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Tardigrades (/ˈtɑːrdɪˌɡreɪdz/), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbär ("little water bear"). In 1777, the Italian biologist Lazzaro Spallanzani named them Tardigrada (/tɑːrˈdɪɡrədə/), which means "slow steppers".
Johann August Ephraim Goeze originally named the tardigrade Kleiner Wasserbär, meaning "little water-bear" in German (today, they are often referred to in German as Bärtierchen or "little bear-animal"). The name "water-bear" comes from the way they walk, reminiscent of a bear's gait. The name Tardigradum means "slow walker" and was given by Lazzaro Spallanzani in 1777.
What are water bears? Are they really bears? This question is easy to answer: no, the only thing that water bears and bears have in common is the fact that both are animals. The shape of a water bear slightly resembles that of true bears, such as the polar bear or the grizzly, but they are most closely related to the huge group called the arthropods, which includes insects, spiders, millipedes, and crabs. However, you cannot see a water bear with the naked eye, because these animals are very tiny. They usually grow to
Just like almost any other creature on our planet, water bears must eat food and breathe air to generate the energy needed for their cells to divide and their bodies to grow. In contrast to true bears, water bears are just too tiny to eat salmon or seals. Honey is also not on their menu. Nevertheless, water bears basically eat everything. While they mainly prefer vegetarian foods like plants and algae, they will also eat microscopic animals.
Unlike most other animals, the bodies of water bears are created following a specific plan. Every type of adult water bear even has exactly the same number of cells. Their cells are continuously dividing, but the water bear is covered by a non-growing and non-flexible sheath, or protective outer covering. As soon as the sheath becomes too tight, water bears will shed the sheath in a process called molting, similar to spiders and snakes. Although both humans and water bears need oxygen to survive, water bears do not breathe the way we do. In fact, they do not even possess respiratory organs like lungs. Water bears take up air through the surfaces of their bodies, just like insects. Water bears can even stop breathing and eating for some time, similar to the process of hibernation that allows other animals, such as polar bears, to slow down their bodily processes to survive the winter months. However, water bears are even more impressive, because not only can they sleep for a couple of months, but they can also become extremely old and thrive in the most extreme places on earth.
Water bears do not only survive the coldest cold or the hottest heat without food and without air to breathe, but they can also go without water and they are resistant to radiation. Since those extreme conditions exist in space, scientists asked themselves whether water bears might even be able to travel in space (Figure 2C). Scientists knew that the high pressure present in the deep sea could be tolerated by water bears, but in space there is a vacuum, with lower pressure compared to earth. Nevertheless, several species of water bears were sent into space and all of them returned home in healthy condition. Moreover, more than 1,000 water bears in dormancy were crash-landed on the moon as passengers of a spacecraft in 2019. It is expected that most of these robust animals have survived the crash and could be revived by water and oxygen in the future.
One part of the genome of water bears has recently been identified and reproduced in a laboratory . When this factor was added to human cells grown in the same laboratory, the human cells tolerated more intense radiation than did human cells without the water bear factor. These early experiments may lead to future applications of water bear factors that could not only be used to protect the human cells against radiation, but possibly also to stabilize drugs or to increase the resistance of crop plants to environmental conditions like drought.
So, now you can see that those little water bears are quite different from the bears we know well. We have learned from these animals that they not only tolerate the most extreme conditions on our planet, they are even capable to survive in Space. Because of these unique properties, water bears are fascinating and among the most interesting model organisms for us to further study.
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Tardigrades (Tardigrada), also known as water bears or moss piglets, are a phylum of small invertebrates. They were first described by the German pastor J.A.E. Goeze in 1773 and given the name Tardigrada, meaning "slow stepper," three years later by the Italian biologist Lazzaro Spallanzani. Tardigrades are short (0.05mm - 1.2mm in body length), plump, bilaterally symmetrical, segmented organisms. They have four pairs of legs, each of which ends in four to eight claws. Tardigrades reproduce via asexual (parthenogenesis) or sexual reproduction and feed on the fluids of plant cells, animal cells, and bacteria. They are prey to amoebas, nematodes, and other tardigrades. Some species are entirely carnivorous! Tardigrades are likely related to Arthropoda (which includes insects, spiders, and crustaceans) and Onychophora (velvet worms), and are often referred to as a "lesser known taxa" of invertebrates. Despite their peculiar morphology and amazing diversity of habitats, relatively little is known about these tiny animals. This makes them ideal research subjects for which students and amateur microscopists may contribute novel data to the field.
