However, sand deposits are not restricted to deserts, and dunes are also found along sea shores, along streams in semiarid climates, in areas of glacial outwash , and in other areas where poorly cemented sandstone bedrock disintegrates to produce an ample supply of loose sand. Subaqueous dunes can form from
3036-746: The Dsup protein to human cultured cells and found that it suppressed X-ray damage to the human cells by around 40%. While the exact mechanism of DNA protection is largely unknown, the results from an August 2020 study suggest that strong electrostatic attractions along with high protein flexibility help form a molecular aggregate, which allows Dsup to shield DNA. The Dsup proteins of tardigrades Ramazzottius varieornatus and Hypsibius exemplaris promote survival by binding to nucleosomes and protecting chromosomal DNA from hydroxyl radicals . The Dsup protein of R. varieornatus also confers resistance to ultraviolet-C by upregulating DNA repair genes that protect
3128-837: The Italian biologist Lazzaro Spallanzani named them Tardigrada ( / t ɑːr ˈ d ɪ ɡ r ə d ə / ), which means 'slow steppers'. They have been found in diverse regions of Earth's biosphere – mountaintops, the deep sea , tropical rainforests , and the Antarctic . Tardigrades are among the most resilient animals known, with individual species able to survive extreme conditions – such as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, radiation , dehydration , and starvation – that would quickly kill most other known forms of life . Tardigrades have survived exposure to outer space . There are about 1,300 known species in
3220-551: The action of water flow ( fluvial processes) on sand or gravel beds of rivers , estuaries , and the sea-bed. Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach . In most cases, the dunes are important in protecting the land against potential ravages by storm waves from the sea . Artificial dunes are sometimes constructed to protect coastal areas. The dynamic action of wind and water can sometimes cause dunes to drift, which can have serious consequences. For example,
3312-533: The body. Some species only defecate when they molt, leaving the feces behind with the shed cuticle. The tardigrade nervous system consists primarily of the brain and four segmental ganglia associated with the four body segments. The brain comprises about 1% of the total body volume. The brain develops in a bilaterally symmetric pattern. Tardigrades have a dorsal brain atop a paired ventral nervous system. The brain includes multiple lobes, mostly consisting of three bilaterally paired clusters of neurons . The brain
3404-418: The closest known relatives of Tardigrada. The oldest remains of modern tardigrades are those of Milnesium swolenskyi , belonging to the living genus Milnesium known from a Late Cretaceous ( Turonian ) aged specimen of New Jersey amber , around 90 million years old. Another fossil species, Beorn leggi , is known from a Late Campanian (~72 million years old) specimen of Canadian amber, belonging to
3496-409: The coastal environment of the sand dune vital to their species' survival. Over the course of time coastal dunes may be impacted by tropical cyclones or other intense storm activity, dependent on their location. Recent work has suggested that coastal dunes tend to evolve toward a high or low morphology depending on the growth rate of dunes relative to storm frequency. During a storm event, dunes play
3588-604: The concave sides of the dunes. These dunes form under winds that blow consistently from one direction (unimodal winds). They form separate crescents when the sand supply is comparatively small. When the sand supply is greater, they may merge into barchanoid ridges, and then transverse dunes (see below). Some types of crescentic dunes move more quickly over desert surfaces than any other type of dune. A group of dunes moved more than 100 metres per year between 1954 and 1959 in China 's Ningxia Province , and similar speeds have been recorded in
3680-405: The corridors between individual dunes. Because all dune arms are oriented in the same direction, and, the inter-dune corridors are generally swept clear of loose sand, the corridors can usually be traversed in between the trailing arms of the dune. However to cross straight over the dune by going over the trailing arms, can be very difficult. Also, traversing the nose is very difficult as well because
3772-454: The deepest ocean trenches, ionizing radiation at doses hundreds of times higher than the lethal dose for a human, and the vacuum of outer space. Tardigrades that live in harsh conditions undergo an annual process of cyclomorphosis , allowing for survival in subzero temperatures. They are not considered extremophilic because they are not adapted to exploit these conditions, only to endure them. This means that their chances of dying increase
