Diane Nelson, a Tardigrade researcher who works in Great Smoky Mountains National Park, used a scanning microscope to take this 3-D image of a Tardigrade.

Tardigrades, often called water bears or moss piglets, are near-microscopic animals with long, plump bodies and scrunched-up heads. They have eight legs, and hands with four to eight claws on each. While strangely cute, these tiny animals are almost indestructible and can even survive in outer space.

Tardigrade is a phylum, a high-level scientific category of animal. (Humans belong in the Chordate phylum — animals with spinal cords.) There are over 1,000 known species within Tardigrade, according to Integrated Taxonomic Information System (ITIS).

Size

These creatures look like the hookah-smoking caterpillar from "Alice in Wonderland." They can range from 0.05 millimeters to 1.2 mm (0.002 to 0.05 inches) long, but they usually don't get any bigger than 1 mm (0.04 inches) long.

Habitat

Water bears can live just about anywhere. They prefer to live in sediment at the bottom of a lake, on moist pieces of moss or other wet environments. They can survive a wide range of temperatures and situations.

Research has found that tardigrades can withstand environments as cold as minus 328 degrees Fahrenheit (minus 200 Celsius) or highs of more than 300 degrees F (148.9 C), according to Smithsonian magazine. They can also survive radiation, boiling liquids, massive amounts of pressure of up to six times the pressure of the deepest part of the ocean and even the vacuum of space without any protection. A 2008 study published in the journal Current Biology found that some species of tardigrade could survive 10 days at low Earth orbit while being exposed to a space vacuum and radiation.

In fact, water bears could survive after humanity is long gone, researchers found. Scientists from Harvard and Oxford universities looked at the probabilities of certain astronomical events — Earth-pummeling asteroids, nearby supernova blasts and gamma-ray bursts, to name a few — over the next billions of years. Then, they looked at how likely it would be for those events to wipe out Earth's hardiest species. And while such catastrophic events would likely wipe out humans, the researchers found little tardigrades would survive most of them, they reported in a study published online July 14, 2017, in the journal Scientific Reports.

"To our surprise, we found that although nearby supernovas or large asteroid impacts would be catastrophic for people, tardigrades could be unaffected," David Sloan, a co-author of the new study and researcher at Oxford, said in a statement. "Therefore, it seems that life, once it gets going, is hard to wipe out entirely. Huge numbers of species, or even entire genera may become extinct, but life as a whole will go on."

Habits

Tardigrades were discovered by a German pastor, Johann August Ephraim Goeze, in 1773. He named them Tardigrada, which means "slow stepper." In 1776, Italian clergyman and biologist Lazzaro Spallanzani discovered that water bears survive extreme conditions by making a transformation.

In many conditions, they survive by going into an almost death-like state called cryptobiosis. They curl into a dehydrated ball, called a tun, by retracting their head and legs. If reintroduced to water, the tardigrade can come back to life in just a few hours.

While in cryptobiosis, tardigrades' metabolic activity gets as low as 0.01 percent of normal levels, and their organs are protected by a sugary gel called trehalose. They also seem to make a large amount of antioxidants, which may be another way to protect vital organs. Water bears also produce a protein that protects their DNA from radiation damage, according to research by the University of Tokyo.

In cold temperatures, they form into a special tun that prevents the growth of ice crystals.

They also have another defense for when they are in water. When the water they live in is low on oxygen, they will stretch out and allow their metabolic rate to reduce. In this state, their muscles absorb oxygen and water well enough that they can survive.

In 2016, scientists revived two tuns and an egg that had been in cryptobiosis for more than 30 years. The experiment was reported in the journal Cryobiology.

Reports from an experiment in 1948 claim that a tun over 120 years old had been revived, but this research has never been duplicated, according to the BBC.

Diet

Tardigrades eat fluid to survive. They suck the juices from algae, lichens and moss. Some species are carnivores and even cannibals — they can prey on other tardigrades, according to the BBC.

Offspring

Tardigrades reproduce through sexual and asexual reproduction, depending on the species. They lay one to 30 eggs at a time. During sexual reproduction, the female will lay the eggs and the males will fertilize them. In asexual reproduction, the female will lay the eggs and then they will develop without fertilization.

Classification/taxonomy

Here is the classification for tardigrades, according to ITIS:

Kingdom: Animalia Subkingdom: Bilateria Infrakingdom: Protostomia Superphylum: Ecdysozoa Phylum: Tardigrada

The Tardigrade phylum branches out into:

3 classes

5 orders

20 families

15 subfamilies

105 genera

4 subgenera

1,018 species

67 subspecies

Conservation status

Tardigrades have not been evaluated by the International Union for Conservation of Nature. They are also not on any other endangered list and have survived five mass extinctions over the course of around a half a billion years, according to National Geographic.

Other facts

The water bear's mouth can telescope outward to reveal sharp teeth that are used to grab onto food.

They swim! Their multiple legs propel water bears forward to reach food.

Tardigrades must have at least a thin coating of water around their bodies to prevent turning into a tun.

Additional resources

Editor's Note: This article was updated on July 14, 2017, with information about a new discovery regarding the resilience of the tardigrade.