Overview

Brief Summary

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)

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Distribution

Worldwide.

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Physical Description

Morphology

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)

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The cuticle can also be blue or white. Some tardigrades have simple eyes, others none at all. Each tardigrade has 4 pairs of legs, usually without a tail.

  • tardigrades.com
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Size

Body length is typically 100 to 150 micrometers, but there are that reach lengths of 1.5 mm (Nelson, 2002; Ruppert et al., 2004).

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Ecology

Habitat

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)

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Dispersal

The dormant state of cryptobiosis facilitates dispersal – the dried “tuns” are easily transported by winds or other animals (Ruppert et al., 2004).

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Trophic Strategy

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).

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Associations

In Great Britain and/or Ireland:
Animal / parasite
Ballocephala verrucospora parasitises live Tardigrada

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Known predators

Tardigrada (Turbellaria, Halacaridae, Tartigrada, Copepoda (Diacyclops, Bryocamptus), Cladocera (Alona, Ilyocryptus), Chironomidae (Mesocricotopus, Micropsectra), Cytheridea) is prey of:
Stercorarius
Larus hyperboreus
Somateria
Gavia stellata
Clangula hyemalis
Pentaneurini
Procladius
Protanypus

Based on studies in:
Norway: Spitsbergen (Coastal)
Finland (Lake or pond, Pelagic)

This list may not be complete but is based on published studies.
  • J. Sarvala, Paarjarven energiatalous, Luonnon Tutkija 78(4-5):181-190, from p. 184 (1974).
  • V. S. Summerhayes and C. S. Elton, Contributions to the ecology of Spitsbergen and Bear Island, J. Ecol. 11:214-286, from p. 232 (1923).
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Known prey organisms

Tardigrada (Turbellaria, Halacaridae, Tartigrada, Copepoda (Diacyclops, Bryocamptus), Cladocera (Alona, Ilyocryptus), Chironomidae (Mesocricotopus, Micropsectra), Cytheridea) preys on:
protozoa
algae
Nematoda
bottom organic matter

Based on studies in:
Norway: Spitsbergen (Coastal)
New Zealand (Grassland)
Finland (Lake or pond, Pelagic)

This list may not be complete but is based on published studies.
  • K. Paviour-Smith, The biotic community of a salt meadow in New Zealand, Trans. R. Soc. N.Z. 83(3):525-554, from p. 542 (1956).
  • J. Sarvala, Paarjarven energiatalous, Luonnon Tutkija 78(4-5):181-190, from p. 184 (1974).
  • V. S. Summerhayes and C. S. Elton, Contributions to the ecology of Spitsbergen and Bear Island, J. Ecol. 11:214-286, from p. 232 (1923).
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Life History and Behavior

Behavior

Behaviour

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).

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Life Expectancy

10 to 100 years (Ruppert et al., 2004).

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Reproduction

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).

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Evolution and Systematics

Evolution

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).

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Functional Adaptations

Functional adaptation

Cryptobiosis protects from extremes: water bear
 

Water bears survive extreme environmental conditions by entering a reversibly suspended metabolic state known as cryptobiosis.

           
  "Most incredible of all, however, is the virtually indestructible nature of tardigrades while they remain in cryptobiosis. In laboratory experiments, cryptobiotic specimens have been chilled in liquid helium to -457°F (-272°C), which is only marginally above absolute zero. They have also been heated to temperatures exceeding 300°F (149°C), exposed to radiation doses far in excess of the lethal dose for humans, immersed in vats of liquid nitrogen, concentrated carbolic acid, hydrogen sulphide, brine, and pure alcohol, and even bombarded by deadly streams of electrons inside an electron microscope. Yet when removed from all of these incredibly hostile environments - which would have proven fatal for any other form of animal life - and moistened with water, these astounding creatures recovered.

"They simply emerge from their cryptobiotic state, rehydrate themselves, and amble away on their four pairs of stubby claw-tipped legs, completely unharmed. Even today, the physiological mysteries behind the tardigrades' unparalleled powers of endurance during cryptobiosis remain unsolved." (Shuker 2001:113)
  Learn more about this functional adaptation.
  • Shuker, KPN. 2001. The Hidden Powers of Animals: Uncovering the Secrets of Nature. London: Marshall Editions Ltd. 240 p.
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Physiology and Cell Biology

Physiology

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).

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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:1,693Public Records:514
Specimens with Sequences:939Public Species:38
Specimens with Barcodes:830Public BINs:120
Species:56         
Species With Barcodes:46         
          
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Barcode data

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Tardigrada

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