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Tardigradawater bears(Also: moss piglets)

By Jeremy Wright

Diversity

Phylum Tardigrada is comprised of over 1,000 species placed into three classes: Heterotardigrada, Eutardigrada, and Mesotardigrada. Class Heterotardigrada includes order Arthrotardigrada, which are mostly marine, as well as order Echiniscoidea, which are terrestrial. Class Eutardigrada includes the primarily terrestrial or freshwater order Parachela and the unarmored, terrestrial order Apochela. Class Mesotardigrada only includes one order, Thermozodia; their existence is questionable, due to the destruction of the type locality of its single species, Thermozodium esakii, which had been found in a Japanese hot spring, and the loss of the original types themselves. There are a variety of morphological characters used to distinguish classes of tardigrades, including presence or absence of cephalic papillae, a cloaca, or malpighian tubules. They are known from all intertidal and subtidal zones, at temperatures ranging from 149°C to -272°C, and from the Arctic to the Antarctic. They can survive extreme environmental conditions, including anoxic conditions, vacuums, and ionizing radiation. (Brusca and Brusca, 2003; Horikawa, 2008; Nelson, 2002; Ruppert, et al., 2004; Zhang, 2011)

Geographic Range

Tardigrades are cosmopolitan, and are found in terrestrial, marine, and freshwater environments from the Arctic to the Antarctic, including great depths and altitudes. (Brusca and Brusca, 2003; Glime, 2010; Nelson, 2002)

Habitat

These animals are found in semi-aquatic habitats, such as water films and leaf litter, as well as deep and shallow freshwater and marine habitats. They are commonly associated with bryophytes (mosses). Their ability to enter anabiotic dormancy enables them to survive extreme environmental conditions. Active specimens have been collected from marine intertidal and subtidal zones at depths of up to 4,690 m, and from lakes up to 150 m deep. They have also been collected in the Himalaya Mountains at altitudes over 6,000 m. Although as a group, tardigrades are broadly cosmopolitan, some species are found only under specific environmental conditions. (Brusca and Brusca, 2003; Glime, 2010; Nelson, 2002)

Systematic and Taxonomic History

The German zoologist Johann August Ephraim Goeze first recognized and described tardigrade species in 1773, giving them the common name "little water bear" ("kleiner Wasserbär" in German), a name that is still used today. The name Tardigrada (meaning "slow walker") was applied to the group in 1777 by the Italian biologist Lazzaro Spallanzani. This name has been in use ever since, no synonyms exist. Tardigrades were first recognized as their own phylum in 1962. (Bordenstein, 2008; Nelson, 2002)

Tardigrades are widely accepted as monophyletic, with a number of morphological and molecular autapomorphies that are diagnostic for the group. Recent molecular evidence suggests that tardigrades are the sister group to a clade composed of arthropods and onychophorans (velvet worms). Together, these three phyla constitute Panarthropoda, which has itself been placed in superphylum Ecdysozoa, a clade containing many other phyla of molting organisms. Interrelationships among many ecdysozoan lineages are still unresolved, with the membership of several phyla still under debate. Within tardigrades, molecular analyses indicate heterotardigrades and eutardigrades represent sister groups, although relationships within Heterotardigrada are poorly resolved and the order may in fact be paraphyletic. (Budd, 2001; Campbell, et al., 2011; Garey, et al., 1999; Kristensen, 1994; Nelson, 2002; Rota-Stabelli, et al., 2010; Telford, et al., 2008)

  • Synapomorphies
    • nerve commissure from the protocerebrum to the first trunk ganglion
    • telescopic mouth and legs
    • mitochondrial gene trnI located between genes trnL1 and trnL2

Physical Description

Tardigrades are small (average 0.1 to 0.5 mm long), bilaterally symmetrical animals, with four pairs of lobopodious legs terminating in adhesive pads, discs, or claws. All tardigrades have intrinsic musculature and some species have telescopic legs. Their bodies are covered by a thin cuticle, which is uncalcified, may be divided into dorsal and lateral plates, and is often ornamented. The cuticle, which is secreted by an underlying epidermal layer, is made of up to seven layers of proteins and chitin, sometimes including a waxy layer, and also lines the rectum and foregut. These animals grow through a series of molts, reaching sexual maturity after three to six instars. Non-marine tardigrades can be very colorful; this color is determined by food in the gut, cuticle pigments, or granular bodies in their hemocoel. (Brusca and Brusca, 2003; Nelson, 2002; Ruppert, et al., 2004)

