Blogger's note: I'm still away from the blog for a few weeks. In the meantime, here is another post from the Artful Amoeba archive. It originally appeared on October 4, 2010. I recently read a delightful leaflet on water bears which gave me a whole new appreciate for their anatomy (some of them have armored plates like tanks!) and diversity. Enjoy!
If you had to name one multi-cellular organism that has survived both the vacuum of space and the full onslaught of solar radiation, could you? There is, in fact, one creature that has done this, and it has done so while accomplishing the (apparently) unrelated feat of being incredibly cute. Tell me this video doesn’t make you want to pick one up and give it a belly rub.
This is a little animal called a water bear, or tardigrade. On the mosses, lichens, forest litter, ponds, beaches, snowbanks (and even hot springs) of the world, this little guy plods along, oblivious to the larger world. At just 100 micrometers (.1 mm) to 1.5 mm long, they are cute on paws. Did you notice the little fingers?
Discovered in the 18th century, these little guys were named water bears for their trundling, bear-like gait — that is, if can you imagine a bear with *four* pairs of legs and a penchant for shriveling up in winter rather than curling up in a cave. Tardigrade, in fact, just means “slow walker”.
Water bears exist at a strange junction between the world of the large and the world of the small. They are multicellular organisms with intestines, brains, eyes, fingers, and a chitinous cuticle that they shed, but in many ways they behave like protists, which are also microorganisms but not animals at all. Some tardigrades don’t defecate until they moult. Others don’t mate until that happens. The fertilized eggs stay behind in the moulted skin and incubate there, or sometimes adhere to a nearby surface. They are also eutelic (you-tell-ik), which just means that every water bear grows exactly the same number of cells, and once that number is reached, they can grow larger only by growing those cells. This isn’t uncommon for microbial life. Their mouths are armed with stylets with which they pierce and suck the delicious contents of plant cells, algae, and small invertebrates.
As for their bizarre survivalism streak, withstanding the vacuum and scorching solar radiation of space seems to be a byproduct of their ability to survive dry spells (they can go for a decade without water), just as it is for the bdelloid rotifers, whom I’ve also covered here. They can reversibly enter a state of suspended animation called cryptobiosis, in which their metabolism screeches to a halt and their water content plunges to a hundredth of normal. This helps protect their DNA, and a sugar called trehalose helps protect their membranes. For further information, see here. In 1997, they were launched into low-earth orbit and survived the vacuum of space for 10 days. Yes, Tardigrades . . . in . . . Space! Several went on to lay and hatch eggs normally. Interestingly, there is even a sci-fi sounding word for their state of suspended animation: when so ensconced, they are called a “tun”.
Taxonomically, water bears are most closely related to arthropods, or all the crustaceans and insects of the world, and onychophorans, the velvet worms. Biologists would say they are one of the bilaterian crown groups, or one of the earliest lineages to split into their own group after animals developed mirror-image symmetry. Other early-diverging animals either had no symmetry (sponges) or were radially symmetrical (jellyfish et al.) You can check out their neighborhood of the life family tree here (look for tardigrada). You may notice they’re also in a group called “ecdysozoa”, which is just a code word for “all the organisms that moult exoskeletons”, which actually does seem to be a true, historical, one-time evolutionary inovation (i.e., synapomorphy in bio-speak), and thus make it a taxonomically valid group.
Final cool factoid: few tardigrades have fossilized, but of those that have, one was named Beorn leggi, which will be delightful to those of you who have read The Hobbit. And in case you were wondering if someone actually had the chutzpah to do it, yes, yes someone did. It was screaming to be done. Behold the plush tardigrade.