"Ordinary words convey only what we know already; it is from metaphor that we can get hold of something fresh," said Aristotle. Light is a wave. Atoms are little billiard balls. Some metaphors take us into whole new realms of understanding. Others just remind us of the essential unity of the world.
And so it was when a biologist friend gave me a bottle containing a few ounces of water, some algae, assorted microscopic organisms, and -- wonder of wonders! -- a few tardigrades. She knew I would be appreciative. On a few occasions over the years, I had mentioned to her how much I would like to see a tardigrade in the flesh. These little creatures, about the size of the period at the end of this sentence, are adorably cute in microphotographs. And now, thanks to my friend, they were cavorting like playful otters in the field of view of my microscope.
Like playful otters! Tardigrades - literally, "slow walkers" - are sometimes called "water bears" because of the way they lumber along bearlike on eight (or six) stumpy appendages, or even more charmingly, "moss piglets." Under the microscope, they do indeed look remarkably like vertebrates of some sort, but they have no bony skeleton. They are invertebrates, related to insects, but so unique they have a phylum all of their own.
Tardigrades do not interest scientists only because they are cute. They are also among the hardiest of multicelled animals, maybe the toughest animals of all. Dry them out and they go into a state of suspended animation in which they can live for -- well, no one knows. When some apparently-lifeless, 120-year-old moss from an Italian museum was moistened, tardigrades rose as if from the dead and scampered about.
They can be frozen at temperatures near absolute zero, heated to 150 degrees centigrade, subjected to high vacuum or to pressure greater than that of the deepest ocean, and zapped with deadly radiation. It is not impossible that tardigrades could survive space travel without a spaceship.
My own curiosity, I confess, was based entirely on the tardigrade's reputation as a water bear or moss piglet. The metaphors are irresistibly seductive. Who can resist a creature the size of a dust-mote that might have stepped right out of Beatrix Potter?