Tardigrades have both smooth and cross-striated muscles, their body walls are made of muscle bands extending between subcuticular attachment points. These bands are typically comprised of a single, or a few muscle cells. Tardigrades move by using their legs, controlled by independent sets of muscles, in a stepping motion, although at least one marine species is known to swim, expanding its cuticle to resemble the bell of a jellyfish. Their nervous systems are metamerous, with a large, dorsal, lobed cerebral ganglion connected to a subesophageal ganglion, subsequently attached to a pair of ventral nerve cords extending posteriorly and connecting a chain of four ganglia, which control their four pairs of legs. Their bodies are covered with sensory bristles or spines, similar to setae, most thickly in their anterior and ventral regions, and their bodies often terminate with long sensory cirri, often known as clava, which are likely chemoreceptive. Two eyespots, made up of five cells, one of which is light sensitive and pigmented, are often, though not always, present. (Brusca and Brusca, 2003; Nelson, 2002; Ruppert, et al., 2004)

These animals have greatly reduced coeloms, so their body cavities function mainly as hemocoels. They have a pair of oral stylets, mouth placement depends on diet: detritivorous tardigrades have ventral mouths while carnivorous and omnivorous tardigrades have terminal mouths. The mouth is connected to a muscular sucking pharynx flanked by a pair of salivary glands. In some species, there are also chitinous rods in the pharynx, which provide support and possibly a masticating action. The pharynx empties into the esophagus, which is connected to a large midgut, where digestion and absorption take place. The midgut opens into a short hindgut, leading to a terminal anus. In freshwater tardigrades, there are three or four large glands, comprised of three to nine cells each, known as malpighian tubules. The exact function of these structures is unknown but may be related to osmoregulation. Tardigrades have no blood vessels or other structures for gas exchange, relying on their body walls and cavities for this function. (Brusca and Brusca, 2003; Nelson, 2002; Ruppert, et al., 2004)

  • Sexual Dimorphism
  • sexes alike

Development

In egg-laying species, females lay anywhere from 1 to 30 fertilized eggs at a time. In aquatic species, eggs are laid in the female’s shed cuticle or glued to a nearby object; if laid within a cuticle, the eggs are most often smooth, while those laid outside the cuticle often have exterior decoration. Eggs of terrestrial species have thick shells that protect the developing embryos from potential desiccation. It is possible for embryos, as well as adults, to enter diapause or cryptobiosis at any point during development, dictated by environmental conditions. Embryonic development is direct and rapid, with holoblastic cleavage. Development may be completed within 14 days, although it may be delayed for as many as 90 days. Young use their stylets to break out of their shells. Upon hatching, they often have fewer claws, spines, and cirri than adults, and lack coloration. These animals are eutelic, having a fixed number of cells in their bodies from birth; as the animal grows, cells grow in size rather than increasing in number. Growth occurs through a series of molts; a molt usually takes 5 to 10 days to complete, occurring more rapidly in early stages of growth, and may be associated with egg laying or defecation. While molting, tardigrades shed their entire cuticles, including their gut linings, and cannot feed. (Brusca and Brusca, 2003; Glime, 2010; Nelson, 2002)

Reproduction

Many terrestrial tardigrades are parthenogenetic or self-fertilizing hermaphrodites, while aquatic species are most typically dioecious. In species that reproduce sexually, each sex has a single gonad, located above the gut. Males have two sperm ducts connecting to a single gonopore, opening in front of the anus or into the hindgut. Females have a single oviduct opening into a single gonopore as well, located in the hindgut (cloaca) or dorsally to the anus, as well as either one (near the cloaca) or two (opening separately) seminal receptacles. (Bertolani, 2001; Brusca and Brusca, 2003; Glime, 2010; Nelson, 2002)

