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Summary Article: Tardigrada
from Encyclopedia of Life Sciences

The Tardigrada comprise a phylum of about 800 described species of minute aquatic animals, showing clear affinities with the arthropods. Many inhabit interstitial or temporary water bodies, surviving periods of drying by cryptobiosis.






Basic Design

The essential features of a tardigrade are shown in Figure 1. Most distinctive are the cylindrical, segmented trunk, usually 100-1000 μm in length, and the four pairs of squat, 'lobopodial' limbs bearing claws, the last pair of limbs being terminal. There is a complex mouthpart apparatus comprising a buccal tube, a pair of piercing stylets and associated salivary glands and a tri-radiate muscular pharynx. The pharynx opens into a large midgut, followed by a shorter hindgut. Opening into the hindgut are three Malpighian tubules that probably serve in ionoregulation and excretion, and the single dorsal gonad. The epidermis is overlain with a flexible chitinous cuticle. Between the alimentary tract and the epidermis is a spacious haemocoel that contains large numbers of spherical haemocytes, approximately 8 μm in diameter. See also: Chitin

The tardigrade nervous system shows a typical protostome pattern. The brain consists of dorsal and paired ventral ganglia, and is followed by paired ventral cords interconnecting four large segmental ganglia. These innervate the corresponding limbs and muscle groups. The name tardigrade is derived from the Latin tardus (slow) and gradus (step) and refers to the lumbering, bear-like gait of these animals; they are sometimes named 'water bears'. The 'rolling' gait arises from the locomotor rhythm in which opposite and adjacent limbs move 180° out of phase. Tardigrade muscles comprise individual cells that originate and insert at discrete points on the cuticle and work against the hydrostatic pressure of the haemocoel to bring about locomotion and shape change. The muscle fibre ultrastructure is intermediate between smooth and striated types and resembles that of onychophorans. See also: Onychophora (Velvet Worms)

Representative Tardigrada. (a) Echiniscus testudo, a common and cosmopolitan heterotardigrade, showing the buccal sensilla and thickened cuticular plates with projecting filaments. Length 250-400 μm. (b) Macrobiotus richtersi, a large eutardigrade (600-1000 μm in length). Note the curved stylets, wide buccal tube and bulbous pharynx with masticatory placoids.

Claw types of tardigrade genera.

Diversity and Life Styles

Two classes of tardigrades are recognized: the Heterotardigrada, which possess a generalized claw structure, cephalic sensilla and often dorsal cuticular plates; and the Eutardigrada, which show simplified cuticular sensilla and always possess paired claws. Heterotardigrades include a marine order, Arthrotardigrada, and a predominantly freshwater/meiofaunal order, Echiniscoidea. The Eutardigrada chiefly inhabit freshwater/meiofaunal biotopes. Specific genera are most easily distinguished by details of the claws and buccopharyngeal apparatus; examples are illustrated in Figures 2 and 3.

Most tardigrades inhabit interstitial water. Habitats include coastal sands (Batillipes, Orzeliscus), marine rock crevices or barnacle shells (Echiniscoides), soil water (Macrobiotus, Hypsibius), and lichens and moss cushions (diverse Echiniscoidea and Eutardigrada). A few species such as Macrobiotus dispar inhabit lakes and rivers. Mosses and lichens frequently support large populations of Echiniscus spp. and several eutardigrade genera. Most tardigrades feed on algae and plant cells, piercing tissues with the protracted stylets and withdrawing intact cells or cytoplasm with the pumping pharynx. Some species such as the cosmopolitan Macrobiotus richtersi, M. hufelandi and Hypsibius prosostomus are facultative carnivores and may attack other tardigrades, rotifers and nematodes; Milnesium tardigradum appears to be exclusively carnivorous. See also: Nematoda (Roundworms); Rotifera

Reproduction peaks in the summer months, but continues throughout the year in warmer latitudes. Tardigrades have separate sexes, although males are generally rare. The females reproduce mostly by diploid parthenogenesis. A typical brood size is 3-15 eggs, exceptionally over 20, and these are often deposited inside the moulted cuticle. The eggs are frequently ornamented with projecting aeropyles, which open into a mesh-like chorion; this probably serves as a respiratory plastron. Meiotic (haploid) eggs are produced seasonally in some populations, but it is not known what factors control the switch between mictic (sexual) and amictic (parthenogenetic) generations. Growth involves periodic shedding of the cuticle (ecdysis) and continues throughout life. Prior to ecdysis, the cuticular buccal apparatus is shed, producing a nonfeeding simplex stage. The new buccal tube and stylets are secreted by the salivary glands and smaller 'claw glands' secrete new claws.


