Tardigrada

These interesting little micrometazoans (ranging from 50 Mm, the smallest juvenile, to 1200 Mm, the largest adult) (Nelson and Higgins, 1990) are also called "water bears" because of their microursine appearance. They were named "Il Tardigrado," literally slow-stepper, because their slow movements resembled those of a tortoise, by the famous Italian abbot and natural history professor Lazzaro Spallanzani in 1776 (Nelson and Higgins, 1990).

Tardigrades are members of the monophyletic group known as Ecdysozoa, a clade of all molting animals that includes nematodes and arthropods (Garey, 2001). Tardigrades are bilaterally symmetrical with four pairs of legs, equipped with claws on the distal end, of various sizes and forms (Fig. 4.10). The sizes and shapes of the claws are used in keying genera and species. Perhaps their greatest notoriety in recent times has come from the marked recuperative powers that they show after having been kept dry in a state of "suspended animation" for many years or even decades. These studies (Crowe, 1975; Crowe and Cooper, 1971; Wright, 2001) have found that tardigrades recover well even after extreme environmental insults such as being plunged into liquid nitrogen. More generally, a series of five types of latency or virtual cessation of metabolism have been described: encystment, anoxybiosis, cryobio-sis, osmobiosis, and anhydrobiosis (Crowe, 1975). All these are subsumed under the more general term "cryptobiosis" (Keilin, 1959), or hidden life, which was first described by Antonie van Leeuwenhoek in a Royal Society lecture in 1702 (Wright, 2001). Being highly resistant, or resilient, to various environmental insults, tardigrades exemplify a recurrent thread throughout biology in general, and soil biology in particular: the selective advantages to "waiting out" a spell of bad microclimate and being able to reactivate and become active in a given patch in the soil, years or decades later.

Tardigrades occur predominantly in the surface 1-3 cm of many grassland soils, but certain genera (e.g., the Macrobiotus-group species), are quite numerous at depths up to 10 cm in subalpine coniferous forest (Ito and Abe, 2001). They may serve as "early-warning devices" for environmental stress. Tardigrades were found to be the most sensitive organism measured in a several-year study of the effects of dry-deposition of SO2 on litter and soil of a mixed-grass prairie ecosystem (Grodzinski and Yorks, 1981; Leetham et al, 1982). They are thought to feed on algal cells and debris in the interstices of moss thalli and probably have a rather broad diet of various microbial-rich bits of

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