Fertilization may be direct, with the male depositing sperm into the female's seminal receptacle or body cavity, or indirect. Indirect fertilization occurs when the male deposits sperm under the female’s cuticle; when she molts, she lays fertilized eggs into her shed cuticle. Courtship behavior has been observed in a few species of tardigrades: a male will stroke a female with his cirri, stimulating her to lay eggs on a grain of sand, and then he spreads his sperm over the eggs. In some species, there are no males known, others have dwarf males. (Bertolani, 2001; Brusca and Brusca, 2003)

Tardigrades reproduce year-round, assuming conditions are favorable. Females lay multiple clutches throughout their life, while males either reproduce once, or several times, depending on the species. If environmental conditions are unfavorable, tardigrades may enter a period of cryptobiosis or diapause. Although sexual reproduction is common, some are parthenogenic or hermaphroditic. Development may be completed in as few as 14 days, although it may be delayed to 30 to 90 days and, in some species, temperature may be a key factor in time to hatching and size at hatching. Some marine tardigrades (such as Halobiotus crispae) living in particularly extreme conditions exhibit cyclomorphosis, with different morphs in the winter and summer seasons. Typically winter forms hibernate and are sexually immature and gregarious; individuals gather to protect themselves against cold temperatures. In those populations, all individuals become sexually mature at the same time, in the beginning of the summer. (Bertolani, 2001; Brusca and Brusca, 2003; Glime, 2010; Nelson, 2002)

Beyond gamete production and, in the case of females, laying eggs, tardigrades are not known to exhibit any parental investment. (Brusca and Brusca, 2003; Glime, 2010)

Lifespan/Longevity

Excluding cryptobiotic periods, tardigrades generally live 3 to 30 months. The aging process may be halted during cryptobiosis. At least one marine genus (Echiniscoides) alternates between active and inactive stages ever six months; this may increase their lifespan by decades. (Brusca and Brusca, 2003; Glime, 2010; Nelson, 2002)

Behavior

The most distinctive behavior of tardigrades is their ability to enter cryptobiotic and anabiotic stages. These include encystment, anoxybiosis, cryobiosis, osmobiosis and anhydrobosis. Encystment is a type of anabiosis, a state of dormancy in which metabolic activity is greatly reduced; it is common in freshwater tardigrades, while in this state, the animals are drought-resistant and can survive for over a year. During encystment, the animal pulls in its legs, excretes excess body water, and secretes a double-walled cuticle to protect itself. The remaining states are considered forms of cryptobiosis, an extreme form of anabiosis, in which there are no outward signs of metabolic activity. When in these states, tardigrades may form tuns: single-walled, barrel-shaped protective forms. Aquatic tardigrades enter anoxybiosis due to low oxygen tension, but can only live in this state for up to about three days before oxygen must be reintroduced. Cryobiosis is induced by very low temperatures and allows the animals to survive freezing and thawing, while osmobiosis is induced by increased osmotic pressures and anhydrobiosis is brought about by the threat of dehydration. Tardigrades in these states are extremely well-protected and, if in a cyst or tun, may be dispersed by winds, currents, or other outside forces. (Brusca and Brusca, 2003; Nelson, 2002)

Communication and Perception

Tardigrade bodies are covered with sensory bristles or spines, similar to setae, most thickly in their anterior and ventral regions. Their bodies often terminate with long sensory cirri, some known as clava, which are likely chemoreceptive. Two eyespots, made up of five cells, one of which is light sensitive and pigmented, are often, though not always, present. (Brusca and Brusca, 2003)

Food Habits

Tardigrades feed on cellular fluids, piercing cell walls with their oral stylets. Food items include bacteria, algae, protozoa, bryophytes, fungi, and decaying plant matter. Larger species are known to feed on protozoa, nematodes, rotifers, and smaller tardigrades. (Brusca and Brusca, 2003; Glime, 2010; Nelson, 2002; Sanchez-Moreno, et al., 2008)

Predation

Predators include other tardigrades, nematodes, snails, mites, oligochaetes, spiders, springtails and insect larvae, as well as aquatic crustaceans and arthropods. They may also be prey to fungal predators. (Glime, 2010; Nelson, 2002; Thorp and Covich, 2010)

