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79-705: 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, 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,

158-504: A body composed of self-similar units, but usually it is the parts of an organism that are referred to as being segmented. Segmentation in animals typically falls into three types, characteristic of different arthropods , vertebrates , and annelids . Arthropods such as the fruit fly form segments from a field of equivalent cells based on transcription factor gradients. Vertebrates like the zebrafish use oscillating gene expression to define segments known as somites . Annelids such as

237-556: A certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were. The most important objective measure in

316-407: A character unique to a sub-set of the crown group. Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan. A classification using this definition may be strongly affected by the chance survival of rare groups, which can make

395-484: 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

474-647: 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

553-509: 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

632-402: A group ("a self-contained unity"): "perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate [a] Stamm [i.e., stock] ( Phylon )." In plant taxonomy , August W. Eichler (1883) classified plants into five groups named divisions,

711-602: A group containing Viridiplantae and the algal Rhodophyta and Glaucophyta divisions. The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Monilophyta, while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for all gymnosperms (i.e. including cycads, ginkgos and gnetophytes), or for conifers alone as below. Since

790-403: 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

869-528: 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

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948-400: A phylum based on body plan has been proposed by paleontologists Graham Budd and Sören Jensen (as Haeckel had done a century earlier). The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by

1027-471: A phylum much more diverse than it would be otherwise. Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species, some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. The kingdom Plantae

1106-706: A phylum, other phylum-level ranks appear, such as the case of Bacillariophyta (diatoms) within Ochrophyta . These differences became irrelevant after the adoption of a cladistic approach by the ISP, where taxonomic ranks are excluded from the classifications after being considered superfluous and unstable. Many authors prefer this usage, which lead to the Chromista-Protozoa scheme becoming obsolete. Currently there are 40 bacterial phyla (not including " Cyanobacteria ") that have been validly published according to

1185-401: A set of characters shared by all its living representatives. This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is based on an arbitrary point of time: the present. However, as it is character based, it is easy to apply to the fossil record. A greater problem is that it relies on

1264-418: 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,

1343-401: A subjective decision about which groups of organisms should be considered as phyla. The approach is useful because it makes it easy to classify extinct organisms as " stem groups " to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display

1422-424: A term that remains in use today for groups of plants, algae and fungi. The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier . Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan . At its most basic, a phylum can be defined in two ways: as a group of organisms with

1501-413: 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

1580-474: Is a paraphyletic taxon, which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such as Protozoa and Chromista in the Cavalier-Smith system . Protist taxonomy has long been unstable, with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by

1659-446: 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

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1738-525: 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

1817-464: Is characterized as the formation of a pair of somites on either side of the midline. This is often referred to as somitogenesis . In vertebrates, segmentation is most often explained in terms of the clock and wavefront model . The "clock" refers to the periodic oscillation in abundance of specific gene products, such as members of the Hairy and Enhancer of Split (Hes) gene family. Expression starts at

1896-405: Is defined in various ways by different biologists (see Current definitions of Plantae ). All definitions include the living embryophytes (land plants), to which may be added the two green algae divisions, Chlorophyta and Charophyta , to form the clade Viridiplantae . The table below follows the influential (though contentious) Cavalier-Smith system in equating "Plantae" with Archaeplastida ,

1975-481: Is generally included in kingdom Fungi, though its exact relations remain uncertain, and it is considered a protozoan by the International Society of Protistologists (see Protista , below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor), which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish

2054-399: 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

2133-681: The Bacteriological Code Currently there are 2 phyla that have been validly published according to the Bacteriological Code Other phyla that have been proposed, but not validly named, include: Segment (biology) Segmentation is a difficult process to satisfactorily define. Many taxa (for example the molluscs) have some form of serial repetition in their units but are not conventionally thought of as segmented. Segmented animals are those considered to have organs that were repeated, or to have

2212-583: 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

2291-565: The Catalogue of Life , and correspond to the Protozoa-Chromista scheme, with updates from the latest (2022) publication by Cavalier-Smith . Other phyla are used commonly by other authors, and are adapted from the system used by the International Society of Protistologists (ISP). Some of the descriptions are based on the 2019 revision of eukaryotes by the ISP. The number of protist phyla varies greatly from one classification to

2370-536: The Drosophila embryo, the anterior - posterior axis is defined by maternally supplied transcripts giving rise to gradients of these proteins. This gradient then defines the expression pattern for gap genes , which set up the boundaries between the different segments. The gradients produced from gap gene expression then define the expression pattern for the pair-rule genes . The pair-rule genes are mostly transcription factors , expressed in regular stripes down

2449-535: 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

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2528-1025: 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

2607-533: 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

2686-427: The leech use smaller blast cells budded off from large teloblast cells to define segments. Although Drosophila segmentation is not representative of the arthropod phylum in general, it is the most highly studied. Early screens to identify genes involved in cuticle development led to the discovery of a class of genes that was necessary for proper segmentation of the Drosophila embryo. To properly segment

2765-475: The onychophora . While not as well studied as in Drosophila and zebrafish , segmentation in the leech has been described as “budding” segmentation. Early divisions within the leech embryo result in teloblast cells, which are stem cells that divide asymmetrically to create bandlets of blast cells. Furthermore, there are five different teloblast lineages (N, M, O, P, and Q), with one set for each side of

