The Tardigrada are microscopic animals that look superficially like miniature eight-legged teddy bears. They live in habitats with at least intermittent moisture and can form resistant resting stages enabling them to endure extreme environmental conditions, including intense heat and cold, radiation, desiccation, even the vacuum of space. As of 2002, about 900 species had been described, but it’s likely that many more are still to be discovered.
(Becquerel, 1950; Crowe et al., 1998; Nelson, 2002; Ruppert et al., 2004; Horikawa et al., 2006; Jönsson et al., 2008)
When environmental conditions become unfavorable, tardigrades can enter a state of dormancy called “cryptobiosis,” which is characterized by desiccation, reduced metabolic rate, and enhanced resistance to conditions such as drought, low oxygen, salinity changes, or extreme temperatures. A dormant tardigrade, called a “tun,” may survive for long periods – 10 to 100 years, alternating between periods of activity and dormancy. (Nelson, 2002; Ruppert et al., 2004).
Tardigrades are bilaterally symmetrical, usually “plump” and roughly cylindrical. Superficial form appears to be organized into five segments: a short head and a trunk of four segments, though the trunk may actually have 4 to 5 segments and the head 3 segments. Stubby legs are borne on each trunk segment, each having 4 to 8 retractile claws at the ends. The epidermis secretes a complex cuticle, which in some species is elaborated with structures like spines, granules, and pores. The cuticle is molted periodically. Tardigrades can be translucent or opaque, and the cuticle or gut may be colored brown, green, orange, pink, red, or yellow (Nelson, 2002; Ruppert et al., 2004)
Because tardigrades are so small, they have a high surface-area to volume ratio, making specialized gas exchange structures unnecessary. Diffusion across the body surface is sufficient. The nervous system is similar to that of arthropods, onychophorans, and annelids, with a lobed dorsal brain and ventral nerve cord with fused ganglia. (Nelson, 2002; Ruppert et al., 2004).
Body length is typically 100 to 150 micrometers, but there are that reach lengths of 1.5 mm (Nelson, 2002; Ruppert et al., 2004).
Tardigrades are found in a variety of wet or moist habitats, including marine and freshwater, moist soils, hot-springs, glaciers, mosses, lichens, leaf-litter. In environments with intermittent moisture, tardigrades can enter a “cryptobiotic phase” that allows them to survive dry periods. (Nelson 2002; Ruppert et al., 2004)
Trophic strategies vary. Most tardigrades feed on the cytoplasm of plant cells. Those living in soil eat algae, probably detritus, and some are predators, feeding on other microscopic soil animals, including nematodes and other tardigrades (Ruppert et al., 2004).
The dormant state of cryptobiosis facilitates dispersal – the dried “tuns” are easily transported by winds or other animals (Ruppert et al., 2004).
10 to 100 years (Ruppert et al., 2004).
Most tardigrades are either female or male (“gonochoric”), though a few species are hermaphroditic. And males are absent in some parthenogenetic genera. (Nelson, 2002; Ruppert et al., 2004).
Evolution and Systematics
Systematics and Taxonomy
“Recent molecular analyses and additional morphological studies of the nervous system have confirmed the phylogenetic position of tardigrades as a sister group of the arthropods.” (Nelson, 2002)
However according to Ruppert et al. (2004), evolutionary relationships are currently uncertain. Though they seem to be closest to arthropods, there is also similarity to cycloneuralians (Kinorhynchans, Loriciferans, Priapulids, Nematodes, and Nematomorphs).