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3864-491: The direction (s) of prevailing winds, are known as lunettes, source-bordering dunes, bourrelets and clay dunes. They may be composed of clay, silt, sand, or gypsum, eroded from the basin floor or shore, transported up the concave side of the dune, and deposited on the convex side. Examples in Australia are up to 6.5 km long, 1 km wide, and up to 50 metres high. They also occur in southern and West Africa , and in parts of
3956-657: The distance between the water line and where vegetation can grow. Coastal dunes can be classified by where they develop, or begin to take shape. Dunes are commonly grouped into either the Primary Dune Group or the Secondary Dune Group. Primary dunes gain most of their sand from the beach itself, while secondary dunes gain their sand from the primary dune. Along the Florida Panhandle, most dunes are considered to be foredunes or hummocks. Different locations around
4048-604: The dune and underlying soils . The stability of the dunes was once attributed to the vegetative cover but recent research has pointed to water as the main source of parabolic dune stability. The vegetation that covers them—grasses, shrubs, and trees—help anchor the trailing arms. In inland deserts, parabolic dunes commonly originate and extend downwind from blowouts in sand sheets only partly anchored by vegetation. They can also originate from beach sands and extend inland into vegetated areas in coastal zones and on shores of large lakes. Most parabolic dunes do not reach heights higher than
4140-513: The dunes and provide horticultural benefits, but instead spread taking land away from native species. Ammophila arenaria , known as European beachgrass, has a similar story, though it has no horticulture benefits. It has great ground coverage and, as intended, stabilized the dunes but as an unintended side effect prevented native species from thriving in those dunes. One such example is the dune field at Point Reyes, California . There are now efforts to get rid of both of these invasive species. As
4232-593: The dunes, washing humus into the slacks, and the slacks may be much more developed than the exposed tops of the dunes. It is usually in the slacks that more rare species are developed and there is a tendency for the dune slacks' soil to be waterlogged where only marsh plants can survive. In Europe these plants include: creeping willow, cotton grass, yellow iris , reeds, and rushes. As for vertebrates in European dunes, natterjack toads sometimes breed here. Dune ecosystems are extremely difficult places for plants to survive. This
4324-592: The dunes. Typically these are heather , heaths and gorses . These too are adapted to the low soil water content and have small, prickly leaves which reduce transpiration. Heather adds humus to the soil and is usually replaced by coniferous trees, which can tolerate low soil pH , caused by the accumulation and decomposition of organic matter with nitrate leaching. Coniferous forests and heathland are common climax communities for sand dune systems. Young dunes are called yellow dunes and dunes which have high humus content are called grey dunes . Leaching occurs on
4416-401: The effective winds associated with the growth and migration of both the parabolic and crescent dunes probably are the most consistent in wind direction. The grain size for these well-sorted, very fine to medium sands is about 0.06 to 0.5 mm. Parabolic dunes have loose sand and steep slopes only on their outer flanks. The inner slopes are mostly well packed and anchored by vegetation, as are
4508-430: The eggs are left inside the shed cuticle to develop, but some species attach them to a nearby substrate. The eggs hatch after no more than 14 days, with the young already possessing their full complement of adult cells . Growth to adult size occurs by enlargement of the individual cells ( hypertrophy ), rather than by cell division. Tardigrades may molt up to 12 times. Tardigrades tend to court before mating. Courtship
4600-505: The entirety of the body. Some tardigrades have three tubular glands associated with the rectum; these may be excretory organs similar to the Malpighian tubules of arthropods , although the details remain unclear. Also, nephridia are absent. The tubular mouth is armed with stylets , which are used to pierce cells, algae , or small invertebrates on which the tardigrades feed, releasing the body fluids or cell contents. The mouth opens into
4692-427: The exception of 62 known species that live exclusively in freshwater, all non-marine tardigrades are found in terrestrial environments. Because the majority of the marine species belongs to Heterotardigrada, the most ancestral class, it confirms the phylum's marine origin. Tardigrades have barrel-shaped bodies with four pairs of stubby legs. Most range from 0.3 to 0.5 mm (0.012 to 0.020 in) in length, although