Tardigrades are able to inhabit the ephemeral interstitial water of mosses and lichens owing to cryptobiosis. This extraordinary process enables the animal's tissues to survive complete loss of liquid water, although some 'bound' water remains associated with macromolecules even at extremely low water potentials. Hazards of dehydration include intracellular concentration of electrolytes, protein denaturation, and loss of the hydrophobic-hydrophilic orienting forces that maintain thermodynamic stability of membrane phospholipid bilayers. Cryptobiotic species replace electrolytes with compatible osmolytes that serve the additional function of preserving macromolecular structure. The most common of these 'membrane protectants' are sugars such as trehalose and mannose, and various polyols, particularly glycerol. As the moss cushion dries and water potential declines, tardigrades contract into a barrel-like tun (see Figure 4) and drying of the cuticle initiates an abrupt decline in permeability. Thus protected from rapid desiccation, the tardigrade begins a rapid synthesis and mobilization of protectants that stabilize proteins and substitute for membrane-associated water over the succeeding hours.

Morphological variation in the buccopharyngeal complex of tardigrade genera.

Scanning electron micrographs of Hypsibius (Rammazottius) oberhaeuseri during desiccation and entry into cryptobiosis: (a) partially contracted; (b) tun (dorsal); (c) tun (ventral). Length of tun approximately 120 μm.

Water is the essential solvent for biochemical pathways, and cryptobiotic organisms in advanced dehydration show a complete cessation of metabolism. Correlated with this is a remarkable decline in senescence: tardigrades have been revived from moss samples dried for over 10 years, and cryptobiotes protected from oxidation by anaerobic conditions or low temperatures may survive indefinitely. Many damaging effects of physical or chemical extremes arise from effects on water, aqueous solute systems and metabolic regulation, and are ameliorated by drying. Examples are susceptibility to heating, freezing, organic solvents, anoxia and radiation. The same sugars and polyols that serve as membrane protectants also protect tissues against freeze-dehydration and function independently as antifreezes. High concentrations of trehalose and glycerol allow some arctic tardigrades to supercool indefinitely without prior dehydration, avoiding intracellular freezing and resultant ice damage. This process is referred to as cryobiosis and is a distinct category of cryptobiosis from dehydration-induced cryptobiosis or anhydrobiosis. See also: Water: Structure and Properties

Both eggs and adults of most 'terrestrial' or meiofaunal tardigrades can exploit cryptobiosis. They share this capacity with bdelloid rotifers, several protozoans and nematodes, all of which frequently occur with tardigrades in simple meiofaunal communities. Many other organisms possess a capacity for cryptobiosis in specific life stages, including fungal spores, bacteria, pollen grains, seeds and some insect eggs and larvae.

Freshwater and soil tardigrades may undergo seasonal encystation in response to an unfavourable chemical environment. This represents another, distinct form of dormancy. Cyst formation involves contraction of the animal within the moulted cuticle and secretion of a protective, heavily tanned cyst cuticle; this is shed at excystation. Cysts show reduced metabolism and modest desiccation tolerance, but details of their physiology are poorly known.

