Tardigrades: Facts about one of the hardiest animals on Earth, and beyond

Water bears live anywhere there's liquid water, including oceans, freshwater lakes and rivers, and the water film that coats mosses and lichens on land.

They can live above 19,600 feet (6,000 meters) in the Himalayas down to ocean depths of more than 15,000 feet (4,700 m), according to the University of Michigan's Animal Diversity Web (ADW).

Related: Tardigrades probably see in black and white

Not all tardigrades live in extreme environments, but they can survive extreme conditions that would kill most other life-forms.

Tardigrades don't live in or on humans, and they are not dangerous to us.

Most tardigrades suck fluids from cells in plants, algae and fungus. They use needle-like structures in their mouths to pierce cell walls and vacuum up the liquid inside.

However, some species can consume tiny moving creatures, such as nematodes and even other tardigrades.

Tardigrades can live in a wide variety of environments because they enter an almost death-like state called cryptobiosis. To enter cryptobiosis, a tardigrade squeezes out more than 95% of the water from its body, retracts its head and legs, and curls into a dried-out tun.

Four things can trigger cryptobiosis: being dried out, freezing, not having enough oxygen and having too much salt.

During cryptobiosis, a tardigrade slows its metabolism to a tiny fraction of normal levels. Unique proteins protect its cells from damage. When a tardigrade expels its body's water, these proteins form a tough, glass-like cocoon around the tardigrade's cells. This keeps its cells safe while the tardigrade is a tun. Then, it can reanimate in water when conditions are more hospitable. Tardigrades evolved this ability hundreds of millions of years ago — perhaps even to survive mass extinctions, a study found.

"Tardigrades are fascinating little beasties," Sandra McInnes, a tardigrade researcher with the British Antarctic Survey, previously told Live Science. "Tardigrades have this ability to cope with extreme environments by shutting down their metabolism. This ability to cope with drying out or freezing is what gives them their durability in the Antarctic."

Tardigrades can spend decades, or even a century, in a tun state before reviving. That means tardigrades can technically live a century or more.

However, tardigrades do have a fatal weakness: They wilt in hot water. Tardigrades can die within a day in water temperatures of about 100 degrees Fahrenheit (38 degrees Celsius), one study found.

Tardigrades can be seen with the naked eye, but just barely. They range from 0.002 to 0.05 inch (0.05 to 1.2 millimeters) long, but they usually don't get bigger than 0.04 inch (1 mm) long, according to the World Tardigrada Database.

A tardigrade's body is made up of only 1,000 cells. In comparison, the human body has many trillions of cells.

If you look under a microscope into the tardigrade's tiny body, you won't find any bones. Instead, tardigrades have a fluid-filled compartment called a hemolymph. Similarly to human blood, the hemolymph is filled with nutrients.

Although water bears lack a spinal cord, they have a nervous system, according to the 2004 book "Forest Canopies." This sends signals between the tardigrade's brain and body and acts similar to a vertebrate's spinal cord.

Water bears have a complete digestive system but no circulatory or respiratory system. Instead, oxygen from the water enters their bodies through their outer coats. To pump nutrients and oxygen around their bodies, they squeeze their muscles.

Tardigrades in a tun state can withstand temperatures as low as minus 328 F (minus 200 C) and hotter than 300 F (149 C). They can also survive some exposure to radiation and boiling liquids and up to six times the pressure of the deepest part of the ocean, according to the Science Education Resource Center at Carleton College in Minnesota.

When dehydrated, some tardigrade species survived a 10-day trip into low Earth orbit and returned to Earth unharmed by solar ultraviolet radiation or the vacuum of space.

Dried-out tardigrades were shot from a high-speed gun, traveling nearly 3,000 feet per second (900 meters per second) and surviving extremely high pressure.

But that research found that several thousands of tardigrades that were in their ball-like tun state and carried on the Israeli lunar mission Beresheet would not have survived after the lander crashed on the moon on April 11, 2019. The shock pressure of the metal lander hitting the moon would've been "well above" the limits tardigrades could survive.