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#17328907849254784-541: The family Hypsibiidae . The related hypsibioidean Aerobius dactylus was found in the same amber piece. The youngest known fossil tadigrade genus, Paradoryphoribius , was discovered in amber dated to about 16 Ma . The enigmatic panarthropod Sialomorpha found in 30-million year old Dominican amber , while not classifiable as a tardigrade, shows some apparent affinities. Tardigrade genomes vary in size, from about 75 to 800 mega base pairs of DNA. Hypsibius exemplaris (formerly Hypsibius dujardini ) has
4876-548: The former is a common result of morphological studies ; evidence for the latter is found in genomic analysis. Water bears ( Tardigrada ) [REDACTED] Velvet worms ( Onychophora ) [REDACTED] Arthropods ( Arthropoda ) [REDACTED] The minute sizes of tardigrades and their membranous integuments make their fossilization both difficult to detect and highly unusual. The only known fossil specimens are those from mid- Cambrian deposits in Siberia ( Orsten fauna ) and
4968-504: The genomic DNA from the damages introduced by UV irradiation. Many organisms that live in aquatic environments feed on species such as nematodes, tardigrades, bacteria, algae, mites, and collembolans . Tardigrades work as pioneer species by inhabiting new developing environments. This movement attracts other invertebrates to populate that space, while also attracting predators. They contribute to ecosystems by being predators of plant parasitic nematodes, which helps decrease biomass within
5060-519: The geological record . All these dune shapes may occur in three forms: simple (isolated dunes of basic type), compound (larger dunes on which smaller dunes of same type form), and complex (combinations of different types). Simple dunes are basic forms with the minimum number of slipfaces that define the geometric type. Compound dunes are large dunes on which smaller dunes of similar type and slipface orientation are superimposed. Complex dunes are combinations of two or more dune types. A crescentic dune with
5152-595: The globe have dune formations unique to their given coastal profile. Coastal sand dunes can provide privacy and/or habitats to support local flora and fauna. Animals such as sand snakes, lizards, and rodents can live in coastal sand dunes, along with insects of all types. Often the vegetation of sand dunes is discussed without acknowledging the importance that coastal dunes have for animals. Further, some animals, such as foxes and feral pigs can use coastal dunes as hunting grounds to find food. Birds are also known to utilize coastal dunes as nesting grounds. All these species find
5244-408: The harsh conditions of the foredune, typically having deep roots which reach the water table , root nodules that produce nitrogen compounds, and protected stoma , reducing transpiration . Also, the deep roots bind the sand together, and the dune grows into a foredune as more sand is blown over the grasses. The grasses add nitrogen to the soil, meaning other, less hardy plants can then colonize
5336-399: The largest species may reach 1.2 mm (0.047 in). The body consists of a head, three body segments each with a pair of legs, and a caudal segment with a fourth pair of legs. The legs are without joints , while the feet have four to eight claws each. The cuticle contains chitin and protein and is moulted periodically. The first three pairs of legs are directed downward along
5428-478: The last ice age under permafrost conditions dominated by sparse tundra vegetation. Star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally. They dominate the Grand Erg Oriental of the Sahara. In other deserts, they occur around
5520-604: The longer they are exposed to the extreme environments, whereas true extremophiles thrive in a physically or geochemically extreme environment that would harm most other organisms. Tardigrades are one of the few groups of species that are capable of suspending their metabolism (see Cryptobiosis ) . While in this state, their metabolism lowers to less than 0.01% of normal and their water content can drop to 1% of normal, and they can go without food or water for more than 30 years, only to later rehydrate, forage, and reproduce. Many species of tardigrade can survive in
5612-515: The margins of the sand seas , particularly near topographic barriers. In the southeast Badain Jaran Desert of China, the star dunes are up to 500 metres tall and may be the tallest dunes on Earth. Oval or circular mounds that generally lack a slipface. Dome dunes are rare and occur at the far upwind margins of sand seas. Fixed crescentic dunes that form on the leeward margins of playas and river valleys in arid and semiarid regions in response to
