GA Milestones EOG Practice Test 8

The Water Bear's Secret

¹Pull a clump of damp moss from a tree trunk, soak it in water, and look at the water under a microscope. With some patience you may spot a plump, slow-moving creature, smaller than a grain of sand, paddling along on eight stubby legs tipped with tiny claws. It looks a little like a bear in a puffy snowsuit. Scientists call it a tardigrade, but most people know it by its nickname: the water bear.

²Water bears have been known to science for about 250 years. In 1773 a German pastor named Johann Goeze described one and called it a "little water bear." A few years later an Italian scientist gave the group its formal name, from Latin words meaning "slow stepper." Since then, researchers have identified more than a thousand species living almost everywhere, from mountaintops to the deep sea floor to the moss in an ordinary backyard.

³What makes tardigrades remarkable is what happens when their world dries out. A tardigrade needs a film of water around it to stay active. As its moss or soil dries, the animal pulls in its head and legs and curls into a tiny, shriveled barrel called a tun. It loses nearly all of the water in its body, and the chemical activity inside its cells slows until it can barely be measured. Scientists call this state cryptobiosis; to an observer, the dried tardigrade seems to have no life in it at all. Yet when water returns, the tun can swell, uncurl, and begin walking again, sometimes within an hour.

⁴The tun state is what has made tardigrades famous. In laboratory tests, tardigrades in the tun state have survived temperatures far below freezing and doses of radiation many times greater than the amount that would kill a person. Some writers have called them the toughest animals on Earth. In 2007, a European space mission carried dried tardigrades into orbit and exposed them to the vacuum of space for ten days, and many of them revived after they returned to Earth. Results like these have made tardigrades a favorite subject of science-fiction stories. Even in these famous tests, though, some of the animals never woke up.

⁵How do the survivors do it? Researchers are still finding out, but special proteins seem to play a large part. One protein, discovered in a tardigrade species from Japan, was nicknamed Dsup, short for "damage suppressor," because it appears to shield the animal's DNA from harm. When scientists added the gene for Dsup to human cells growing in a laboratory, those cells suffered less damage from X-rays than ordinary cells did. Other tardigrade proteins may turn into a glass-like gel as water disappears, holding the parts of each cell in place until water returns.

⁶Headlines sometimes call tardigrades "indestructible," but that word is misleading. An active, water-filled tardigrade is a delicate animal that can be killed by conditions far less extreme than those described above. Even the tun offers no guarantee: a tardigrade needs time to prepare for drying, and one that dries out too quickly may never wake up. In many experiments a large share of the animals do not survive. Their famous toughness belongs to one special state, not to every moment of their lives.

⁷Still, the tun has caught the attention of scientists far beyond biology. Many vaccines and medicines must be kept cold from the factory to the clinic, which is difficult in places without reliable electricity. Some researchers are testing whether tardigrade proteins could help such medicines survive being dried and stored without refrigeration.

⁸The next time you walk past a patch of moss, it may be worth a second look. Somewhere inside it, a creature you cannot see may be waiting, dried and silent, for the next rain.