These animals can survive periodic drying that occurs naturally in lichens by entering a special metabolic state called cryptobiosis. During cryptobiosis microinvertebrates weather dry conditions by suspending all but the most vital life functions. These microscopic animals range from 0.1 to 1.0 millimeters and live wherever trees and lichens grow. Samples of lichens with as many as five species of tardigrades as well as rotifers, nematodes mites, small insect larvae and various protozoa have been found. Tardigrades are distributed worldwide and thrive in diverse habitats including marine, terrestrial and freshwater environments.
Terrestrial tardigrades become active only when surrounded and rehydrated by water.
Reanimated tardigrades cling to substrate and search slowly for food. This loss of body water is called anhydrobiosis and leads to a cryptobiotic state in which tardigrades assume a desiccated form called a tun. Tardigrades can survive for months or even years as tuns, remaining inactive until reanimated with water. When moistened, a 120 year-old museum specimen of dried moss yielded tardigrades showing signs of life. The hardy lichens, and the community living on them, may provide a useful bioindicator for air pollution since they derive their water and essential nutrients mainly from the atmosphere rather then from the soil.