In the Animal Olympics event of Surviving Horrible Things, tardigrades would take home the gold, silver, and the bronze. These eight-legged, microscopic creatures — also known as "water bears" — are probably the hardiest animals in existence, able to live through extreme heat, cold, pressure, radiation, and even the vacuum of space. Now, these water-dwelling critters have set a new personal best, with cryobiologists from Japan successfully reviving a tardigrade after it had been frozen for more than 30 years. (The tardigrade's previous record was nine years.) What's more, the defrosted creatures even managed to reproduce; with one laying 19 eggs of which 14 successfully hatched.
When the going gets tough, tardigrades slow down
The water bears were able to survive this time in the cooler thanks to a process known as cryptobiosis. This is a state of extreme hibernation that many microscopic creatures use to survive difficult environments, such as those lacking water or air. In the tardigrade's case, the creature's metabolism slows down to less than 0.01 percent of its normal rate; it sheds nearly all of the water in its body, and curls up into a "tun" state — forming a tiny, indestructible pellet. It's thought that some tardigrade species replace the water in their cells with natural antifreeze (glycerol) or crystalline sugars to preserve their structure. Getting rid of the water in this way is essential, as it's often a vector for environmental harm; water damages cells as it freezes, for example.
Here's a video showing dormant tardigrades reviving from their tun state:
In the case of these recently-revived tardigrades (of the species Acutuncus antarcticus), the creatures were originally collected in November 1983 in moss samples from East Antarctica. The samples were stored at −20 °C after collection, and thawed in May last year over several days. Individual tardigrades were collected from the moss, with two of the creatures — nicknamed Sleeping Beauty (SB)-1 and SB-2 — judged to be alive. (Their bodies were still curled up in the "tun" state, while the others had stretched out again in miniature rigor mortis.) These individuals were then placed on a culture plate, immersed in water (Volvic), and left in the dark to think things over.
As you might expect, the tardigrades came to very, very slowly. Imagine taking a 31-year sleep and then instead of a hot cup of coffee when you wake up, all you're given is a sip of water and a bit of algae to munch on. Here are the scientists from Tokyo's National Institute of Polar Research (NIPR) describing the process for SB-1:
"SB-1 first showed slight movement in its 4th pair of legs on the first day after rehydration. This progressed to twisting of the body from day 5 along with movement in its 1st and 2nd pairs of legs, but the movements remained slow. After starting to attempt to lift itself on day 6, SB-1 started to slowly crawl on the agar surface of the culture well on day 9, and started to eat the algal food provided the culture plate on day 13."
After taking nearly two weeks to come around, SB-1 started developing eggs on day 21, depositing them in five clutches by day 45. (Some female tardigrades can reproduce asexually.) SB-2, unfortunately, didn't make it. It started moving at about the same time as SB-1, but stopped eating around two weeks in and died on day 20. An egg that had also been retrieved from the frozen moss sample was revived as well, hatching into a fully functional tardigrade on day six, which then went on to lay its own eggs just eight days later. This is a species that just doesn't know how to quit.
One of the revived tardigrades next to a 0.1 millimeter scale. The green material in its body is the consumedTe algae. (Image credit: National Institute for Polar Research; Tsujimotoa, Satoshi Imuraa, Hiroshi Kandaa, et al)
"We want to unravel the mechanism for long-term survival."
The scientists involved in the study (published last Thursday in the journal Cryobiology) say it will help understand how tardigrades can survive such extreme conditions. "We want to unravel the mechanism for long-term survival by looking into damage to tardigrades’ DNA and their ability to repair it," Megumu Tsujimoto, a postdoctoral researcher at NIPR, told The Asahi Shimbun.
Some researchers have suggested that tardigrades' survival skills are actually borrowed from other creatures' DNA. The theory is that when the water bears go into a "tun" state their DNA is damaged, and when they recover, they incorporate genes from bacteria, plants, and even fungi. One recent paper suggested that as much as a sixth of all tardigrade genes are borrowed from other organisms, but this claim is highly-disputed, with another group of scientists criticizing the original findings for using contaminated samples. The long and the short of it is that we still have a long way to go before we truly understand the hardy tardigrade.