Tardigrade
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 SEM image of a tardigrade
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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 Wasserbär = little water bear). The name Tardigrada means "slow walker" and was given by Spallanzani in 1777. 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.
More than 1,000 species of tardigrades have been described. Tardigrades occur over the whole world, from the high Himalaya (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).
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[edit] Anatomy and morphology
Tardigrades have a body with four segments, not counting the head, four pairs of legs (reduced to three in the genus Hexapodibius) 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 we still can find a true coelom 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 have the same number of cells) and oviparous.
[edit] Ecology and life history
[edit] Feeding ecology
Most tardigrades are phytophagous or bacteriophagous, but some are predatory (e.g. Milnesium tardigradum).[citation needed]
[edit] Physiology
[edit] Extreme environments
Tardigrades are very hardy animals; scientists have reported their existence in hot springs, on top of the Himalaya, 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 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).[2]
- 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 1000-2000 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.[citation needed]
- Dehydration - When a piece of dried-up moss which had been kept in a museum for 120 years was moistened, tardigrades successfully recovered from the 'tuns' in the moss.[3]
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.[4][5]
[edit] Evolutionary relationships and history
Recent DNA and RNA sequencing data indicate that tardigrades are the 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 (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 species Beorn leggi has been recorded from Canadian Cretaceous amber. Aysheaia from the middle Cambrian Burgess shale might be related to tardigrades.
[edit] References
- ^ Tardigrada (TSN 155166). Integrated Taxonomic Information System.
- ^ Ramel, G. (2005-11-11). The Water Bears (Phylum Tardigrada).
- ^ http://www.microscopy-uk.org.uk/mag/artmay99/dwbear.html
- ^ Franceschi, T. (1948). Anabiosi nei tardigradi. Bolletino dei Musei e degli Istituti Biologici dell'Università di Genova 22: 47–49.
- ^ Jönsson, K. I. & R. Bertolani (2001). Facts and fiction about long-term survival in tardigrades. Journal of Zoology 255: 121–123.
[edit] External links
- Tardigrada Newsletter
- Tardigrades - Pictures and Movies
- The Edinburgh Tardigrade project
- Tardigrade Appreciation Headquarters
- Tardigrades (English/German)
- The incredible water bear!
- Tardigrade video
- Tardigrade Reference Center