  • Known Predators
    • Bulimulus guadalupensis (Class Gastropoda, Phylum Mollusca)
    • Ballocephala pedicellata (Order Entomophthorales, Phylum Zygomycota)
    • Lecophagus antarcticus (Order Pezizales, Phylum Ascomycota)
    • Lecophagus longispora (Order Pezizales, Phylum Ascomycota)
    • Lecophagus muscicola (Order Pezizales, Phylum Ascomycota)
    • insects (Class Insecta, Phylum Arthropoda)
    • mites (Subclass Acari, Class Arachnida)
    • nematodes (Phylum Nematoda)
    • oligochaetes (Subclass Oligochaeta, Phylum Annelida)
    • spiders (Order Aranae, Class Arachnida)
    • springtails (Subclass Collembola, Subphylum Hexapoda)
    • tardigrades (Phylum Tardigrada)

Ecosystem Roles

In some environments, tardigrades may be a primary consumer of nematodes, greatly affecting population sizes. Some species (Milnesium tardigradum and Ramazzottius oberhaeuseri, in particular) may carry a symphoriont protozoan species (Pyxidium tardigradum). Many tardigrade species living in mossy environments carry fungal parasites. (Drechsler, 1951; Glime, 2010; Sanchez-Moreno, et al., 2008; Thorp and Covich, 2010)

Mutualist Species
  • Pyxidium tardigradum (Phylum Ciliophora)
Commensal/Parasitic Species
  • Catenaria anguillulae (Division Chytridiomycota, Kingdom Fungi)
  • Entomophthorales sp. (Subphylum Entomophthoromycotina, Kingdom Fungi)
  • Haptoglossa mirabilis (Kingdom Chromalveolata)
  • Harposporium anguillulae (Division Eumycota, Kingdom Fungi)

Economic Importance for Humans: Positive

Beyond scientific research and a small hobbyist following, there are no positive effects of tardigrades on humans. (Brusca and Brusca, 2003)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no known adverse effects of tardigrades on humans. (Brusca and Brusca, 2003)

Conservation Status

As a cosmopolitan phylum, there is little concern that tardigrades will become endangered, and currently, there are no conservation initiatives focused on any specific tardigrade species. There is evidence, however, that pollution may adversely affect their populations, as poor air quality, acid rain, and concentrations of heavy metals in bryophyte habitats have led to decreases in some populations. (Glime, 2010)

  • IUCN Red List [Link]
    Not Evaluated

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.

Glossary

Antarctica

lives on Antarctica, the southernmost continent which sits astride the southern pole.

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

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Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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Neotropical

living in the southern part of the New World. In other words, Central and South America.

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Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

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Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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abyssal

on or near the ocean floor in the deep ocean. Abyssal regions are characterized by complete lack of light, extremely high water pressure, low nutrient availability, and continuous cold (3 degrees C).

agricultural

living in landscapes dominated by human agriculture.

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

bog

a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

crepuscular

active at dawn and dusk

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

detritivore

an animal that mainly eats decomposed plants and/or animals

diapause

a period of time when growth or development is suspended in insects and other invertebrates, it can usually only be ended the appropriate environmental stimulus.

diurnal
  1. active during the day, 2. lasting for one day.
embryonic diapause

At about the time a female gives birth (e.g. in most kangaroo species), she also becomes receptive and mates. Embryos produced at this mating develop only as far as a hollow ball of cells (the blastocyst) and then become quiescent, entering a state of suspended animation or embryonic diapause. The hormonal signal (prolactin) which blocks further development of the blastocyst is produced in response to the sucking stimulus from the young in the pouch. When sucking decreases as the young begins to eat other food and to leave the pouch, or if the young is lost from the pouch, the quiescent blastocyst resumes development, the embryo is born, and the cycle begins again. (Macdonald 1984)

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

freshwater

mainly lives in water that is not salty.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

holarctic

a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.

internal fertilization

fertilization takes place within the female's body

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

marsh

marshes are wetland areas often dominated by grasses and reeds.

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

mycophage

an animal that mainly eats fungus

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oceanic vent

Areas of the deep sea floor where continental plates are being pushed apart. Oceanic vents are places where hot sulfur-rich water is released from the ocean floor. An aquatic biome.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

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oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

parthenogenic

development takes place in an unfertilized egg

pelagic

An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).

planktivore

an animal that mainly eats plankton

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

reef

structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

semelparous

offspring are all produced in a single group (litter, clutch, etc.), after which the parent usually dies. Semelparous organisms often only live through a single season/year (or other periodic change in conditions) but may live for many seasons. In both cases reproduction occurs as a single investment of energy in offspring, with no future chance for investment in reproduction.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

suburban

living in residential areas on the outskirts of large cities or towns.