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#17328907849255704-654: The nose is usually made up of loose sand without much if any vegetation. A type of extensive parabolic dune that lacks discernible slipfaces and has mostly coarse grained sand is known as a zibar . The term zibar comes from the Arabic word to describe "rolling transverse ridges ... with a hard surface". The dunes are small, have low relief, and can be found in many places across the planet from Wyoming (United States) to Saudi Arabia to Australia. Spacing between zibars ranges from 50 to 400 metres and they do not become more than 10 metres high. The dunes form at about ninety degrees to
5796-637: The organisms to be sent by the U.S.-based Planetary Society on the Russian Fobos-Grunt mission's Living Interplanetary Flight Experiment to Phobos ; however, the launch failed. In August 2019, scientists reported that a capsule containing tardigrades in a cryptobiotic state may have survived for a while on the Moon after the April 2019 crash landing of Beresheet , a failed Israeli lunar lander , but in May 2021 it
5888-440: The prevailing wind which blows away the small, fine-grained sand leaving behind the coarser grained sand to form the crest. Occurring wherever winds periodically reverse direction, reversing dunes are varieties of any of the above shapes. These dunes typically have major and minor slipfaces oriented in opposite directions. The minor slipfaces are usually temporary, as they appear after a reverse wind and are generally destroyed when
5980-478: The resultant direction of sand movement (hence the name "longitudinal"). Some linear dunes merge to form Y-shaped compound dunes. Formation is debated. Ralph Bagnold , in The Physics of Blown Sand and Desert Dunes , suggested that some seif dunes form when a barchan dune moves into a bidirectional wind regime, and one arm or wing of the crescent elongates. Others suggest that seif dunes are formed by vortices in
6072-475: The ridge crest. Seif dunes are linear (or slightly sinuous) dunes with two slip faces. The two slip faces make them sharp-crested. They are called seif dunes after the Arabic word for "sword". They may be more than 160 kilometres (100 miles) long, and thus easily visible in satellite images (see illustrations). Seif dunes are associated with bidirectional winds. The long axes and ridges of these dunes extend along
6164-415: The sand in the dune migrates forward. In plan view, these are U-shaped or V-shaped mounds of well-sorted, very fine to medium sand with elongated arms that extend upwind behind the central part of the dune. There are slipfaces that often occur on the outer side of the nose and on the outer slopes of the arms. These dunes often occur in semiarid areas where the precipitation is retained in the lower parts of
6256-425: The sand particles move leeward; the leeward flux of sand is greater than the windward flux. Conversely, if sand hits from below, sand particles move windward. Further, if the wind is carrying sand particles when it hits the dune, the dune's sand particles will saltate more than if the wind had hit the dune without carrying sand particles. Coastal dunes form when wet sand is deposited along the coast and dries out and
6348-438: The sea. A nabkha , or coppice dune, is a small dune anchored by vegetation. They usually indicate desertification or soil erosion, and serve as nesting and burrow sites for animals. Sub-aqueous ( underwater ) dunes form on a bed of sand or gravel under the actions of water flow. They are ubiquitous in natural channels such as rivers and estuaries, and also form in engineered canals and pipelines. Dunes move downstream as
6440-440: The segments that are homologous to the head region in arthropods. All adult tardigrades of the same species have the same number of cells (see eutely ). Some species have as many as 40,000 cells in each adult, while others have far fewer. The body cavity consists of a haemocoel , but the only place where a true coelom can be found is around the gonad . No respiratory organs were found, with gas exchange able to occur across
6532-417: The sides and are the primary means of locomotion, while the fourth pair is directed backward on the last segment of the trunk and is used primarily for grasping the substrate. Tardigrades lack several Hox genes and a large intermediate region of the body axis. In insects, this corresponds to the entire thorax and the abdomen. Practically the whole body, except for the last pair of legs, is made up of just
6624-657: The smallest below 0.1 mm (0.0039 in). Newly hatched tardigrades may be smaller than 0.05 mm (0.0020 in). For comparison, grass pollen is typically 0.025–0.04 mm (0.00098–0.00157 in). Tardigrades are often found on lichens and mosses — for example, by soaking a piece of moss in water. Other environments in which they are found include dunes and coasts generally, soil , leaf litter , and marine or freshwater sediments, where they may occur quite frequently, up to 25,000 animals per litre (95,000 animals per gallon). One tardigrade, Echiniscoides wyethi , may be found on barnacles . With
