Slow walkers

1 November 2010

Tardigrades occur worldwide. Members of this phylum are often referred to as "water bears" because of their bear-like appearance and slow lumbering gait, though in Latintardigrada means "tortoise-like movement". Tardigrades were first described by German zoologist Johann August Ephraim Goeze in 1773 (kleiner Wasserbär = little water bear). The name Tardigrada means "slow walker" and was given by Italianbiologist Spallanzani in 1777. Tardigrades have been relatively neglected byinvertebrate zoologists and so far about 800 species have been described from marine, freshwater and terrestrial habitats.

Although most tardigrades live in terrestrial habitats, they are all aquatic because they need a layer of water around them in order to be active. They live in a wide variety of habitats including soils, leaf litter, moss, lichens, flowering plants, algae and even in hot springs. Some tardigrades live on the legs of isopods, on the gills of mussels or as parasites on the epidermis of barnacles orholothurians. They must live in areas with sufficient aeration because they respire across their cuticle and are sensitive to low oxygen levels. In moss, tardigrades can reach densities of 2,000,000 per square meter!

Tardigrades feed on plant or animal cells, bacteria, algae, decaying plant matter and some of them are carnivorous on small invertebrates. They feed by piercing the cell wall (or cell membrane) with their stylets and using their muscularpharynx to suck out the fluids inside the cell. The foregut and hindgut are lined with extensions of the cuticle.Digestion takes place in the midgut because it has only a single layer of epithelial cells and is not lined with cuticle.

Tardigrades are preyed upon by nematodes, other tardigrades, mites, spiders, insect larvae, parasitic protozoa, andfungi.

When environmental conditions start to deteriorate, tardigrades enter a latent state until conditions improve. A latent state is a state in which metabolism, growth and reproduction are reduced or cease temporarily while resistance to environmental extremes (cold, heat, drought, chemicals, ionizing radiation) increases. Tardigrades in a latent state can survive temperatures up to 272°C, high vacuums, ionizing radiation, outer space, and long periods with no oxygen. One tardigrade was revived from its latent state after 120 years! Since aging ceases in the latent state, entering a latent state can greatly increase the life span of the animal. Tardigrades have five routes which they employ to enter latency: encystment, anoxybiosis, cryobiosis, osmobiosis and anhydrobiosis.

Yolk amd myself were deeply impressed...

Citation

Biodiversity Institute of Ontario (Lead Author);Paul D. N. Hebert (Contributing Author);Leszek Bledzki (Topic Editor) . "Tardigrada". In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth August 25, 2008; Last revised Date August 25, 2008; Retrieved November 1, 2010 <http://www.eoearth.org/article/Tardigrada>