Tardigrade

A tardigrade is a type of microscopic animal noted for its ability to survive in extreme environments. Also known as water bears, the tiny invertebrates have chubby segmented bodies, heads that resemble short snouts, and clawed toes on each of their eight legs. While they predominantly reside in freshwater lakes and in water droplets covering vegetation, tardigrades have also been found in almost every environment on Earth. They have been observed on the slopes of mountains, in tropical rain forests, in the depths of the ocean, and in the coldest climate on the planet. Tardigrades have evolved a distinctive survival mechanism that allows them to enter a death-like protective state. In this condition, they can withstand extreme heat and cold, severe radiation, massive pressure changes, and even the vacuum of space.

Background

Tardigrade fossils have been found dating back more than five hundred million years—about the time when the first creatures began colonizing the land. In those half billion years, they have survived all five major extinction events that have affected Earth. In 1773, German pastor Johann August Ephraim Goeze first discovered tardigrades. He called them water bears because they reminded him of miniature bears. Three years later, an Italian scientist named Lazzaro Spallanzani found that the creatures seemed to spring to life after he added water to sediment from a rain gutter. Spallanzani named them tardigrada, or "slow steppers," because of their slow movements.

The term tardigrade does not refer to a single species; rather, it is a classification known as a phylum, a category of animal with a common ancestor and with similar anatomical characteristics. Humans, for example, are part of the phylum chordate, which comprises animals with spinal cords. Tardigrades are related to spiders, insects, and a type of invertebrate called a velvet worm. Scientists have classified about one thousand different species of tardigrade. Some survive on fluids from plant cells, animal cells, and bacteria; others are carnivorous or even feed on other tardigrades. Some species reproduce asexually, while others require a male to fertilize eggs laid by a female.

Overview

Tardigrades range from about 0.05 millimeters to 1.2 millimeters in length. Their primary environment is in the sediment on the bottom of freshwater lakes and in moisture found on lichen and moss. However, they have also been discovered in more extreme environments. Some have been found on the ocean floor, in the boiling waters near a Japanese hot spring, more than 18,000 feet (5,486 meters) up the side of a Himalayan mountain, and in the frigid reaches of Antarctica.

In the early twentieth century, scientists proved that tardigrades could survive exposure to temperatures as high as 304 degrees Fahrenheit (151 degrees Celsius) for fifteen minutes. Further experiments showed they could withstand extreme cold of up to -328 degrees Fahrenheit (-200 degrees Celsius). Some of the hardier species of tardigrade have been found to survive temperatures as low as -459.04 degrees Fahrenheit (-272.8 degrees Celsius)—slightly above absolute zero, the temperature at which atoms theoretically stop moving. Tardigrades have also been exposed to normally fatal doses of X-ray, gamma, and ultraviolet radiation and survived. They have been subjected to atmospheric pressures six times greater than any normally found on Earth and lived through that as well.

In 2007, researchers with the European Space Agency sent thousands of tardigrades aboard a satellite into Earth's orbit for ten days. All were exposed to the vacuum of space. Some were shielded from the intense solar radiation, while others were not. When the experiment returned to Earth, 68 percent of the tardigrades solely exposed to space survived, and some even laid eggs while in orbit. The ones left unprotected against the radiation had a much higher mortality rate; however, some did manage to withstand even those conditions. In 2021, tardigrades traveled to the International Space Station for a long-term study of their genes and other qualities that enable them to survive extreme conditions.

To accomplish these feats of survival, tardigrades enter a near-lifeless state of suspended animation. The condition was first identified in 1922 by German scientist H. Baumann who called it a tönnchenform, a term later shorted to tun. A tardigrade enters the tun state by dehydrating itself and curling up into a ball. Its metabolism slows drastically to near 0.01 percent of normal levels, a process called cryptobiosis. While in this state, a set of proteins unique to tardigrades begin to form a glassy substance that cocoons the creature's internal structures, protecting them from harm. Known as tardigrade-specific intrinsically disordered proteins (TDPs), these proteins are normally jelly-like until the tardigrade begins drying out. When the TDPs crystallize, they are believed to trap other proteins and molecules inside the protective shell, preventing them from being damaged or destroyed by extreme conditions.

While in the tun state, tardigrades do not require food or water and can remain dormant for years. After being exposed to water, tardigrades can be revived in the span of an hour. Scientists have successfully revived tardigrades that have been in the tun state for more than thirty years. A report from 1948 claimed a tardigrade had been resuscitated after more than a century, though further research has been unable to verify the claim.

While transitioning into a tun state increases the tardigrade's chances of survival, it does not make it indestructible. The chances of dying increase the longer the tardigrade remains exposed to extreme conditions. In its balled-up, inactive state, the creature is also more susceptible to predators, such as amoebas, nematodes, and other tardigrades. Some species of tardigrade do not exhibit the ability to enter a suspended state or survive in harsh environments. These species, known as anthrotardigrades, are among the oldest known form of tardigrade and live only in the ocean. Scientists theorize that the relatively stable environment beneath the sea did not lead to the same evolutionary survival skills found in land- and freshwater-dwelling tardigrades.

In 2024, biologists in China reported that they had identified a new species of tardigrade and subjected it to doses of radiation several times higher than the lethal dose for humans. The biologists found a gene that enabled the exposed tardigrades' cells to produce antioxidant pigments. The pigments protected the exposed cells from radiation damage, thus enabling the tardigrade to survive. The researchers then treated human cells with the same antioxidant pigments and discovered that the treated cells were better able to survive radiation exposure as well.

Bibliography

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