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Tardigrades are often called Water Bears, or Moss Piglets.  They are so odd that an extraterrestrial origin has even been suggested.  There is more about this later in the article.

They are normally referred to as being microscopic, but the biggest of them are just within what a Human can see without aid, being up to about one millimetre long.

Eight Legs

Tardigrades have eight stumpy legs.  They walk over sand grains and plants looking like tiny bears.


Some tardigrades eat small animals and algae whole, while others suck out the cell contents of these things.

Discovery and Classification

They were discovered by an amateur microscopist in 1773.  Biologists had some trouble trying to fit this new animal into their classification systems.  Now they have been given their own phylum, the Tardigrada.  When classifying living organisms, the highest classification is the Kingdom, for example the plant kingdom or the animal kingdom.  Apart from subkingdom, the second highest classification is the phylum.  The fact that the Tardigrades have been given such as high classification (Vertebrates only rate the classification of sub-phylum) suggests that they are not closely related to anything else. 

However, they have some similarities to both the annelids and the arthropods.  This suggests that they are of Earthly origin. Proponents of Panspermia might disagree with this.

There are about 750 species of tardigrade that have been described.  It is absolutely certain that there are others not yet scientifically described, and it is very likely that there are a lot more undiscovered ones than discovered tardigrades.


Tardigrades are all over the place.  There are probably millions outside my window in our front yard, living in moss and other things.  The fact that most of the yard gets hot and dry in the summer would not matter to these creatures.

Tardigrades live in the Antarctic dry valleys and the depths of the Atlantic Ocean.

Many of them are aquatic, living in fresh or saltwater, but others live in dry areas, but to be active they need some liquid water.


Tardigrades can dry out and be apparently dead, but will revive when water in available again. This ability is called cryptobiosis. In this state, Tardigrades are resistant to a wide range of things including cold down to at least minus 200 degrees C (minus 328 degrees F).  They can also survive heating to 151 degrees C (304 degrees F) for a short time, and radiation thousands of times what a Human can survive.

Tardigrades in Space

Tardigrades are so tough, especially in the dry state that the possibility was considered of them being able to survive in outer space.


A project was stated to test this, called Tardigrades In Space or TARDIS for short.

It was found that the dry tardigrades had no trouble with the vacuum of space, but there was some effect of the radiation, although many of then even survived this.  The two species tested were different in this regard.

There is little doubt that if a piece of the Earth was knocked off by a meteorite with tardigrades in it that the tardigrades could survive in space and if there were things like algae with them, they could colonise suitable habitats on other planets or moons.

Survival on Mars

A meteorite from the Earth could take a little as a year to reach Mars, or it could take longer, even billions of years.  No habitat so far discovered on Mars could support tardigrades, but our exploration of the Red Planet is not very well advanced.

By Goldstein lab - tardigrades (originally posted to Flickr as water bear) [CC-BY-SA-2.0 (], via Wikimedia Commons.
This file is licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license.
Hypsibius dujardini
By Rpgch at en.wikipedia [CC-BY-SA-2.5 (], from Wikimedia Commons.
This file is licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license.
Scanning electron micrograph of an adult tardigrade (water bear).