Water Bears

Meet the Water Bear

A tardigrade is a microscopic animal with a plump, segmented body and eight stubby legs, and each leg ends in a set of tiny claws. Most adults measure only about half a millimeter at the very most, which is roughly the size of a grain of fine salt, so a person usually needs a microscope to see one properly. Because its slow, lumbering walk resembles that of a bear, a German scientist named Johann Goeze called it the little water bear in 1773, and a few years later an Italian scientist named Lazzaro Spallanzani gave it the name tardigrade, which means slow stepper. Tardigrades are not insects and they are not worms. They form a phylum of their own, which is one of the major branches of the animal kingdom, and their closest relatives include the arthropods and the velvet worms. Altogether, these relatives make up a gigantic group of animals that includes every insect, spider, and crab on Earth, which makes the tardigrade a distant cousin of an enormous number of familiar creatures.

Scientists have identified more than a thousand species of tardigrade, and the animals live almost everywhere on the planet. They have been collected from mountaintops in the Himalayas, from the sandy bottoms of the ocean, from hot springs, from polar ice, and from the damp moss that grows on a city sidewalk. Most of them need a thin film of water around their bodies in order to move, eat, and breed, so the moist spaces inside moss, lichen, and leaf litter are particularly good places to find them. Some feed by piercing the cells of plants or algae with a pair of sharp, needle-like stylets and sucking out the contents, while others hunt even smaller animals, such as rotifers and nematodes. Many females lay eggs, and a few species can reproduce without any males. Their eggs are often decorated with intricate spikes and patterns, and scientists sometimes use these distinctive shapes to tell the different species apart.

Under a microscope a tardigrade looks like a tiny balloon with legs, and it has a simple brain, a mouth at the front of its head, and a stomach that is often visible through its transparent skin, so a person can sometimes see the food moving inside it. Its body is covered by a flexible outer layer called a cuticle, which the animal sheds and replaces several times during its life as it grows. Strangely enough, a tardigrade does not breathe through lungs or gills, because oxygen simply soaks inward through its skin, which is one reason the animal must stay so small and why it needs to keep that skin wet. Fossils of tardigrades that are about five hundred million years old have been found, which means that the group has existed for far longer than the dinosaurs. That remarkable history suggests that tardigrades have found a dependable way to endure the dramatic changes that have transformed the planet over hundreds of millions of years.

Anyone can look for tardigrades at home with a little patience. The usual method is to collect a small clump of moss or lichen from a wall or a tree, soak it in clean water for a few hours, and then squeeze the water into a dish, which can be examined under a microscope at fairly low magnification. With luck, a slow-moving creature will come into view, clutching at the dish with its claws as it lumbers along. A tardigrade usually moves at a speed of only a few body lengths a minute, so it takes a long time to cross a drop of water. Careful observers sometimes see one stop to feed, or tumble over, or tug at a speck of debris that is much bigger than its own head. Patience is essential, because the animals are so transparent that they are easy to overlook against a bright background.

Built to Survive

The real fame of the tardigrade comes from its ability to survive conditions that would kill nearly any other animal. When its surroundings begin to dry out, a tardigrade pulls in its head and legs, curls into a shriveled little barrel called a tun, and loses almost all of the water in its body, so that the water it keeps may amount to only a few percent of its weight. In this condition its metabolism, which is the set of chemical reactions that keeps a creature alive, slows to a level that is very hard to detect. Scientists call this state of suspended life cryptobiosis, which means hidden life. The animal is not dead, and it is not exactly alive either, since it simply waits, sometimes for years, until water returns. Remarkably, a tun can remain in this condition through periods of drought that would be fatal to nearly every other animal in the same patch of moss.

When water finally touches a tun, the revival can begin within minutes. The animal swells, extends its legs, and starts to move again, usually in an hour or less, and it may soon be eating and laying eggs as though nothing had happened. Researchers have revived tardigrades from dried museum samples that were many years old. In 2016, a Japanese team reported that tardigrades from Antarctic moss, which had been frozen for about thirty years, were brought back to life, and one of them even produced eggs that hatched. Scientists believe that a tardigrade protects itself while it dries by making special sugars and proteins that act like a glass, holding the delicate parts of its cells in place until water allows them to function again. Researchers are still investigating exactly how this protective chemistry works, and every year they uncover additional details about the molecules involved. It is a clever trick of nature, and it works very well.

A dried tardigrade can withstand extreme temperatures that would destroy ordinary animals. In laboratory tests, tuns have survived being chilled to within a few degrees of absolute zero, the coldest temperature that is possible anywhere in the universe, at about minus 273 degrees Celsius. They have also endured being heated briefly to about 150 degrees Celsius, which is well above the boiling point of water. Tardigrades can survive pressures that are many times greater than the pressure at the bottom of the deepest ocean, and some can tolerate doses of radiation that would be deadly to a person. Experiments have exposed them to radiation levels hundreds of times higher than the amount that would kill a human being, and a considerable number of the animals still lived. Researchers have identified a protein in one species, which they call Dsup, short for damage suppressor, that appears to shield the animal's DNA from radiation damage.

In 2007, tardigrades became the first animals known to survive exposure to open space. European scientists sent dried specimens into orbit on a spacecraft called FOTON-M3 and exposed some of them directly to the vacuum of space for about ten days, along with the harsh ultraviolet light of the sun. When the capsule returned to Earth, researchers added water, and many of the animals that had been shielded from the strongest light revived, while a smaller number of those exposed to the full glare of the sun also came back, and some of those later laid eggs. This experiment demonstrated that a complex multicellular animal can endure conditions that seem impossible, although it does not mean that tardigrades live in space, only that their dried bodies can survive a short visit. The tardigrades themselves were not harmed in any lasting way by the journey, and the scientists regarded the outcome as a genuine surprise, since nobody had previously demonstrated that an animal could withstand such an environment.

It is worth noticing that tardigrades are not truly indestructible, because an active tardigrade that has not dried out is a delicate animal, and it can be killed by very high heat, by strong chemicals, or by a lack of water if it dries too fast to form a tun. Only the tun stage can resist the most extreme conditions, and even then there are limits to what the animal can survive. Nevertheless, the tardigrade has taught scientists a surprising amount, because the sugars and proteins that protect it are being studied as possible ways to store vaccines and medicines without refrigeration. A creature smaller than a grain of salt, living quietly in a patch of moss, may hold a few secrets that could someday help people in ways that no one has yet imagined. An active tardigrade does not live forever, and it can be harmed like any other animal.

Tardigrades also remind us how much of the living world is invisible to ordinary eyes. A single handful of moss from a forest floor or a rooftop may contain hundreds of these animals, along with countless bacteria, fungi, and other microscopic creatures, all of them carrying on their lives within a landscape that is only a few inches across. For a tardigrade, a tuft of moss is a vast and complicated jungle, and a single raindrop is as large as a lake, which is an extraordinary way to imagine the ordinary places around us. The most amazing fact about the water bear is perhaps this one: it can pause its life for years, set aside nearly all of its water, and then simply start again. Few other animals can do anything remotely like it, and scientists are still discovering the extraordinary chemistry that makes the performance possible. People first described water bears more than two hundred and fifty years ago, and they are still full of surprises.