Tardigrade

Tardigrades (commonly known as water bears) comprise the phylum Tardigrada. They are small, segmented animals, similar and probably related to the arthropods. 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 Spallanzani in 1777. 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 900 species of tardigrades have been described. Tardigrades occur over the whole 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.

Water bears are able to survive in extreme environments that would kill almost any other animal. They can survive temperatures close to absolute zero, temperatures as high as 151°C (303°F), 1,000 times more radiation than any animal, nearly a decade without water, and can also survive in a vacuum like that found in space. 

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. 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. Most species of tardigrades have about 40,000 cells in their adult body. .

Feeding ecology
Most tardigrades are phytophagous or bacteriophagous, but some are predatory (e.g. Milnesium tardigradum).

Extreme environments
Tardigrades are very hardy animals; 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 what is 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.

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 or being chilled for days at -200°C, or for a few minutes at -272°C. (1° warmer than absolute zero).
 * Radiation — as shown by Raul M. May from the University of Paris, tardigrades can withstand 5,700 grays or 570,000 rads of x-ray radiation. (Ten to twenty grays or 1,000-2,000 rads could be fatal to a human).
 * Pressure — they can withstand the extremely low pressure of a vacuum and also very high pressures, many times greater than atmospheric pressure. It has recently been proven that they can survive in the vacuum of space. Recent research has notched up another feat of endurability; apparently they can withstand 6,000 atmospheres pressure, which is nearly six times the pressure of water in the deepest ocean trench.
 * Dehydration - tardigrades have been shown to survive nearly one decade in a dry state.

Recent experiments conducted by Cai and Zabder have also shown that these water bears can undergo chemobiosis — a cryptobiotic response to high levels of environmental toxins. However, their results have yet to be verified.

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 tartigrades.

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.

Aysheaia from the middle Cambrian Burgess shale might be related to tardigrades.