These ambling, eight-legged microscopic “bears of the moss” are cute, ubiquitous, all but indestructible and a model organism for education
Tardigrades’ best-known feature is their brute, dogged ability to survive spectacularly extreme conditions. A few years ago, the Discovery network show Animal Planet aired a countdown story about the most rugged creatures on Earth. Tardigrades were crowned the “Most Extreme” survivor, topping penguins in the Antarctic cold, camels in the dry oven of the desert, tube worms in the abyss and even the legendarily persistent cockroach.
But extreme survivorship applies only to some species of terrestrial tardigrades. Marine and aquatic tardigrades did not evolve these characteristics because their environments are stable. It appears that the extravagant survival adaptations have been selected in direct response to rapidly changing terrestrial microenvironments of damp flora subject to rapid drying and extreme weather.
Terrestrial tardigrades have three basic states of being: active, anoxybiosis and cryptobiosis. In the active state, they eat, grow, fight, reproduce, move and enact the normal routines of life. Anoxybiosis occurs in response to low oxygen. Tardigrades are quite sensitive to oxygen tension. Prolonged asphyxia results in failure of the osmoregulatory controls that regulate body water, causing the tardigrade to puff up like the Michelin Man and float around for a few days until its habitat dries out and it can resume active life.
Cryptobiosis is a reversible ametabolic state—the suspension of metabolism—that has inevitably been compared to death and resurrection. In cryptobiosis, brought on by extreme desiccation, metabolic activity is paralyzed due to the absence of liquid water. Terrestrial water bears are only limnoterrestrial—aquatic animals living within a film of water found in their terrestrial habitats. Moss and lichens provide spongelike habitats featuring a myriad of small pockets of water and, like sponges, these habitats dry out slowly. As its surroundings lose water, the tardigrade desiccates with them. It has no choice. The creature loses up to 97 percent of its body moisture and shrivels into a structure about one-third its original size, called a tun. In this state, a form of cryptobiosis called anhydrobiosis—meaning life without water—the animal can survive just about anything.
Tardigrades have been experimentally subjected to temperatures of 0.05 kelvins (–272.95 degrees Celsius or functional absolute zero) for 20 hours, then warmed, rehydrated and returned to active life. They have been stored at –200 degrees Celsius for 20 months and have survived. They have been exposed to 150 Celsius, far above the boiling point of water, and have been revived. They have been subjected to more than 40,000 kilopascals of pressure and excess concentrations of suffocating gasses (carbon monoxide, carbon dioxide, nitrogen, sulfur dioxide), and still they returned to active life. In the cryptobiotic state, the animals even survived the burning ultraviolet radiation of space.
Challenging student scientists to ponder the astonishing durability of tardigrades brings their understanding of physics, chemistry and biology into play. They recall that water expands as it approaches the freezing point, which is why ice floats. At 4 degrees celsius the expansion of water exerts sufficient force to split boulders, rupture metal containers and explode living cells. A cell is more than 95 percent water. The rupturing forces and icy microshards that form in frozen cells are the same that cause frost bite.
How can water bears survive all that? my new student and, perhaps, future colleague asks.
Really, deep down, we’re still puzzled about a lot of it, I respond.
The survival attributes of tardigrades are in fact quite appropriate for an organism that makes its home in mosses and lichens (bryophytes), which provide them with just a thin layer of protection. Bryophytes are subject to the environmental extremes experienced on a planet bathed in solar radiation. They may receive varying periods of direct ultraviolet exposure and are never far from drying out as ambient conditions change.