Tardigrades: Profile and Information

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, and Ephraim Goeze in 1773.

In 1777, an Italian biologist, Lazzaro Spallanzani, 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 are 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 modern and likely have a much earlier origin, as they diverged from their relatives in Cambria more than 500 million years ago.

Scientific Classification

When fully grown, tardigrades are usually about 0.5 mm (0.02 in). They are 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 was 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 how they walk, reminding them of a bear’s gait. The name Tardigradum translates to “slow walker” and was given in 1777 by Lazzaro Spallanzani.

Description 

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.

Habitat 

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 liter). Tardigrades may be found on barnacles in the case of Echiniscoides wyethi.

Anatomy 

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 may be 1.2 mm (0.047 in).

The body comprises 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 homologous to the arthropod head region.

All adult tardigrades of the same species have the same number of cells. Some species have as many as 40,000 cells per adult, while others have fewer. The body cavity comprises a hemocoel, 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 used to pierce plant cells, algae, or small invertebrates on which the latex feeds release 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 is attached to a short esophagus and then to an intestine that occupies a large part of the body’s length, 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 molt, leaving the feces behind with the cuticle shed.

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

The cord has one ganglion per segment, each producing lateral nerve fibers 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, distinguishes species.

Reproduction 

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 one duct opening either on top of the anus or directly inside the rectum, which forms a cloaca.

Tardigrades are oviparous, and usually, fertilization is external. The mating occurs during the molting 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 tardigrade species (e.g., Milnesium tardigradum).

Tardigrades share morphological characteristics with many species, which differ largely 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 worldwide. Tardigrade 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 many 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 others, not counting their time in dormant states.

Physiology

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.

Certain animals can be found in milder environments, such as wetlands, rivers, and meadows, while some can be found on stone walls and roofs. Tardigrades are most common in humid conditions but can remain active anywhere and 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 only taking advantage of these conditions to survive.

This means that the more they are exposed to harsh environments, the more their risk of dying increases. 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 organisms capable of suspending their metabolism (see cryptobiosis).

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

Many tardigrade 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 primarily depended on the high levels of non-reducing sugar trehalose that protect their membranes.

However, recent research indicates that tardigrades have a special 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:

Temperature

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.

Pressure

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

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

Dehydration

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

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

Radiation

Tardigrades can tolerate 1,000 times more radiation than other animals, with 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. One reason is their ability to effectively repair the damage to their DNA resulting from this exposure.

Irradiation of tardigrade 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 hatched at the middle stage. In contrast, eggs irradiated at a late stage hatched at a rate indistinguishable from controls.

Environmental toxins are known to undergo chemobiosis, a cryptobiotic response to high 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 previously dehydrated and some 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 surviving 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 on the Moon after Beresheet, a failed Israeli lunar lander, crashed in April 2019.

Taxonomy

Scientists have performed morphological and molecular studies to explain how tardigrades correlate with other ecdysozoan animal lineages.

Two possible placements have been proposed: tardigrades are 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 various molecular studies is a long-branch attraction artifact.

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, a simpler head composition, and no posterior head appendages.

Still, they share their columnar cuticle construction with modern tardigrades. Scientists claim 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 genus and family, Beorn leggi, but it resembles several living specimens of the Hypsibiidae family.

Evolution

There are several lines of evidence that a larger ancestor secondarily miniaturizes tardigrades, perhaps a lobopodian, and possibly similar to Aysheaia, which several analyses 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 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 tardigrade 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 resulted from horizontal gene transfer.

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

This research also found a high expression of unexplained tardigrade 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 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.