Tardigrades have been known to survive the following extreme conditions: Show credits Hide Scanning electron micrograph of Tardigrada, or water bear. Image Credit: Rick Gillis and Roger J. Haro Department of Biology University of Wisconsin - La Crosse.
Animals as small and soft as tardigrades seldom have legs and almost never bother walking. For example, round worms of similar size and body type thrash about, slithering their doughy forms over unpredictable substrates. Yet the water bear, a micro-animal so distinct that scientists were forced to assign it to its own phylum, uses eight stubby legs to improbably propel itself through marine and freshwater sediment, across desert dunes, and beneath the soil.
Now, a new study in PNAS analyzes tardigrade gaits and finds that water bears walk in a manner most closely resembling that of insects 500,000 times their size. The discovery implies the existence of either a common ancestor or an evolutionary advantage that explains why one of the smallest and squishiest creatures evolved to walk just like larger, hard-bodied insects.
Another possibility is that there is no ancestral connection between tardigrades and arthropods, but that the unrelated groups of organisms independently arrived at the same walking and running strategies because they were evolutionarily advantageous. Perhaps the best way to navigate unpredictable terrain with a microscopic body is to plod like a water bear.
Tardigrades, often called water bears or moss piglets, are near-microscopic aquatic animals with plump, segmented bodies and flattened heads. They have eight legs, each tipped with four to eight claws or digits, and somewhat resemble the hookah-smoking caterpillar from "Alice in Wonderland." Though tardigrades are disarmingly cute, they are also nearly indestructible and can even survive in outer space.
Tardigrades were discovered in 1773 by the German zoologist Johann August Ephraim Goeze, who dubbed them "little water bear." Three years later, Italian biologist Lazzaro Spallanzani named the group "Tardigrada," or "slow stepper," for their toddling gait, according to the Science Education Resource Center at Carleton College (opens in new tab) (SERC). There are currently about 1,300 known tardigrade species within the Tardigrada phylum (a classification category) according to the Integrated Taxonomic Information System (opens in new tab) (ITIS), a resource for species names and classifications created by a partnership of U.S. federal agencies.
Water bears have an unusual strategy for surviving harsh conditions: They enter an almost death-like state called cryptobiosis, expelling more than 95% of the water from their bodies, retracting their heads and legs and curling into a dehydrated tun.
Water bears can range from 0.002 to 0.05 inches (0.05 to 1.2 millimeters) long, but they usually don't get any bigger than 0.04 inches (1 mm) long, according to the World Tardigrada Database (opens in new tab).
As their name implies, water bears live just about anywhere there's liquid water, inhabiting the ocean, freshwater lakes and rivers, and the water film that coats terrestrial mosses and lichens. They can survive a wide range of environments: from altitudes of over 19,600 feet (6,000 meters) in the Himalayan mountain range to ocean depths more than 15,000 feet (4,700 m) below the surface, according to the University of Michigan's Animal Diversity Web (opens in new tab) (ADW).
Not all tardigrades live in extreme environments, but water bears are known for surviving extreme conditions that would kill most other forms of life, by transforming into a dehydrated ball known as a tun.
Embryos typically are fully developed within 14 days of fertilization, though their development can last up to 90 days depending on environmental conditions such as dryness and temperature, according to ADW. Young tardigrades do not have a larval stage and resemble miniature adults upon hatching, though they usually have fewer claws and spines than fully-grown water bears do. The youngsters grow in several stages by molting their external cuticle "skin," and each molt can take five to ten days to complete. 2b1af7f3a8