"Tardi what?" A question I am often asked, generally elicited by the SEM pictures of tardigrades on my office walls. With that opening, I get to explain an animal that you cannot see, has never been heard of, and causes you no harm. My story is about the adventure of discovery, the excitement of working with animals that few humans have ever seen. You fell into my trap when you picked up this booklet, so take a few minutes, read what follows, and maybe you too will be intrigued by Tardigrades, Bears of the Moss.

Tardigrada is one of several little known phyla of invertebrates located between the nematodes (roundworms) and the arthropods (crustacea, insects, ticks and mites). They are small, 0.2-0.5 mm in length, about the size of a dot made with a "fine" mechanical pencil. A big tardigrade can be seen with the naked eye, but the light must be just right.

Tardigrades look like miniature caterpillars with five body segments and four pairs of clawed legs (Figures 1 & 2). Like "higher" animals they have digestive, excretory, and nervous systems; separate sexes; and well-developed muscles. Like "lower" animals, they lack respiratory and circulatory systems. Instead, they breathe through their skin or cuticle and the whole body acts as a pump to circulate fluids.

The word tardigrade means "slow walker" which describes their rather sluggish, clumsy movement. They use their short legs and claws to cling to a substrate and waddle along like "Water Bears" or "Moss Piglets" (Kinchin 1994). Tardigrades are very common and have been found on every continent (McInnes 1994). They have been recorded in every biotope: both salt and fresh water; the humidity of rain forests, the altitude of mountains, the dryness of desserts, and the isolation of remote islands and Antarctic nunatacks.

All tardigrades are aquatic. They need to be in water to live, to find food, to breathe, to reproduce, and to move. There are marine, freshwater, and limno-terrestrial species. The latter are subject of this booklet and live in the water droplets trapped in the space between the leaves of moss cushions, the thalli of lichens, and leaf litter. Here they share a micro-world with other organisms (collembola, mites, rotifers, and nematodes) and endure extreme environmental cycles from flood to drought.

The reason tardigrades are "interesting" is that hey have developed ways to survive these environmental swings. They survive times of flood or lack of oxygen by swelling up like a balloon (anoxybiosis) and floating around for a few days. They survive until the environment dries, then return to the active state for feeding, growing, and reproducing (Figure 3).

When the environment dehydrates in dry weather, tardigrades desiccate into a reversible state of metabolic suspension called cryptobiosis. They shrivel to about one-third their former size into a wrinkled "tun" (Figure 3). Individuals have been observed to come and go from the cryptobiotic state repeatedly and tardigrades have been reported to survive more than 100 years (Kinchin 1994).

Cryptobiosis is of great interest in the study of cryogenics and tardigrades have been subjected to laboratory experiments which verified their ability to survive. Tardigrades have tolerated temperatures below freezing at 0.05K (-272.95 degrees C) for 20 hours and -200 degrees C for 20 months. They have survived 120 degrees C, pressures of 1000 atmospheres, and high vacuums. In the cryptobiotic state, tardigrades have shown resistance to hydrogen sulfide, carbon dioxide, ultraviolet light, and X-rays (Kinchin 1994). We could speculate that tardigrades could be transported through outer space in their existing form.

Despite these "capabilities," tardigrades are still little understood. In the 200 years since the waterbear was first described, we have not identified any specific medical, commercial, or environmental effect of tardigrades. We have identified three Classes, five Orders, 15 Families, 94 genera and more than 750 species (Table 1).