Tardigrades are known colloquially as water bears or moss piglets. They are eight-legged, semi-segmented micro-animal phyla of water-dwelling.

They were originally described by Johann August, the German Zoologist, Ephraim Goeze in 1773. In 1777 Lazzaro Spallanzani, an Italian biologist called them Tardigrada, which means “slow steppers.”

They have been found everywhere, from the mountaintops to the deep seas and mud volcanoes, and from the tropical rainforests to the Antarctic.

Tardigrades, among the most resilient animals known, with individual species capable of surviving extreme conditions—such as air deprivation, exposure to extreme temperatures, extreme pressures (both high and low), radiation, dehydration, and starvation—that would have quickened.

Tardigrades have survived outer space exposure—about 1,300 known species from the phylum Tardigrada, part of the Ecdysozoa superphylum. The early known true members of the group are known from Cretaceous amber in North America.

Still, they are essentially modern forms, and therefore are likely to have a much earlier origin, as they diverged from their relatives in Cambria more than 500 million years ago.

Scientific classification


Tardigrades, when fully grown, are usually about 0.5 mm (0.02 in). They usually short and plump, with four pairs of legs, each ending in claws (usually four to eight) or suction plates.

They are prevalent in lichens and mosses and feed on plant cells, algae and small invertebrates. They can be viewed under a low-power microscope when collected.

Johann August Ephraim Goeze originally referred to as the late little Wasserbär, meaning “little water-bear” in German (today it is often termed in German as Bärtierchen or “little bear-animal”).

The name “water bear” was derived from the way they walk, which reminded them of a bear’s gait. The name Tardigradum translates to “slow walker” and was given in 1777 by Lazzaro Spallanzani.


The largest adult may have a body length of 1.5 mm (0.059 in) and the smallest less than 0.1 mm. Newly hatched tardigrades may be smaller than 0.05 mm.


Tardigrades are frequently found on lichens and mosses. Other environments include dunes, seasides, soil, leaf beds, and marine or freshwater sediments, which can frequently occur (up to 25.000 animals per litre). Tardigrades may be found on barnacles in the case of Echiniscoides wyethi.


The Tardigrades have a barrel-shaped body with four pairs of stubby legs. The majority ranges from 0.3 to 0.5 mm (0.012 to 0.020 in) in length, although the largest species maybe 1.2 mm (0.047 in).

The body comprises of a head, three body segments with a pair of legs each, and a caudal segment with a fourth leg pair. The legs are without joints, and the legs have four to eight claws each.

The cuticle contains protein and chitin and is periodically ground. The first three pairs of legs are directed downwards along the sides and are the means of movement, while the fourth pair is directed back to the last segment of the trunk and is used primarily to grasp the substrate.

Tardigrades do not have several Hox genes and a large intermediate area of the body axis. This corresponds to the whole chest and abdomen in insects. The whole body, except the last pair of legs, consists of segments that are homologous to the arthropod head region.

All adult tardigrades of the same species have the same number of cells. Some species have as much as 40,000 cells per adult, while others have far fewer. The body cavity is made up of a haemocoel, but the only place where the true coelom can be discovered is around the gonad.

No respiratory organs are discovered, with gas exchange capable of occurring throughout the entire body. Some late stages have three tubular glands associated with the rectum, which may be excretory organs similar to the Malpighian arthropod tubules, although the details remain unclear. Nephridia is also missing.

The tubular mouth is armed with styluses that are used to pierce plant cells, algae, or small invertebrates on which the latex feeds releases body fluids or cell contents. The mouth opens to a triradiate, muscular, sucking pharynx.

The stylus is lost when the animal grinds, and a new pair is secreted from a pair of glands lying on either side of the mouth.

The pharynx attached to a short oesophagus, and then to an intestine that occupies a large part of the body’s length, which is the main digestive site.

The intestine opens, through a short rectum, to the anus at the end of the body. Some species defecate when they moult, leaving the faeces behind with the cuticle shed.

