Tardigrades

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? Water Bears
Scientific classification
Kingdom: Animalia Phylum: Tardigrada Spallanzani, 1777
Classes
Heterotardigrada Mesotardigrada Eutardigrada

Tardigrades (Tardigrada), or "water bears", are a phylum of small, segmented animals, similar and related to the Arthropods. Tardigrades were first described by J.A.E. Goeze in 1773 ('kleiner Wasserbär'=little waterbear). 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. Fresh 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.

Contents 1 Location and Environment 2 Tardigrades and Extreme Environments 3 Tardigrade characteristics 4 Relationships 5 See also 6 References 7 External links

Location and Environment

There are about 750 species 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.

Architect Eugene Tsui built the Tsui House in Berkeley, California, a structure that mimics the tardigrade form, with emphasis on resilience to extreme conditions.

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. One Tardigrade was recorded to be in this state for over 120 years (but although the revival process started, the animal did not successfully revive); 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 of x-ray radiation. (Five grays 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. In theory, they could even survive the vacuum of space, though the possibility of it is slim.

References:

1. Franceschi, T. (1948): Anabiosi nei tardigradi. Bolletino dei Musei e degli Istituti Biologici dell'Università di Genova 22:47-49
2. Jönsson, 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
• Cuticle with chitin, molting
• Males and females present (though some species are parthenogenetic)
• Four pair 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.

See also

• Soil life
• Bryophyte

References

• http://www.itis.usda.gov (TSN: 155166)

External links

• The Edinburgh Tardigrade project
• Tardigrade Appreciation Headquarters
• Tardigrades (English/German)
• The incredible water bear!
• Tardigrade video
• Tsui Design and Research Inc.

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