By Leslie Mullen
All life on Earth is based on water. The water in our cells constantly needs to be replenished, and if water is not available, we could die. Some organisms, however, have evolved to adapt to a loss of water ñ in essence, to cheat death until water reappears.
One such organism is a microscopic animal called a "tardigrade." J.A.E. Goeze, who published the first paper on tardigrades in 1773, said, "Strange is this animal ... because it resembles a bear in miniature." Because of this description, and because tardigrades normally live in water, they are also known as "water bears."
Tardigrades look more like a candy Gummy Bear than a grizzly bear ñ they have the bright orange, red or green colors of Gummy Bears, and a gummy surface texture. Their inflated round bodies have four pairs of stubby little legs. They use these clawed limbs to walk, grasping onto lichen or moss as they amble along.
If their environment dries up, the tardigrades undergo a process called anhydrobiosis (life without water). A sugar called trehalose moves into their cells to replace the lost water, and the tardigrade curls into a little ball called a "tun." Their metabolism lowers to a death-like 0.01% of normal, or is entirely undetectable. Depending on how long they have been in an anhydrobiotic state, tardigrades can become active again within a few minutes to a few hours after exposure to water.
Anhydrobiosis is just one type of a range of adaptable techniques called cryptobiosis. The other types of cryptobiosis are cryobiosis (cold temperatures), osmobiosis (salt water), and anoxybiosis (reduction of oxygen). Cryptobiotic animals were first documented in 1702 by Anton van Leeuwenhoek, when he observed tiny life forms in sediment collected from rooftops. He dried the "animalcules" to preserve them, and when he later added water he saw the creatures begin to move around. (The animals van Leeuwenhoek studied were probably rotifers ñ a microscopic organism that uses a wheel-like organ to swim and feed).
Because of their ability to withstand hostile conditions, tardigrades and other cryptobiotic organisms are of interest to astrobiologists. Some tardigrades can survive in temperatures as low as minus 200 degrees Celsius (minus 328 F). Others can survive temperatures as high as 151 degrees C (304 F). Tardigrades can survive the process of freezing or thawing, as well as changes in salinity, extreme vacuum pressure conditions, and a lack of oxygen. Tardigrades also are resistant to levels of X-ray radiation that are hundreds of times more lethal to humans and other organisms.
This resilience stems from the tardigradeís ability to survive without water. While the water in our cells is necessary for our survival, it also makes us extremely vulnerable. Whatever affects water also affects the cells of our bodies. When tardigrades are in a state of anhydrobiosis ñ when their cells contain no water ñ they become resistant to many of the things that normally would be fatal to water-based creatures."When tardigrades dry up into the little barrel called a tun, they become amazingly resistant to just about everything," says Jim Garey, an evolutionary biologist at the University of South Florida. "Tardigrades can survive such extreme conditions for long periods of time. Live tardigrades have been regenerated from dried-up mosses more than 100 years after they were collected."
There are other creatures on Earth, called "extremophiles," that are able to live in extreme environmental conditions. Extremophiles can live in boiling hot, extreme cold, salty, and dry conditions ñ in short, all the conditions that tardigrades can survive. Tardigrades, however, are not extremophiles.
"Tardigrades are not true extremophiles because they are not adapted to live in extreme conditions," says Garey. "They can merely survive exposure to such conditions. The longer they undergo such exposure, the greater their chance of dying. Tardigrades are always waiting for something better."
Tardigrades, rotifers, nematodes and other microscopic cryptobiotics were intensively documented during the eighteenth century. Since then, however, interest in these tiny creatures has waned. One problem with studying tardigrades today, says Garey, is that some of the information about these creatures is over 100 years old ñ collected long before the advent of modern scientific techniques and instruments.
So far, about one thousand tardigrade species have been documented, but that number may be misleading. Some tardigrade species may have been "discovered" more than once by different sources. Some species may have been lumped into more common categories because of the poor descriptions typical of earlier studies.
"Most of us who describe new species recognize that - since there is no international database yet - we canít be sure of the count," says William Miller, a biologist at Chestnut Hill College in Philadelphia. "The taxonomy of tardigrades is continuing to evolve. As we become more detailed, we are discovering more differences. As we explore new places on the Earth, we discover more new species."
Although their most typical home is the thin film of water that
coats mosses and lichens, tardigrades have been found in a vast range of habitats - in marine, fresh water, and semi-aquatic terrestrial environments ranging from tropical rainforests to the Arctic Ocean. Scientists have reported finding tardigrades in hot springs, on top of the Himalayas, and under a 5 meter layer of solid ice.It is thought that tardigrades are widely distributed because they are carried on the wind, still clinging to their little bits of dried moss. This theory seems to be supported by the discovery of tardigrades on remote volcanic islands, where they could only have been deposited by wind or birds.