The brain is developing in a bilaterally symmetrical pattern. The brain contains multiple lobes, mostly made up of three bilaterally paired clusters of neurons. The brain is connected to a large ganglion beneath the oesophagus, from which the body has a double ventral nerve cord.

The cord has one ganglion per segment, each producing lateral nerve fibres that run into the limbs. Many species have a pair of rhabdomeric pigment-cup eyes, and many sensory bristles are on the head and body.

Tardigrades can often be found by soaking a piece of moss in the water.

Tardigrades all have a buccopharyngeal apparatus (a swallowing device made of muscles and spines that activates the inner jaw and begins to digest and move along the throat and intestines) which, along with the claws, is used to distinguish species.


Although some species are parthenogens, males and females are usually present, although females are often larger and more common. Both sexes have a single gonad situated above the intestine.

In males, two ducts run from the testes, opening through a single pore in front of the anus. On the other hand, the females have a one duct opening either just ontop the anus or directly inside the rectum, which forms a cloaca.

Tardigrades are oviparous, and usually, fertilization is external. The mating occurs during the moulting process, with the eggs 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 completely discards the cuticle. In most cases, eggs are left to develop inside the shed cuticle, but some species attach them to the nearby substrate.

Eggs will hatch after no more than 14 days, with young people already possessing their full complement of adult cells. Growth to adult size is due to the enlargement of individual cells (hypertrophy) rather than cell division. Tardigrades may be molten up to 12 times.

Ecology and History of Life


Most tardigrades are phytophagous (plant eaters) or bacteriophage (bacteria eaters), but some are carnivorous to the extent that they eat smaller tardigrades species (e.g., Milnesium tardigradum).

Tardigrades share morphological characteristics with many species, which differ in large part by class. Biologists have difficulty finding verification among late-earth species because of this relationship.

These animals are most closely related to the early development of arthropods. Tardigrade fossils have gone as far back as the Cretaceous period in North America.

Tardigrades are considered to be cosmopolitan and can be found in regions all over the world. Tardigrades eggs and cysts are so long-lasting that they can be carried long distances at the feet of other animals.

The Tardigrades survived the five mass extinctions. This gave them a plethora of survival characteristics, including the ability to survive situations that would have been fatal to almost all other animals (see next section).

Tardigrade lifespans range from 3–4 months for some species, to 2 years for other species, not counting their time in dormant states.


In hot springs, over the Himalayas (6,000 m; 20,000 ft, above sea level), tardigrades were recorded by researchers to the deep sea (-4,000 m; − 13,000 ft) and from polar regions to the equator, under firm ice layers, and in ocean sediments.

In milder environments, such as wetlands, rivers and meadows, certain animals can be found, while some can be found on stone walls and roofs. Tardigrades are most common in humid conditions but can remain active anywhere it is possible to retain at least some moisture.

Due to astrophysical phenomena such as gamma-ray bursts or massive meteorite collisions, tardigrades are thought to be able to endure even full global mass extinction events.

Some of them can tolerate incredibly cold temperatures of up to 1 K (-458 ° F; -272 ° C) (close to absolute zero), while others can tolerate extremely hot temperatures of up to 420 K (300 ° F; 150 ° C) for many minutes.

They are not called extremophiles because they are not accustomed, to take advantage of these conditions only to survive.

This means that the more they are exposed to harsh environments, their risk of dying increase. At the same time, true extremophiles thrive in a physical or geochemically extreme environment that would harm most other organisms.

Tardigrades are one of the few classes of organisms capable of suspending their metabolism (see cryptobiosis).

When in this condition, their metabolism drops to less than 0.01 per cent and their water content can drop to 1 per cent of normal, and they can go without food or water for more than 30 years, only to rehydrate, forage, and reproduce later.

Many tardigrades species may live in a dehydrated state for up to five years, or in rare cases longer. Depending on the environment, they can reach this state through anhydrobiosis, cryobiosis, osmobiosis, or anoxybiosis.