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

taiga

Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

tundra

A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

visual

uses sight to communicate

year-round breeding

breeding takes place throughout the year

young precocial

young are relatively well-developed when born

References

Bertolani, R. 2001. Evolution of the reproductive mechanisms in tardigrades: A review. Zoologischer Anzeiger, 240: 247-252.

Bordenstein, S. 2008. "Tardigrades (Water Bears)" (On-line). Microbial Life Educational Resources. Accessed March 24, 2013 at http://serc.carleton.edu/microbelife/topics/tardigrade/index.html.

Brusca, R., G. Brusca. 2003. Invertebrates (2nd Edition). Sunderland, MA: Sinauer Associates.

Budd, G. 2001. Tardigrades as ‘stem-group arthropods’: the evidence from the Cambrian fauna. Zoologischer Anzeiger, 240: 265-279.

Campbell, L., O. Rota-Stabelli, G. Edgecombe, T. Marchioro, S. Longhorn, M. Telford, H. Phillipe, L. Rebecchi, K. Peterson, D. Pisani. 2011. MicroRNAs and phylogenomics resolve the relationships of Tardigrada and suggest that velvet worms are the sister group of Arthropoda. Proceedings of the National Academy of Sciences of the United States of America, 108/38: 15920-15924.

Drechsler, C. 1951. An entomorphthoraceous tardigrade parasite producing small conida on propulsive cells in spicate heads. Bulletin of the Torrey Botanical Club, 78/3: 183-200. Accessed March 13, 2013 at http://www.jstor.org/discover/10.2307/2481973?uid=3739832&uid=2129&uid=2&uid=70&uid=4&uid=3739256&sid=21101756270653.

Garey, J., D. Nelson, L. Mackey, J. Li. 1999. Tardigrade phylogeny: congruence of morphological and molecular evidence. Zoologischer Anzeiger, 238: 205-210.

Glime, J. 2010. Bryophyte Ecology: Volume 2, Bryogological Interaction. Houghton, MI: Michigan Technological University and the International Association of Bryologists. Accessed March 13, 2013 at http://www.bryoecol.mtu.edu/.

Horikawa, D. 2008. "What are tardigrades?" (On-line). Research on the World's Toughest Animals. Accessed March 13, 2013 at http://tardigrades.net/e-kumamushi.html.

Kristensen, R. 1994. The phylogenetic position of the Tardigrada. Sixth International Symposium on Tardigrada, 6: 25.

Nelson, D. 2002. Current status of the Tardigrada: Evolution and ecology. Integrative and Comparative Biology, 42/3: 652-659.

Rota-Stabelli, O., E. Kayal, D. Gleeson, J. Daub, J. Boore, M. Telford, D. Pisani, M. Blaxter, D. Lavrov. 2010. Ecdysozoan mitogenomics: evidence for a common origin of the legged invertebrates, the Panarthropoda. Genome Biology and Evolution, 2: 425-440.

Ruppert, E., R. Fox, R. Barnes. 2004. Invertebrate zoology: A functional evolutionary approach (7th Edition). Belmont, CA: Thomson-Brooks/Cole.

Sanchez-Moreno, S., H. Ferris, G. Noemi. 2008. Role of tardigrades in the suppressive service of a soil food web. Agriculture, Ecosystems, and Environment, 124: 187-192. Accessed March 13, 2013 at http://plpnemweb.ucdavis.edu/nemaplex/FerrisPublications/pdf%20files/156Sanchez-etal2008.pdf.

Telford, M., S. Bourlat, A. Economou, D. Papillon, O. Rota-Stabelli. 2008. The evolution of the Ecdysozoa. Proceedings of the Royal Society B: Biological Sciences, 363: 1529-1537.

Thorp, J., A. Covich. 2010. Ecology and Classification of North American Freshwater Invertebrates. London, England: Academic Press.

Zhang, Z. 2011. Animal biodiversity: An introduction to higher-level classification and taxonomic richness. Zootaxa, 3148: 7-12.

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