Garey believes that the tardigradeís preference for mosses and lichen is due to the wet/dry cycles these plants undergo. In areas that donít experience wet/dry cycles, tardigrades tend to be out-competed by other animals like nematodes. But nematodes are not as good at surviving without water as are tardigrades.
"In mosses and lichen, with their wet and dry cycles, tardigrades have found their ecological niche, " says Garey. "While other organisms like nematodes and rotifers can also undergo anhydrobiosis, tardigrades are the most efficient at the process - they do it best."
Like other animals, the ancestors of tardigrades probably first appeared during the Cambrian explosion, 540 million years ago. Tardigrades share a common ancestor with arthropods, nematodes, and onychophorans ("velvet worms"), because these animals all grow by molting (shedding their cuticle outer layer). These molting animals are classed together under the name Ecdysozoa.
Arthropods are a hugely diverse group of organisms ñ they include such different animals as centipedes, lobsters, and fruit flies - but they all have jointed appendages and a hard exoskeleton. Like arthropods, tardigrades have leg-like appendages that they use to move around - but unlike arthropods, tardigrade appendages are unjointed.
Tardigrades and nematodes both have a spear-like mouth part called a "stylet" that they use to pierce their prey and suck their juices as though through a straw. But tardigrades have two stylets, while nematodes only have one (arthropods, meanwhile, have jaws).
Tardigrades are thought to be the most closely related to onychophorans, caterpillar-like invertebrates that share traits with both arthropods and annelids (worms). Both tardigrades and onychophorans have unjointed appendages that terminate in claws. But tardigrades lack the antennae, jaws, and respiratory system of the onychoporans.
While tardigrades have been classified as nematodes, arthropods or onychophorans in the past, today tardigrades have their own separate phylum, Tardigrada.
Scientists arenít sure exactly when the tardigrade phylum first emerged. For one thing, there arenít many tardigrade fossils.
"A few examples of tardigrades that look just as they do today have been discovered encased in cretaceous amber," says Miller. "That would place them at about 100 million years old in their present form. Few other records exist because of their small size and soft bodies; they do not fossilize well and are even more difficult to see."
Garey is studying the DNA of tardigrades to pin down where they belong in the evolutionary diagram called the Tree of Life. An organismís position in the Tree can indicate when they appeared in the course of evolutionary history. But placing a precise date on their emergence has proved to be difficult.
"Itís hard to really date the emergence of tardigrades, nematodes, and other such animals," says Garey. "Nematodes, for instance, have faster evolution rates than other animals. Also, dating based on nucleotide substitutions ñ so-called ëmolecular clockí dating ñ results in dates ranging as far as 700 million to 1.5 billion years ago."
By studying tardigrade DNA, Garey and his team also hope to figure out how all the different tardigrade species are related to each other.
"The tardigrades most likely originally evolved in the ocean, and only later colonized fresh water and terrestrial habitats," says Garey. "One group of tardigrades ñ the Heterotardigrada - is mostly marine but has some terrestrial members. The other group of tardigrades ñ the Eutardigrada - is exclusively terrestrial. An interesting question is whether the eutardigrades evolved from the terrestrial heterotardigrades or whether terrestrial tardigrades evolved twice."
The ability of terrestrial tardigrades to undergo cryptobiosis has led some to suggest that they could be transferred by Panspermia - that is, between different planets via meteorites. Although he finds the concept highly unlikely, Garey says, "If you had to pick an animal candidate, Iíd pick a tardigrade."
Perhaps future research will lend some credibility to this idea. Miller is mentoring a group of students in the NASA Student Involvement Program, and they have proposed a project to fly tardigrades on the space shuttle. This project could determine how tardigrades are affected by low gravity and test whether tardigrades can survive in space.
What Next?
The dispersion of tardigrades is not well understood, and demands closer study. For instance, tardigrades seem to be more common in Temperate and Polar Regions than in the Tropics, but no one knows why. And some habitats that would seem to suit tardigrades perfectly are found not to support any tardigrade populations.
"In the wild, populations of tardigrades are patchy," says Garey. "You might find one area that is rich in tardigrades, while another nearby is completely barren. We donít know why, so more research needs to be done in this area."
Because most of the research on tardigrades has been done in Europe, tardigrade populations in South America, Australia, Asia, Africa, and North America are not as well documented. The same is true for the oceans: marine tardigrades have a higher diversity, and therefore may have more species, than tardigrades on land, but so far the marine environment is mostly unexplored.
"We know of 140 marine tardigrade species, but there are probably thousands more," says Garey.
To study the ecology of tardigrades in the wild, you first have to find them. Their small size makes identifying and collecting tardigrades a challenge, but Garey and his team are developing methods to extract DNA from the sediment in which the microscopic animals live.
Miller, meanwhile, is working on the description of several new species of tardigrades, and has a number of canopy, ecological, diversity, and taxonomic projects under way. He also is working on National Science Foundation grant proposals to study the tardigrades of China, Australia, and North America.
