IF SOMEONE WERE TO ASK YOU IF you had ever seen a water bear, you would probably give them a rather disbelieving look. Chances are that not one person in 10,000 has even heard of water bears, leave alone seen one. One reason for this is that you need a microscope to see these delightful denizens of water films. Known to specialists as "tardigrades", these bizarre animals occupy a variety of habitats. A few kinds inhabit the space between grains of sand in the sea or on beaches; some inhabit similar habitat in soils. Some freshwater species live in the bottom detritus, or on aquatic algae or mosses. The easiest place to find them is in moss soaked in water. Water bears are universally distributed, being found from mountain heights to ocean depths, and from the icy poles to tropical climes.
The majority of tardigrades, however, live in the water films surrounding the "leaves" of land-bound mosses and lichens. The average tardigrade is about half a millimeter in length, but some giants grow to the enormous size of 1.2 millimeters.
You may wonder what water bears eat. There have been a few reports of tardigrades attacking nematodes, rotifers and even other water bears. But the vast majority of kinds are specially designed to eat mosses and algae. However, they don't eat in the normal sense, being more like mosquitoes designed to suck. They are equipped with a magnificent little apparatus with which they pierce individual plant cells one by one and suck out the contents. During feeding, the mouth is placed against a victim plant cell, and two superfine, sharper than needle-sharp stylets are pushed against the cell wall and puncture it. The contents of the cell are then sucked out by a special organ called the pharyngeal bulb.
In the scientific world, water bears are best known for their logic-defying ability to withstand extreme drying and exposure to incredibly low temperatures. This ability is given the grandiose name of cryptobiosis. During periods of dry weather, when moss becomes dust dry, the tardigrade inhabitants pull in their legs, lose water, shrivel up and enter a cryptobiotic state. Note the following amazing statement:
The life processes in cryptobiotic organisms are not readily detectable, and… the properties of such organisms while they are in suspended animation defy some of the standard definitions of what being alive entails. Nonetheless, the animals are far from dead in their dried state… (Crowe, J. H. and Cooper, A. F. Jr. Cryptobiosis, December 1971, Scientific American Offprints 1237, p. 2).
In normal life, water accounts for 85% of the body weight. In cryptobiosis, water makes up only 3% of the body weight!
Some fascinating things have been discovered by researchers. It has been estimated that a tardigrade would have a life span of less than a year if it never entered the cryptobiotic state. However, one that alternates active with cryptobiotic periods might survive for as long as sixty years!
It is not known just how long tardigrades can exist in a state of suspended animation. But some known cases defy imagination. For instance, a museum specimen of moss that had been kept dry for one hundred and twenty years yielded a number of tardigrades. When the animals were moistened, a few of them revived, but all died within a few minutes.
Just as amazing as their invincibility against drying is their seeming invulnerability to cold. In experiments, tardigrades survived when they were chilled, not just for a few minutes but for days, to minus 200 degrees Centigrade! More recently, Henri Becquerel, from the University of Paris, beat all records for resistance to cold. (Not of himself, but of tardigrades). With cryogenic apparatus, he found that water bears could survive exposure to temperatures on the very brink of Absolute Zero — 0.008 degrees Kelvin, in fact! This is beyond comprehension.
Cryptobiosis is a mystery. It is widely held that water is essential to the maintenance of three-dimensional structures in many large biological molecules, including certain proteins and DNA. If these molecules are dehydrated, some of them are irreversibly damaged. Also, water is essential in maintaining the structure of cell membranes. One should expect that drying would do irreversible harm to these structures. Tardigrades lose so much water that damage seems inevitable. Yet, when the rain comes, water bears bounce happily back to life. God knows how.