6716-484: The soil. Dune A dune is a landform composed of wind- or water-driven sand . It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex . A large dune complex is called a dune field , while broad, flat regions covered with wind-swept sand or dunes, with little or no vegetation, are called ergs or sand seas . Dunes occur in different shapes and sizes, but most kinds of dunes are longer on
6808-402: The steeper slip face facing downstream. Ripple marks preserved in sedimentary strata in the geological record can be used to determine the direction of current flow, and thus an indication of the source of the sediments. Dunes on the bed of a channel significantly increase flow resistance, their presence and growth playing a major part in river flooding . A lithified (consolidated) sand dune
6900-604: The stoss side, and slides down the lee side. A side of a dune that the sand has slid down is called a slip face (or slipface). The Bagnold formula gives the speed at which particles can be transported. Five basic dune types are recognized: crescentic, linear, star, dome, and parabolic. Dune areas may occur in three forms: simple (isolated dunes of basic type), compound (larger dunes on which smaller dunes of same type form), and complex (combinations of different types). Barchan dunes are crescent-shaped mounds which are generally wider than they are long. The lee-side slipfaces are on
6992-663: The summer. The converse is true in areas with harsher summer weather. There are many threats to these coastal communities. Some coastal dunes, for example ones in San Francisco, have been completely altered by urbanization; reshaping the dune for human use. This puts native species at risk. Another danger, in California and places in the UK specifically, is the introduction of invasive species. Plant species, such as Carpobrotus edulis , were introduced from South Africa in an attempt to stabilize
7084-413: The tardigrade Kleiner Wasserbär , meaning 'little water-bear' in German (today, they are often referred to in German as Bärtierchen 'little bear-animal'). The name water-bear comes from the way they walk, reminiscent of a bear 's gait . The name Tardigradum means 'slow walker' and was given by Lazzaro Spallanzani in 1777. The largest adults may reach a body length of 2.0 mm (0.079 in),
7176-569: The tardigrade is known as a " tun ". Mitochondria and muscle contraction due to mitochondria are essential for tardigrade to enter the "tun" state of anhydrobiosis. Tardigrades can survive in extreme environments that would kill almost any other animal. Extremes at which tardigrades can survive include those of: Research published in 2020 shows that tardigrades are sensitive to high temperatures. Researchers showed it takes 48 hours at 37.1 °C (98.8 °F) to kill half of active tardigrades that have not been acclimated to heat. Acclimation boosted
7268-409: The temperature needed to kill half of active tardigrades to 37.6 °C (99.7 °F). Tardigrades in the "tun" state fared a bit better, tolerating higher temperatures. It took heating to 82.7 °C (180.9 °F) to kill half of "tun"-state tardigrades within one hour. Longer exposure time decreased the temperature needed for lethality, though. For 24 hours of exposure, 63.1 °C (145.6 °F)
7360-414: The throat and intestine ) which, along with the claws, is used to differentiate species. Although some species are parthenogenic , both males and females are usually present, although females are frequently larger and more common. Both sexes have a single gonad located above the intestine. Two ducts run from the testes in males, opening through a single pore in front of the anus. In contrast, females have
7452-509: The town of Eucla, Western Australia , had to be relocated in the 1890s because of dune drift. The modern word "dune" came into English from French around 1790, which in turn came from Middle Dutch dūne . A universally precise distinction does not exist between ripples, dunes, and draas , which are all deposits of the same type of materials. Dunes are generally defined as greater than 7 cm tall and may have ripples, while ripples are deposits that are less than 3 cm tall. A draa
7544-560: The undescribed Cambrian “Orsten” tardigrade, are parasitic. Tardigrades share morphological characteristics with many species that differ largely by class. Biologists have a difficult time finding verification among tardigrade species because of this relationship. These animals are most closely related to the early evolution of arthropods . Tardigrade fossils go as far back as the Cretaceous period in North America. Tardigrades are considered cosmopolitan and can be located in regions all over
