Tardigrades. What are they? Their name means ‘slow walkers’ and their common name is ‘water bear’ because they do have a rather bear-like appearance under the microscope, if you ignore the fact that they have eight legs. They have also been called moss piglets. They aren’t noticed by most people because tardigrades are so minute in size, being less than two millimeters in length.
Because tardigrades have 6 or 8 legs and are segmented, they were once classified as arthropods, but further studies showed that tardigrades are different enough to deserve their own phylum. For one thing, they have a worm-like pharynx more similar to a nematode than an arthropod. They may be distant relatives of the velvet worms (Onychophora) and certainly both phyla are extremely old, with fossils dating back to the Cambrian Period. This was some 500 million years ago, when sponges, worms and jellyfish dominated the world’s ecosystems and arthropods were only just evolving.
There are about a thousand species of tardigrades living in aquatic and marine sediments all over the world. They need water, so on land they are found in moist habitats, such as mosses and lichens or in moist soils and leaf litter. Most are herbivores although a few are predatory. In some habitats they can be extremely numerous with up to thousands of individuals in a litre of sediments. Most tardigrades have eight clawed legs but one family has only six legs and these end in disks, spoons and shovels (Family Batillipedidae). This aberrant and specialised group is adapted for living between sand grains in marine and estuarine environments.
What impresses me about tardigrades is how tough they are. They can survive freezing, sometimes for months or years at a time, so they are found from 6000 meters up in the Himalayas and in extreme Arctic and Antarctic regions. There aren’t many terrestrial animals that can survive in Antarctica, but where mosses and lichens grow, tardigrades can be found crawling around, the bears and cows (or piglets) of their small ecosystems. At the other extreme, water bears can survive in unbelievably hot conditions, having been found basking in hot springs that would cook us like lobsters. They don’t actually survive in these conditions as much as outlast them by forming cysts or ‘tuns’. The tun, named after the wine cask it resembles, is virtually indestructible. When conditions become suitable, the tun hatches and the tardigrade resumes its life.
In order to test just how tough tardigrades are, scientists have subjected them to mini-Olympic competitions. Instead of highest, fastest and furthest, the tardigrade olympics consists of hottest, coldest and most extreme. In the laboratory, water bears have survived temperatures approaching absolute zero. They can survive a few minutes at -272 deg C and a few hours at the relatively balmy temperature of -200 C. Gold medal material for sure. They have survived short exposures to temperatures as high as 150 deg C. They survived having all the air sucked out of their container to leave them in a vacuum. So scientists sent them into space, where they survived not only the vacuum but the lethal levels of radiation. At the opposite pressure extreme, they have survived exposure to pressures of up to 6000 atmosheres, which is higher than the pressures found in the deepest ocean trenches. Ok, they did it by encysting and going into a hibernation state inside their little wine casks, but the point is, they survived to live again.
Water bears need water for normal life but they can also survive long periods of dehydration by going into this state of suspended animation. Chemicals in their tissues prevent the destruction of cells. Just add water and the tardigrades come back to life. They can also survive being dipped in noxious chemicals, exposure to levels of radiation a thousand times stronger than that needed to kill a human, and severe changes in salinity. They are truly ‘extremophiles’, animals that can tolerate extreme conditions. They do this by a process called cryptobiosis, in which metabolism stops but can restart again when conditions improve. The tardigrade rolls up into a ball inside its tun and goes into a death-like state. The water in its cells is replaced with a sugar called trehalose, which keeps the cells from being damaged. When conditions improve, the tardigrade slowly comes back to life over a few hours and just picks up where it left off, days, months are even years before. Tardigrades are the Rip van Winkles of the invertebrate world.
So if you need an animal to send into space, to colonise Mars or to survive with the cockroaches after we annihilate ourselves, I suggest you check out the water bears. They may be tiny but, boy, are they tough!
References:
http://serc.carleton.edu/microbelife/topics/tardigrade/
http://tardigradesinspace.blogspot.com/
check out http://www.tardigrada.net/main.htm for more information and access to research on these fascinating animals
http://en.wikipedia.org/wiki/Tardigrada
