Tardigrade

| image = Waterbear.jpg | image_caption = The tardigrade Hypsibius dujardini | status = LR/lc | status_system = IUCN2.3 | authority = Spallanzani, 1777 | display children = 1 }}

Tardigrades (commonly known as water bears or moss piglets) form the phylum 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 , the smallest below 0.1 mm. Freshly hatched larvae may be smaller than 0.05 mm.

More than 1,000 species of tardigrades have been described. Tardigrades occur over the entire world, from the high Himalayas (above , 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.

Tardigrades are able to survive in extreme environments that would kill almost any other animal. Some can survive temperatures of , close to absolute zero, temperatures as high as , 1,000 times more radiation than other animals, and almost a decade without water. In September 2007, tardigrades were taken into low Earth orbit on the FOTON-M3 mission and for 10 days were exposed to the vacuum of space. After they were returned to Earth, it was discovered that many of them survived and laid eggs that hatched normally.

Anatomy and morphology

Tardigrades have barrel-shaped bodies with four pairs of stubby legs. Most range from in length, although the largest species may reach . The body has four segments (not counting the head), four pairs of legs without joints, and feet with four to eight claws each. The cuticle contains chitin and is moulted periodically.

Tardigrades are eutelic, with all adult tardigrades of the same species having the same number of cells. Some tardigrade species have as many as about 40,000 cells in each adult's body, 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. There are no respiratory organs, with gas exchange able to occur across the whole 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.

The brain includes multiple lobes, and is attached to a large ganglion below the oesophagus, 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 there are numerous sensory bristles on the head and body.

Reproduction

Although some species are parthenogenetic, both males and females are usually present, each with a single gonad located above the intestine. Two ducts run from the testis in males, opening through a single pore in front of the anus. In contrast, females have a single duct opening either just above the anus or directly into the rectum, which thus forms a cloaca.

Physiology

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.

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.

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 (73 K), or for a few minutes at -272 °C (~1 degree above absolute zero). Dehydration – tardigrades have been shown to survive nearly 10 years in a dry state. When encountered by extremely low temperatures, their body composition goes from 85% water to only 3%. As water expands upon freezing, dehydration ensures the tardigrades do not get ripped apart by the freezing ice (as waterless tissues cannot freeze). Radiation – tardigrades can withstand median lethal doses of 5,000 Gy (gamma-rays) and 6,200 Gy (heavy ions) in hydrated animals (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. In September 2007, a space launch (Foton-M3) 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 solar radiation.

Evolutionary relationships and history

Recent DNA and RNA sequencing data indicate that tardigrades are the sister group to the arthropods and Onychophora. These groups have been traditionally thought 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 well 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.

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.

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

Tardigrades have sometimes been linked to the prehistoric oddity Opabinia as its closest living relative.

Genomes and genome sequencing

Tardigrade genomes vary in size, from about 75 to 800 megabase pairs of DNA. The genome of a tardigrade species, Hypsibius dujardini, is being sequenced 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.

See also

Oncopoda

Living Interplanetary Flight Experiment

References

External links

Tardigrada Newsletter

Tardigrades – Pictures and Movies

The Edinburgh Tardigrade project

NJ Tardigrade Survey

Tardigrades (English/German)

The incredible water bear!

Tardigrade Reference Center

Tardigrades in space

Tardigrade data and analysis

NASA's Tardigrade Research

A short film about tardigrade research from NPR's Science Friday

Swiss Center of Tardigrade Research – Ecology, Physiology and Evolutionary Biology of Tardigrades

Category:Extremophiles