7636-471: The upstream slope is eroded and the sediment deposited on the downstream or lee slope in typical bedform construction. In the case of sub-aqueous barchan dunes, sediment is lost by their extremities, known as horns. These dunes most often form as a continuous 'train' of dunes, showing remarkable similarity in wavelength and height. The shape of a dune gives information about its formation environment. For instance, rivers produce asymmetrical ripples, with
7728-534: The western United States, especially Texas. U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. These dunes are formed from blowout dunes where the erosion of vegetated sand leads to a U-shaped depression. The elongated arms are held in place by vegetation; the largest arm known on Earth reaches 12 km. Sometimes these dunes are called U-shaped, blowout , or hairpin dunes, and they are well known in coastal deserts. Unlike crescent shaped dunes, their crests point upwind. The bulk of
7820-402: The wind and lead to the deposition of sand grains. These small "incipient dunes or "shadow dunes" tend to grow in the vertical direction if the obstacle slowing the wind can also grow vertically (i.e., vegetation). Coastal dunes expand laterally as a result of lateral growth of coastal plants via seed or rhizome . Models of coastal dunes suggest that their final equilibrium height is related to
7912-483: The wind next blows in the dominant direction. Draas are very large-scale dune bedforms; they may be tens or a few hundreds of metres in height, kilometres wide, and hundreds of kilometres in length. After a draa has reached a certain size, it generally develops superimposed dune forms. They are thought to be more ancient and slower-moving than smaller dunes, and to form by vertical growth of existing dunes. Draas are widespread in sand seas and are well-represented in
8004-623: The world. The eggs and cysts of tardigrades are so durable that they can be carried great distances on the feet of other animals. Tardigrades have survived all five recognized mass extinctions due to their plethora of survival characteristics, including the ability to survive conditions that would be fatal to almost all other animals (see the next section). The lifespan of tardigrades ranges from three to four months for some species, up to two years for other species, not counting their time in dormant states. Scientists have reported tardigrades in hot springs , in glacial cryoconite holes, on top of
8096-399: Was done on how these animals survived such extreme conditions. It was found that intrinsically disordered proteins (IDPs) were highly expressed in response to desiccation in tardigrades. Additionally, three new IDPs were found to be specific to tardigrades and coined tardigrade specific proteins (TDPs). These TDPs may maintain the structure of membranes by associating with the polar heads of
8188-487: Was enough to kill half of the "tun"-state tardigrades. In 2021 scientists reported they had cooled down a tardigrade of the species Ramazzottius varieornatus to 10 millikelvin above absolute zero, and put it under extremely low pressure of 0.000006 millibars. After 420 hours, the animal was brought back to life again. Tardigrades are the first known animal to survive after exposure to outer space. In September 2007, dehydrated tardigrades were taken into low Earth orbit on
8280-448: Was high, many of these produced viable embryos. In contrast, hydrated samples exposed to the combined effect of vacuum and full solar UV radiation had significantly reduced survival, with only three subjects of Milnesium tardigradum surviving. Also, it was found that the space vacuum did not have a significant effect on egg-laying in either R. coronifer or M. tardigradum . However, M. tardigradum exposed to UV radiation had
8372-566: Was reported that they were unlikely to have survived the impact. In recent years, there has also been increased speculation regarding tardigrades' ability to survive on Mars without any life support systems, but it would still "need stuff to eat" to survive. Scientists have conducted morphological and molecular studies to understand how tardigrades relate to other lineages of ecdysozoan animals. Two plausible placements have been proposed: tardigrades are either most closely related to Arthropoda and Onychophora , or to nematodes . Evidence for
8464-441: Was thought to be dependent on high levels of the nonreducing disaccharide trehalose , which is commonly seen in other organisms that survive desiccation, and tardigrades have trehalase genes . However, it has been seen that in both tardigrades and bdelloid rotifers , there is only a partial capability to synthesize trehalose in quantities that may contribute to desiccation tolerance. In response to this finding, more research
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