Fossil History and Phylogeny

Two clear tardigrades have been found: single specimens of a eutardigrade and heterotardigrade from Cretaceous amber. However, recent fossil microarthropods from the Middle Cambrian of Siberia include a possible tardigrade. If verified, this would push the origins of the phylum back to at least 520 million years ago. Unfortunately, the fossils in question are already miniaturized animals and give little clue as to the wider affinities of tardigrades. Their closest relatives among Palaeozoic fossils are probably the lobopodial arthropods from the Cambrian deposits in Chengjiang, China and the Burgess Shale, British Columbia. These include the many-legged marine Burgessochaeta, Aysheaia, Luolishania and Xenusion, which bear superficial resemblance to onychophorans. Although these indicate that lobopodial arthropods were widespread during the Cambrian, they again provide no information on when tardigrades first evolved. In the absence of additional fossil evidence, we depend on comparative studies of extant species to understand tardigrade phylogeny. See also: Arthropoda (Arthropods); Burgess Shale; Fossil Record: Quality

Tardigrade characters and their phylogenetic implications



Derived characters

Paired, ventral nerve cords

Metameric segmentation

Small size/eutely

Spiral cleavage

α-Chitin procuticle

Tri-radiate pharynx

Embryonic protocoely

Paired limbs per segment

Buccal/stylet complex

Schizocoely (?)



rRNA sequence homology

Malpighian tubules

Brain/CNS structure

Several workers have proposed affinities between tardigrades and the 'aschelminth' phyla based on ultrastructural details of the gut and pharynx, and their capacity for cryptobiosis. Proposed affinities with aschelminthes are mostly superficial similarities that fail to support homology under closer scrutiny. Cryptobiosis might appear a compelling phylogenetic character in view of its evident complexity, but it occurs in all five kingdoms (Prokaryota, Protista, Fungi, Animalia, Plantae) and several animal phyla; it is thus of doubtful value as a phylogenetic marker. More seriously, perhaps, proponents of aschelminth affinities ignore the clear protostome traits seen in tardigrade embryology and in the nervous system. The most comprehensive molecular analyses based on 18S rRNA sequence data support the existence of a Tardigrada-Arthropoda clade and do not indicate close relationships with the aschelminth phyla. Several features of tardigrades represent probable consequences of miniaturization and adaptation to an interstitial habit, including eutely and the lack of excretory organs in heterotardigrades; this probably explains some convergent similarities with aschelminthes. Selected examples of tardigrade characters and their phylogenetic implications are listed in Table 1. See also: Cladistics; Molecular Phylogeny Reconstruction

Presently we cannot say very much about the phylogenetic position of tardigrades within the Arthropoda. Molecular evidence, comparative embryology and ultrastructural details of the cuticle and spermatozoa do not support clear relationships with the main euarthropod lineages (Crustacea, Tracheata, Chelicerata). It seems most reasonable to classify tardigrades, along with the onychophorans, as lobopodial arthropods and sister taxa to the Euarthropoda. If the Arthropoda is to retain phylum status, the Tardigrada and Onychophora should therefore be ranked as subphyla. See also: Chelicerata (Arachnids, Including Spiders, Mites and Scorpions); Crustacea (Crustaceans); Onychophora (Velvet Worms)

The systematics of the Tardigrada were initially laid out by E. Marcus, who named the modern classes Heterotardigrada and Eutardigrada. While the taxonomic characters outlined above provide the basis for a thorough systematic breakdown of orders and families, more basal relationships are speculative. Heterotardigrades are generally treated as the more primitive class on account of their diversity in marine habitats. However, marine eutardigrades are known. Whether the Eutardigrada are the more derived group, and if so where they arose within the Heterotardigrada, are questions that should become clearer with accumulating molecular evidence.

