Tardigrade
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[Photo] SEM image of a tardigrade. Created by Rick Gillis and Roger J. Haro Department of Biology University of Wisconsin - La Crosse.

Tardigrades or 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 Wasserb??r = little water bear). The name Tardigrada means "slow walker" and was given by Spallanzani in 1776. Tardigrades are small animals. 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.

Most tardigrades are phytophagous or bacteriophagous, but some are predators (e.g. Milnesium tardigradum). The species Beorn leggi has been recorded from Canadian Cretaceous amber. Aysheaia from the middle Cambrian Burgess shale might be related to tardigrades.

Location and environment
More than 1000 species have been described. Tardigrades occur over the whole world, from the high Himalaya (above 6000 m) to the deep sea (below 4000 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/l).

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. Basically, all tardigrades need to do is remain moist in order to be active. They are therefore most common in moist environments.

Tardigrades and extreme environments
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 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 whilst in this state:

Temperature???Tardigrades can survive being heated for a few minutes to 151 °C or being chilled for days at -272.8 °C (almost absolute zero).
Radiation??? Shown by Raul M. May from the University of Paris, Tardigrades can withstand 5700 grays or 570,000 rads of x-ray radiation. (Five grays or 500 rads would 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 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.

References:

Franceschi, T. (1948): Anabiosi nei tardigradi. Bolletino dei Musei e degli Istituti Biologici dell'Universit?? di Genova 22:47-49
J??nsson, K. I. and Bertolani, R. (2001): Facts and fiction about long-term survival in tardigrades. Journal of Zoology 255:121-123

Tardigrade characteristics
Body with four segments and head
Ventral nervous system with one ganglion per segment
Multilobed brain
Triradiate muscular sucking pharynx
Stylets
A single gonad
Instead of a coelom they have a haemocoel, the only place where we still can find a true coelom is around the gonad (coelomic pouch).
Cuticle with chitin, molting
Males and females present (though some species are parthenogenetic)
Four pairs of legs (in genus Hexapodibius reduced to three) without joints
Feet with claws or toes
Oviparous

Relationships
Recent DNA and RNA sequencing data indicate that Tardigrades are the closest sister group to the Arthropods, even closer than Onychophora. These groups have been traditionally thought of as close relatives of the Annelids, but newer schemes consider them Ecdysozoa, together with the roundworms 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.

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