Recently I was gifted with the opportunity to expose a couple of seven-year-olds to this mostly unknown but utterly ubiquitous group of animals. The children in question are my cousins, and they had asked me to "tell them about water animals." Such a vague question, I felt, deserved a specific, totally unexpected answer: not a story about dolphins, sharks, or even octopuses, but the introduction of an eight-legged, millimeter-long aquatic bear.
Tardigrades, despite their excess appendages, have been reminding microscopists of bears since their discovery by a German pastor in 1773. Despite the animals' poor publicity with the general public, there are a variety of excellent tardigrade web resources, so I will limit myself to a brief but fervent encomium.
Eight legs, but no true joints; tardigrade appendages are really just lobes coming off the body. No respiratory or circulatory system; tardigrades get their oxygen through their skin, and pass it around their bodies by diffusion. Best of all, tardigrades are masters of cryptobiosis, a phenomenon which might be described as uber-hibernation. They shut off their metabolism to become inanimate objects known as a tuns (the etymology is a mystery to me). A tun can withstand almost stupid extremes of temperature, pressure, radiation, etc. This is actually a special type of cryptobiosis called anhydrobiosis, which translates pretty literally as "life without water." Contradiction in terms? Apparently not. Because of both the obvious biomedical applications, and astrobiologists' interest in survival at extremes, research into tardigrade tuns is very sexy.
However, when in their normal hydrobiotic state, tardigrades are fond of liquids. They sustain themselves by puncturing the cells of their tiny prey, plant and animal, with two sharp hollow teeth, and sucking out the juices. This habit earns them the title suctorial feeders. (A beautiful phrase--please use it the next time you're sipping some nutrition through a straw.)
Their suctorial habits, along with the four to eight claws that terminate each appendage, could make tardigrades a truly ghastly science fiction monster, if they were enlarged by about four orders of magnitude. On the other hand, a tardigrade enlarged by only three orders of magnitude would make an excellent prototype for Stitch (as in Disney's Stitch, Lilo and). Apparently I'm not the first person to be struck by the similarity, but sadly I can find no official corroboration.
Actually, better than using them as sci-fi monsters would be to market tardigrades (at the size they already are) as the new sea monkey. And dammit, it looks like someone else beat me to this idea, too--see the first comment. However, it may not be practical to sell tardigrades, particularly to a web-savvy populace, due to the number of webpages out there that describe how to collect your own tardigrades from the backyard.
The real problem with keeping tardigrades is that you can't see them without a microscope. Even with a relatively cheap dissecting scope, your pets won't be much to look at. For really good tardigrade viewing, you need a rather expensive electron microscope.
I would like to propose a better candidate for The New Sea Monkey: another little known group of animals, the pseudoscorpions. They are conveniently small but not too small to appreciate with the naked eye. A cheap magnifying glass, while not necessary, enhances the experience. Like other arachnids, they prey on pests, such as lice and clothes moth larvae, and should therefore be fairly easy to feed. They live surprisingly long for a little thing--a couple of years--so you could get much more attached to them than to the brine shrimp.
But I digress from the phylum under discussion. I shall end with one last tardigrade link, because it is funny.