2844-403: 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,

2923-502: 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

3002-428: The posterior end of the embryo and moves towards the anterior , creating travelling waves of gene expression. The "wavefront" is where clock oscillations arrest, initiating gene expression that leads to the patterning of somite boundaries. The position of the wavefront is defined by a decreasing posterior-to-anterior gradient of FGF signalling. In higher vertebrates including Mouse and Chick , (but not Zebrafish ),

3081-747: 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

3160-497: The Zygomycota phylum. Its members would be divided between phylum Glomeromycota and four new subphyla incertae sedis (of uncertain placement): Entomophthoromycotina , Kickxellomycotina , Mucoromycotina , and Zoopagomycotina . Kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi. Protista

3239-458: The above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group. Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine

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3318-695: The animal kingdom Animalia contains about 31 phyla, the plant kingdom Plantae contains about 14 phyla, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships among phyla within larger clades like Ecdysozoa and Embryophyta . The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon ( φῦλον , "race, stock"), related to phyle ( φυλή , "tribe, clan"). Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as

3397-535: The annelids, as with the arthropods, the body wall, nervous system, kidneys, muscles and body cavity are generally segmented. However, this is not true for all of the traits all of the time: many lack segmentation in the body wall, coelom and musculature. Although perhaps not as well understood as Drosophila , the embryological process of segmentation has been studied in many vertebrate groups, such as fish ( Zebrafish , Medaka ), reptiles ( Corn Snake ), birds ( Chicken ), and mammals ( Mouse ). Segmentation in chordates

3476-528: 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

3555-417: 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

3634-452: 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

3713-412: The development of other structures, such as muscles, across the basic segments. Lower vertebrates such as zebrafish do not require retinoic acid repression of caudal Fgf8 for somitogenesis due to differences in gastrulation and neuromesodermal progenitor function compared to higher vertebrates. In other taxa, there is some evidence of segmentation in some organs, but this segmentation is not pervasive to

3792-428: 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

3871-509: 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

3950-426: 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

4029-534: 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

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4108-507: The first publication of the APG system in 1998, which proposed a classification of angiosperms up to the level of orders , many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e.g. subclasses . Wolf plants Hepatophyta Liver plants Coniferophyta Cone-bearing plant Phylum Microsporidia

4187-552: 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

4266-453: The full list of organs mentioned above for arthropods and annelids. One might think of the serially repeated units in many Cycloneuralia , or the segmented body armature of the chitons (which is not accompanied by a segmented coelom). Segmentation can be seen as originating in two ways. To caricature, the 'amplification' pathway would involve a single-segment ancestral organism becoming segmented by repeating itself. This seems implausible, and

4345-501: 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

4424-401: 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

4503-416: 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 the sides and are

4582-451: The length of the embryo. These transcription factors then regulate the expression of segment polarity genes , which define the polarity of each segment. Boundaries and identities of each segment are later defined. Within the arthropods, the body wall, nervous system, kidneys, muscles and body cavity are segmented, as are the appendages (when they are present). Some of these elements (e.g. musculature) are not segmented in their sister taxon,

4661-594: 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

4740-413: The midline. The N and Q lineages contribute two blast cells for each segment, while the M, O, and P lineages only contribute one cell per segment. Finally, the number of segments within the embryo is defined by the number of divisions and blast cells. Segmentation appears to be regulated by the gene Hedgehog , suggesting its common evolutionary origin in the ancestor of arthropods and annelids. Within

4819-411: The next. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae, but other systems consider these phyla part of Protista. In addition, less popular classification schemes unite Ochrophyta and Pseudofungi under one phylum, Gyrista , and all alveolates except ciliates in one phylum Myzozoa , later lowered in rank and included in a paraphyletic phylum Miozoa . Even within

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4898-657: 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

4977-539: The other hand, the highly parasitic phylum Mesozoa was divided into two phyla ( Orthonectida and Rhombozoa ) when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes . This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa in clades without any formal ranking of group size. A definition of

5056-434: 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 the segments that are homologous to

5135-475: The relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not. For example, the bearded worms were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids , so the phyla were merged (the bearded worms are now an annelid family ). On

5214-654: 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

5293-472: The soil. Phylum In biology , a phylum ( / ˈ f aɪ l əm / ; pl. : phyla ) is a level of classification or taxonomic rank below kingdom and above class . Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions,

5372-412: 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),

5451-566: 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

5530-409: 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)

5609-413: 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

5688-562: 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

5767-509: The wavefront also depends upon an opposing anterior-to-posterior decreasing gradient of retinoic acid which limits the anterior spreading of FGF8 ; retinoic acid repression of Fgf8 gene expression defines the wavefront as the point at which the concentrations of both retinoic acid and diffusible FGF8 protein are at their lowest. Cells at this point will mature and form a pair of somites. The interaction of other signaling molecules, such as myogenic regulatory factors , with this gradient promotes

5846-621: 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

5925-399: 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

6004-484: 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

6083-445: 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

6162-561: 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

6241-437: 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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