An illustration of a tardigrade from 1861
Tardigrades (also known as waterbears or moss piglets) are water-dwelling, segmented micro-animals, with eight legs.
They were first described by the German pastor Johann August Ephraim Goeze in 1773. The name Tardigrada (meaning "slow stepper") was given three years later by the Italian biologist Lazzaro Spallanzani. Since 1778, over 1,150 tardigrade species have been identified.
Tardigrades are classified as extremophiles, organisms that can thrive in a physically or geochemically extreme condition that would be detrimental to most life on Earth. For example, tardigrades can withstand temperatures from just above absolute zero to well above the boiling point of water, pressures about six times greater than those found in 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. They can go without food or water for more than 10 years, drying out to the point where they are 3% or less water, only to rehydrate, forage, and reproduce.
Usually, tardigrades are about 0.5 mm (0.020 in) long when they are fully grown. They are short and plump with four pairs of legs, each with four to eight claws also known as "disks". The animals are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a very-low-power microscope, making them accessible to students and amateur scientists.
Tardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. It is an ancient group, with fossils dating from 530 million years ago, in the Cambrian period.
About 1,150 species of tardigrades have been described. Tardigrades occur throughout the world, from the Himalayas (above 6,000 m (20,000 ft)), to the deep sea (below 4,000 m (13,000 ft)) 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 liter). Tardigrades often can be found by soaking a piece of moss in spring water.
Scientists have reported tardigrades in hot springs, on top of the Himalayas, 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 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 10 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 nonreducing sugar trehalose, which protects their membranes. In this cryptobiotic state, the tardigrade is known as a tun.
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