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Tardigrades (commonly known as water bears) form the Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773 (kleiner Wasserbär = little water bear). The name Tardigrada means "slow walker" and was given by Lazzaro Spallanzani in 1777. The name water bear comes from the way they walk, reminiscent of a bear's gait. The biggest adults may reach a body length of 1.5 mm, the smallest below 0.1 mm. Freshly hatched larvae may be smaller than 0.05 mm.

More than 1000 species of tardigrades have been described. Tardigrades occur over the entire world, from the high Himalayas (above 6,000 m), to the deep sea (below 4,000 m) and from the polar regions to the equator.

The most convenient place to find tardigrades is on lichens and mosses. Other environments are dunes, beaches, soil, and marine or freshwater sediments, where they may occur quite frequently (up to 25,000 animals per litre). Tardigrades often can be found by soaking a piece of moss in spring water.^3]

Tardigrades are polyextremophiles and are able to survive in extreme environments that would kill almost any other animal. Some can survive temperatures of -273°C, close to absolute zero ^[4], temperatures as high as 151 °C (303 °F), 1,000 times more radiation than other animals such as humans^[5], nearly a decade without water, and even the vacuum of space.^[6]

Anatomy and Morphology

Tardigrades have a body with four segments (not counting the head), four pairs of legs without joints, and feet with claws or toes. The cuticle contains chitin and is moulted. They have a ventral nervous system with one ganglion per segment, and a multilobed brain. Their pigment-cup eyes are rhabdomeric.^[7]

Instead of a coelom they have a haemocoel. The only place where a true coelom can be found is around the gonad (coelomic pouch). The pharynx is of a triradiate, muscular, sucking kind, armed with stylets. Although some species are parthenogenetic, males and females are usually present, each with a single gonad. Tardigrades are eutelic (all adult tardigrades of the same species are believed to have the same number of cells) and oviparous. Some tardigrade species have as many as about 40,000 cells in each adult's body, others have far fewer. ^[8][9]

Ecology and Life History

Most tardigrades are phytophagous (plant eaters) or bacteriophagous (bacteria eaters), but some are predatory^[10] (e.g. Milnesium tardigradum).^[11]

Physiology

Extreme Environments

Tardigrades are polyextremophiles; scientists have reported their existence in hot springs, on top of the Himalayas, under layers of solid ice and in ocean sediments. Many species can be found in a milder environment like 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.

Modern tardigrades

Tardigrades are one of the few groups of species that are capable of reversibly suspending their metabolism and going into a state of cryptobiosis. Several species regularly survive in a dehydrated state for nearly ten years. Depending on the environment they may enter this state via anhydrobiosis, cryobiosis, osmobiosis or anoxybiosis. While in this state their metabolism lowers to less than 0.01% of normal and their water content can drop to 1% of normal. Their ability to remain desiccated for such a long period is largely dependent on the high levels of the non-reducing sugar trehalose, which protects their membranes. In this cryptobiotic state the tardigrade is known as a tun.^[12]

Tardigrades have been known to withstand the following extremes while in this state:

• Temperature — tardigrades can survive being heated for a few minutes to 151 °C (424 K), or being chilled for days at –200 °C (70 K), or for a few minutes at –272 °C. (~1 degree above absolute zero).^{[13][citation needed]}
• Pressure — they can withstand the extremely low pressure of a vacuum and also very high pressures, more than 1200 times atmospheric pressure. It has recently been demonstrated that tardigrades can survive the vacuum of open space and solar radiation combined for at least 10 days.^[6] Recent research has notched up another feat of endurance: they can withstand 6,000 atmospheres, which is nearly six times the pressure of water in the deepest ocean trench. ^[14]
• Dehydration — tardigrades have been shown to survive nearly one decade in a dry state.^[15]
• Radiation — Tardigrades can withstand median lethal doses of 5000 Gy (gamma-rays) and 6200 Gy (heavy ions) in hydrated animals ^[16]. (5 to 10 Gy could be fatal to a human). The only explanation thus far for this ability is that their lowered water state provides fewer reactants for the ionizing radiation.

Recent experiments conducted by Cai and Zabder have also shown that these tardigrades can undergo chemobiosis — a cryptobiotic response to high levels of environmental toxins. However, their results have yet to be verified.^[17][18] In September 2008, a space launch showed that tardigrades can survive the extreme environment of outer space for 10 days. After being rehydrated back on earth, over 68% of the subjects protected from high-energy UV radiation survived and many of these produced viable embryos, and a handful survived full exposure to the sun.^[6]

Evolutionary Relationships and History

Recent DNA and RNA sequencing data^[19] indicate that tardigrades are the sister group to the arthropods and Onychophora. These groups have been traditionally thought^[20] of as close relatives of the annelids, but newer schemes consider them Ecdysozoa, together with the roundworms (Nematoda) and several smaller phyla. The Ecdysozoa-concept resolves the problem of the nematode-like pharynx as we ll as some data from 18S-rRNA and HOX (homeobox) gene data, which indicate a relation to roundworms.

The minute sizes of tardigrades and their membranous integuments make their fossilization both difficult to detect and highly unlikely. The only known fossil specimens comprise some from mid-Cambrian deposits in Siberia and a few rare specimens from Cretaceous amber.^[21]

The Siberian tardigrades 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. It is considered that they probably represent a stem group of living tardigrades.^[21]

The rare specimens in Cretaceous amber comprise Milnesium swolenskyi, from New Jersey, the oldest, whose claws and mouthparts are indistinguishable from the living M. tartigradum; and two specimens from western Canada, some 15–20 million years younger than M. swolenskyi. Of the two latter, one has been given its own genus and family, Beorn leggi (the genus named by Cooper after the character Beorn from The Hobbit by J. R. R. Tolkien and the species named after his student William M. Legg); however, it bears a strong resemblance to many living specimens in the family Hipsiblidae.^[21][22]

Aysheaia from the middle Cambrian Burgess shale has been proposed as a sister-taxon to an arthropod-tardigrade clade.^[23]

Genomes and Genome Sequencing

Tardigrade genomes vary in size, from about 75 to 800 megabase pairs of DNA.^[24] The genome of a tardigrade species, Hypsibius dujardini, is being sequenced^[25] at the Broad Institute. Hypsibius dujardini has a compact genome and a generation time of about two weeks, and it can be cultured indefinitely and cryopreserved.^[26]

The Phylum Tardigrada is a member of the Superphylum Panarthropoda. Here is the complete "parentage" of Tardigrada:

• Domain: Eukaryota Whittaker & Margulis,1978 - eukaryotes
  • Kingdom: Animalia Linnaeus, 1758 - animals
    • Subkingdom: Bilateria (Hatschek, 1888) Cavalier-Smith, 1983 - bilaterians
      • Branch: Protostomia Grobben, 1908 - protostomes
        • Infrakingdom: Ecdysozoa Aguinaldo Et Al., 1997 Ex Cavalier-Smith, 1998 - ecdysozoans
          • Superphylum: Panarthropoda
            • Phylum: Tardigrada (Spallanzani, 1777) Ramazzotti, 1962 - Tardigrades