Tardigrades (Latin: Tardigrada) also known as water bears are microscopic animals (their body size vary from 0.05 to 1.2 mm). In the contemporary taxonomy they are considered as one of the invertebrate Phyla (that means they are not insects, mites or crustaceans, they are just tardigrades). There are over 800 described species so far, however it is estimated that the total number of tardigrade species may exceed 10 000.

All tardigrades have four pairs of legs and a complicated feeding (buccal) apparatus. They come in the variety of colours (transparent, white, red, orange, yellow, green, purple, black). Water bears show a variety of reproductive modes (including parthenogenesis and hermaphroditism). Some species lay richly ornamented eggs. Click here for some photos of terrestrial tardigrades and their eggs.

Tardigrades on moss - a plate from the book "Tardigrada" by Ernst Marcus, 1929 (click the miniature to view the full size scan).

One of the main reason for which water bears are considered weird, is their ability to survive in extreme conditions. When they are in the active stage (i.e. when they crawl around, eat and reproduce) they are not more tough than any other animal. However, when the conditions worsen water bears can dry or freeze - this peculiar form of existence is called anabiosis. When anabiotic, tardigrades look like they are dead - they don't move, don't breath and they metabolism is undetectable. In this stage they can be exposed to very high X-ray radiation of 570 000 rads (500 would kill a human!), very high pressure or vacuum (even space vacuum, which means they could travel in the interplanetary space), finally they can withstand temperatures as high as over 150 degrees centigrade and as low as minus 272.8 degrees centigrade (almost the absolute zero)!

Thanks to those record-breaking properties, Tardigrades can inhabit a huge variety of environments - from the ocean floors (4 km below the sea level) up to the high mountain tops (even higher than 6 km above the sea level). Although all tardigrades in the active stage need water, apart from the marine and freshwater environment, they can be found in terrestrial habitats too. The only requirement is that these habitats have liquid water at least seasonally (like moss cushions, lichens, soil, even ice surface). As you may already suspect, Tardigrades live on all continents.

Because when tardigrades are in the anabiotic stage, they are "dead but still alive", they can - as Dr. Robert Schill said at the X International Symposium on Tardigrada - "teach us something very important about the nature of life".

Tardigrada in several languages (without non-English characters): Czech: Zelvusky, Danish: Bjornedyr, Dutch: Beerdiertjes, French: Tardigrades, German: Bartierchen, Hungarian: Medveallatkak, Lithuanian: Letunai (or Vandens meskuciai), Polish: Niesporczaki, Spanish: Tardigrados, Suomi (Finnish): Krahukaiset.

You can find more information about Tardigrada on websites listed in the section 'Links'. We also recommend this paper (with a long list of references) as a source of basic scientific information on freshwater/terrestrial tardigrades:

Nelson, D.R. & Marley, N.J. (2000) The biology and ecology of lotic Tardigrada. Freshwater Biology, 44(1): 93-108.