Their ability to stay desiccated for such long periods was thought to be primarily dependent on the high levels of non-reducing sugar trehalose that protect their membranes.

However, recent research indicates that tardigrades have a special form of disordered protein that serves a similar purpose: it replaces water in the cells.

It takes a glassy, vitrified state when the animals dry out. Their DNA is further shielded from radiation by a protein called “dsup” (short for damage suppression). In this cryptobiotic state, the tardigrades are known as a tun.

Tardigrades can live in harsh conditions that would kill almost any other species. The extremes in which tardigrades will survive include:


Late temperatures will survive:

  • At 151 ° C (304 ° F) for a few minutes
  • 30 years at −20 ° C (−4 ° F)
  • Several days at −200 ° C (−328 ° F; 73 K)
  • A few minutes at-272 ° C (−458 ° F; 1 K)

The research reported in 2020 indicates that tardigrades are susceptible to high temperatures. Researchers have shown that it takes 48 hours at 98.8 ° F (37.1 ° C) to destroy half of the active latexes that have not been acclimatized to the heat.

Acclimation raised the temperature required to kill half of the active late stages to 99.7 ° F (37.6 ° C). Tardigrades went a little better in the tun state, tolerating higher temperatures. It took heat to 180.9 ° F (82.7 ° C) to kill half of the late tune in 1 hour.

Longer exposure time, however, reduced the temperature required for lethality. For a full day of exposure, 145.6 ° F (63.1 ° C) was adequate to kill half of the late tunic.


It can withstand very low vacuum pressure and even very high pressure, more than 1,200 times the ambient pressure.

Some animals can also withstand the pressure of 6,000 atmospheres, which is almost six times the pressure of water in the Mariana Trench, which is the deepest ocean trench.


the longest-lived latex has been shown to survive in a dry state for almost ten years, although there is one leg movement record, not widely considered to be a “survival” in a 120-year-old dried moss specimen.

When in extremely low temperatures, the body composition varies from 85 per cent water to just 3 per cent. Since the water expands after freezing, dehydration means that the expansion does not break the tardigrade tissues of freezing ice.


Tardigrades can tolerate 1,000 times more radiation than other animals, median lethal doses of 5,000 Gy (of gamma rays) and 6,200 Gy (of heavy ions) in hydrated animals (5 to 10 Gy may be fatal to humans).

The only explanation found in earlier studies for this ability was that their water-lower state provides less ionizing radiation reactants.

Subsequent research has shown, however, that tardigrades, when hydrated, remain highly resistant to shortwave UV radiation relative to other species, and that one reason for this is their ability to effectively repair the damage to their DNA resulting from this exposure.

Irradiation of tardigrades eggs collected directly from the natural substrate (moss) showed a strong dose-related response, with a steep decline in hatchability at doses up to 4 kGy above which no eggs were hatched.

Eggs were more radiation-resistant at a late stage of development. No eggs irradiated at an early stage of development hatched, and only one egg hatched at the middle stage, while eggs irradiated at a late stage hatched at a rate that was indistinguishable from controls.

Environmental toxins are known to undergo chemobiosis, a cryptobiotic response to high levels of environmental toxins. However, these laboratory findings have not yet been confirmed since 2001.

Survival after being exposed to outer space

Tardigrades are the first known species to live after exposure to outer space. In September 2007, dehydrated Tardigrades were taken to low Earth orbit on the FOTON-M3 mission carrying a payload of BIOPAN astrobiology.

For ten days, groups of tardigrades, some of them previously dehydrated, some of them not, were exposed to the harsh vacuum of outer space, or vacuum and solar UV radiation.

Back on Earth, more than 68% of subjects shielded from solar UV radiation were re-animated within 30 minutes of rehydration, while subsequent mortality was high; many of them developed viable embryos.

On the other hand, hydrated samples exposed to combined vacuum and maximum solar UV radiation had dramatically decreased survival, with only three subjects survived by Milnesium tardigradum.