Further Reading
  • Bertolani, R (1987) Biology of Tardigrades. Proceedings of the 4th International Symposium on Tardigrades. Selected Symposia and Monographs I. Modena: Mucchi Editore, Collana U.Z.I.
  • Dastych, H (1988) The Tardigrada of Poland. Monografie Fauny Polski 16: 1-255.
  • Dewel, RA and Dewel, WC (1996) The brain of Echiniscus viridissimus Peterfi, 1956 (Heterotardigrada): a key to understanding the phylogenetic position of the tardigrades and the evolution of the arthropod head. Zoological Journal of the Linnean Society 116: 35-49.
  • Garey, JR Krotec, M Nelson, DR and Brooks, J (1996) Molecular analysis supports a tardigrade-arthropod association. Invertebrate Biology 115: 79-88.
  • Giribaret, G Carranza, S Baguna, J Riutort, M and Ribera, C (1996) First molecular evidence for a Tardigrada-Arthropoda clade. Molecular Biology and Evolution 13: 76-84.
  • Higgins, RP (ed.) (1975) Proceedings on the First International Symposium on Tardigrades Memorie dell' Istituto Italiano di Idrobiologia 32 (Sppl.): 1-469.
  • Kinchin, IM (1994) The Biology of Tardigrades. London: Portland Press.
  • Marcus, E (1929) Tardigrada. In: Bronn, HG (ed.) Klassen et Ordnungen des Tierreichs 5, section 4(3), pp 1-608.
  • Morgan, CI and King, PE (1976) British Tardigrades. Synopses of the British Fauna No. 9, Linnean Society of London. London: Academic Press.
  • Nelson, DR (1982) Proceedings of the 3rd International Symposium on Tardigrades, Johnson City, TN, 1980. East Tennessee State University Press.
  • Ramazzotti, G and Maucci, W (1983) Il Phylum Tardigrada (III edizione riveduta e aggiornata). Memorie dell' Istituto Italiano di Idrobiologia 41: 1-1012.
  • Weglarska, B (1979) Proceedings of the Second International Symposium on Tardigrades. Zeszyty naukowe Uniwersytetu Jagiellonskiego, 25: 1-197.
  • Wright, JC Westh, P and Ramlov, H (1992) Cryptobiosis in Tardigrada. Biological Reviews of the Cambridge Philosophical Society 67: 1-29.
  • Glossary

    External openings in the chambered chorion of tardigrade eggs and in many insect eggs; they permit gas exchange with minimal water loss.


    Containing the stiff polysaccharide chitin (poly-N-acetylglucosamine); the α-chitin procuticle is a probable derived character of the Arthropoda.


    The proteinaceous outer shell of many arthropod eggs.


    A constancy of cell number in specific tissues or organ systems, common when structures are secondarily simplified (e.g. through miniaturization).


    The open blood system of arthropods and molluscs comprising a system of sinuses. It is formed from the embryonic union of the haemal and coelomic systems in arthropods.


    A limb constructed as a lobe-like outgrowth of the body wall and lacking rigid sections and articulations.

    Malpighian tubules

    Long excretory tubules opening into the midgut-hindgut junction in insects and myriapods; the similar glands of eutardigrades are probably not homologous.


    The aquatic fauna inhabiting the interstitial water of soils, marine or freshwater sediments, moss cushions, etc.


    Comprising an ancestral species and all of its descendants. A monophyletic group should be definable by one or more shared characters or synapomorphies, initially derived in the ancestor. Following the cladistic school of Willi Hennig, true phylogenetic taxa or clades should be restricted to monophyletic groups.

    Onychophora (also velvet worms or peripatus)

    A phylum/subphylum of carnivorous lobopodial arthropods inhabiting litter in tropical and subtropical latitudes.


    The production of eggs by mitosis (from either a haploid or diploid female), which develop into new offspring without fertilization.


    A thin, nonwettable air layer, usually trapped by hydrophobic hairs or a chambered cuticle, and providing a large surface area for gas exchange into the spiracular system of aquatic insects. Alternatively, it may simply facilitate gas exchange through a thick cuticle, as in the chorionated eggs of tardigrades and aquatic insects.


    A superphylum including the Annelida, Arthropoda and Mollusca (and several smaller phyla), sharing several fundamental characters including paired ventral nerve cords, spiral determinate cleavage, embryonic protostomy (blastopore becomes mouth), mesoderm derived from the 4d cell, and planktonic larvae (when present) of the trochophore type. Current molecular phylogenies now generally divide the prototomia into 2 or more superphyla including the Gastroneuralia and Lophotrochozoa.

    Jonathan C Wright
    Pomona CollegeClaremont, California, USA
    Wiley ©2007

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