What are Water Bears?

Mary McMahon

Water bears, also known as tardigrades, are unique microscopic organisms which can be found in almost every environment on Earth. Despite the fact that they are widely distributed and they demonstrate an incredible ability to survive in intense conditions, most people don't know very much about them. If you have a microscope, you can probably find some local water bears.

Water bears can often be found in samples of lichen.
Water bears can often be found in samples of lichen.

The first water bears were observed in 1773, by Johann Goeze, a German clergyman and zoologist. He observed small creatures in water samples when he magnified them, and he called them little “water bears” after their lumbering movements and bear-like appearance. Under high magnification, these creatures really do look remarkably like bears, although they have two extra sets of feet and obviously segmented bodies. Their faces are also quite expressive, and some beautiful prints of magnified tardigrades can be found in natural history museums.

A small clump of moss may have a few thousand water bears.
A small clump of moss may have a few thousand water bears.

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Some people call these animals moss piglets, since they are often found in samples of lichen and moss. Scientists believe that water bears are related to arthropods and annelids, a large phylum of segmented worms. An adult tardigrade has five body segments, including the head, along with four sets of legs. The feet have distinctive claws which show up remarkably well under magnification.

The thing that makes water bears remarkable is their ability to survive in extreme situations. These creatures can tolerate far more radiation than most other organisms can, and they can also survive in temperature extremes, high pressure areas like hydrothermal vents deep under the ocean, and in the vacuum of space. They can also be dehydrated for up to a decade without any ill effects. Tardigrades also appear to be capable of dealing with many environmental toxins.

Because of their amazing versatility, water bears can be found in incredibly diverse environments all over the world. Scientists have also conducted tests with these hardy animals, subjecting them to extreme pressure, intense cold, and severe radiation exposure. The average water bear can take as much as 570 times the amount of radiation it takes to kill a human.

Water bears have another interesting trait; they are eutelic organisms, meaning that mature adults retain the same number of cells throughout their lives. Once a tardigrade reaches maturity, growth is accomplished by cell expansion, rather than cell division. A typical adult water bear has around 40,000 cells in its body.

Tardigrades are also known as "water bears" because of the way they appear under a microscope.
Tardigrades are also known as "water bears" because of the way they appear under a microscope.

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Discussion Comments


While I find microbiology weird and usually sort of gross to look at pictures of, I actually think water bears are sort of cute. If they had a different name, though, like water slugs, I imagine I wouldn't. Names are important.


I had never even heard of a water bear until we studied them in microbiology. Water bears must have moisture to stay alive but they can hibernate for up to a century if their habitat dries out. When they are in their hibernating state, they are called tuns. Recent research on the water bears indicates that they can withstand pressure of 6000 atmospheres which is almost 6 times the pressure of water in the deepest ocean.

They can also survive the vacuum of open space and solar radiation combined for up to ten days.

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