In May 2011, Italian scientists sent the last flight of the Endeavor space shuttle onboard the International Space Station along with the extremophiles on STS-134.

They concluded that microgravity and cosmic radiation “did not significantly affect the survival of tardigrades in flight, and believe that tardigrades represent a useful animal for space research.”

In November 2011, they were among the species to be sent to Phobos by the U.S.-based Planetary Society of the Russian Fobos-Grunt project Living Interplanetary Flight Experiment; In August 2019, scientists announced that a capsule containing tardigrades in a cryptobiotic state could have lasted for a while on the Moon after Beresheet, a failed Israeli lunar lander, crashed in April 2019.


Scientists have performed morphological and molecular studies to explain how tardigrades are correlated with other lineages of ecdysozoans animals.

Two possible placements have been proposed: tardigrades are either most closely related to Arthropoda and Onychophora, or nematodes.

Evidence for the former is a typical consequence of morphological studies; evidence of the latter can be found in some molecular analyses.

The latter hypothesis was dismissed by recent microRNA and expressed sequence tag analysis. The grouping of tardigrades nematodes observed in a variety of molecular studies is a long-branch attraction artefact.

Within the group of arthropods (called panarthropods and containing Onychophora, tardigrades and euarthropods), three patterns of relationship are possible: tardigrade sister to Onychophora plus arthropods (lobopods hypothesis); Onychophora sister to tardigrades plus arthropods (tactopods hypothesis); and Onychophora sister to tardigrades.

The minute sizes of the tardigrades and their membranous integuments make their fossilization difficult to detect and extremely unusual. The only known fossil fossils are those from the mid-Cambrian deposits in Siberia and a few rare fossils of Cretaceous amber.

Siberian tardigrades fossils vary in a variety of respects from living tardigrades. They have three pairs of legs rather than four, they have a simpler head composition, and they have no posterior head appendages.

Still, they share their columnar cuticle construction with modern tardigrades. Scientists claim that they constitute a stem community of people living late in life.

Rare specimens of Cretaceous amber have been found in two places in North America. Milnesium swolenskyi, from New Jersey, is the oldest of the two; its claws and mouthpieces are indistinguishable from the living M. Late graduation.

Other collections of amber are from western Canada, about 15–20 million years older than M. Swolenskyi, guy. One of the two specimens from Canada received its own genus and family, Beorn leggi, but bears a clear resemblance to several living specimens of the Hypsibiidae family.


There are several lines of evidence that a larger ancestor secondarily miniaturizes tardigrades, perhaps a lobopodian, and possibly similar to Aysheaia, which several analyzes place close to the divergence of the late lineage.

Alternative theories derive tactopod from the grade of dinocarides and opabinia.

Genomes and gene sequence

Tardigrade genomes vary in size, ranging from about 75 to 800 megabase DNA pairs. Hypsibius exemplaris (formerly called Hypsibius dujardini) has a compact genome of 100 megabase pairs and a generation period of around two weeks; it can be cultivated indefinitely and cryopreserved.

The genome of Ramazzottius varieornatus, one of the most stress-tolerant tardigrades animals, was found in 2015 by researchers from the University of Tokyo.

Although previous studies claimed that about one-sixth of the genome had been acquired from other species, it is now known that less than 1.2% of its genes were the result of horizontal gene transfer.

They also found proof of a loss of gene pathways known to promote stress-related damage.

This research also found a high expression of unexplained tardigrades proteins, including the Damage Suppressor (Dsup), which was discovered to protect against DNA damage from X-ray radiation. The same researchers applied the Dsup protein to human cultured cells and found that it suppressed about 40% of the X-ray damage to human cells.

Environmental significance

Many organisms living in marine ecosystems feed on species such as nematodes, latex, bacteria , algae, mites, and collembolans.

Tardigrades are acting as pioneer organisms by inhabiting new emerging ecosystems. This movement attracts other invertebrates to populate this space, while also attracting predators.

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