Water bears are a microscopic, semi-transparent, multi-cellular, species living somewhere near you. The real name for water bears (and that of their phylum) is Tardigrade. They are one of the most interesting, yet least-known, creatures that live in your backyard.

Tardigrades live mostly in moss and lichen. They need a film of water around them or they can't breathe, hence they are much more likely to be found in a rainy place, like Oregon, than a dry place, like Texas.

Tardigrades look an awful lot like bears. They are brownish and have an obvious front and back, based on which way they walk around. The obvious difference is that they have eight legs, and they are almost transparent (which means you can see their guts - euww!). On each of their eight legs, they have up to eight claws. That is basically all you can see with the naked eye, a brownish body and its guts, which look like a couple of circles. They can grow up to about 1mm and can be seen more clearly through a microscope.

Water bears feed on the fluids in plant and animal cells. Sometimes they eat entire other organisms, like rotifers and even, on occasion, other water bears. They can pierce the walls or membranes of cells and they possess a tube to eat the internal contents of their prey.

Water bears are both a male and a female tardigrades (though some subspecies of tardigrades only have females). After the eggs are laid, and hatched, out comes another adult tardigrade. There is no larva stage. Also, they never gain any cells after birth. The large distribution of tardigrades can be explained by the fact that their clusters of eggs are light enough to be carried by wind or by animals.

Though tardigrades need a film of water to breathe, if they get dry they go into something very much like hibernation. Except that they can stay in it for 120 years and then, with just a single drop of water, they can be revived and start walking again, just like new. This is called cryptobiosis. What really happens when a water bear goes into cryptobiosis is that it lowers its metabolism to 0.01% of normal. Also, it lowers its water content to less than 1%. While in a state of cryptobiosis, a tardigrade can resist temperatures of up to 151°C and down to minus 200°C. They can also survive in a vacuum (like space, not the household appliance). This is one of the coolest facts about the tardigrade.

Living in environments as diverse as the frozen artic sea and the moss of the tropical rain forests, the tardigrade is a very interesting animal. With 400 species of tardigrade, we can be amazed for a long time.

Further Reading

You can find more information on this and many other species at the Oregon Museum of Science and Industry site.