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Milnesium tardigradum

By Ami Dhaduk and David Kam

Geographic Range

Milnesium tardigradum is a cosmopolitan, carnivorous eutardigrade species found throughout Europe, North America, Central, East and Southeast Asia, Oceania, and Antarctica. (Beasley and Miller, 2007; Horikawa and Higashi, 2004; Mehlen, 1969; Miller, et al., 1994; Tumanov, 2006)

Habitat

Milnesium tardigradum requires moist surroundings for gas exchange, reproduction, and feeding, and therefore is most commonly found on temperate-zone mosses (Grimmia tergestina and Bryum argenteum, for example), and lichens (both epiphytic and rock-borne). This species survives even in environmental extremes; they have been found in the Antarctic and at up to 2250 meters above sea level (in Central Asia). In times of drought, M. tardigradum may undergo active dehydration until conditions become more favorable. This species is most often found in the top-most layers of mosses, rather than lower layers with less drainage and constant moisture. Individuals residing on moss cushions or algae share their habitat with ciliates, nematodes, and bdelloid rotifers, species which also act as prey. Though typically terrestrial, some individuals have been found near or in freshwater biomes. (Beasley and Miller, 2007; Hallas, 1978; Mehlen, 1969; Suzuki, 2003; Wright, 1991)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • temporary pools
  • Range elevation
    2250 (high) m
    7381.89 (high) ft

Physical Description

This tardigrade is relatively large (some specimens up to 0.7 mm long). Like all tardigrades, M. tardigradum exhibits a plump, cylindrical, bilaterally-symmetrical body, with a head followed by four segments. Each segment has a pair of stumpy, unjointed legs with double claws (secreted by glands within the legs). The morphology of the claws is an important diagnostic feature for this species. The mouth is located ventrally and anteriorly on the head and is connected to the sucking pharynx by the buccal tube. This muscular pharynx serves to draw in prey, which are often swallowed whole. The digestive and reproductive systems run longitudinally throughout the body, terminating posteriorly in an anus and gonophore, respectively. The body is completely sheathed in a permeable chitinous cuticle through which gas exchange occurs, hence this species' preference for moist environments. No additional specialized respiratory system is present. (Pechenik, 2010; Suzuki, 2003)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    0.2 to 0.7 mm
    0.01 to 0.03 in

Development

Females may lay clutches of anywhere from 1-12 eggs. Once laid, time until hatching is 5-16 days. Milnesium tardigradum do not undergo metamorphosis, continuously molting instead. Larvae typically molt twice before they attain sexual maturity, and then may molt up to five additional times (each molt occuring after egg laying). Molting begins when an individual ejects its pharynx and associated structures, including the buccal tube and the esophagus, then seals its mouth. Larvae then find a place to hide to undergo ecdysis. Time between molts varies widely, depending on individuals' nutrition. (Suzuki, 2003)

Reproduction

It is currently unknown whether mating in Milnesium tardigradum is seasonal or occurs year-round. Mating systems have not been reported in the literature, but reproduction by parthenogenisis (females only) has been documented. (Suzuki, 2003)

Milnesium tardigradum achieves reproductive maturity after the 2nd-instar stage, which is usually at 8-12 days old. Females may lay clutches of anywhere from 1-12 eggs (each approximately 80 µm long), with an average of 6.9 eggs per clutch. Once laid, time until hatching is 5-16 days. This species exhibits parthenogenesis, a common tactic in freshwater invertebrate species. The environmental cue responsible for the laying of male eggs has yet to be demonstrated. (Suzuki, 2003)

  • Breeding interval
    The frequency with which this species reproduces is currently unknown.
  • Breeding season
    No specific breeding season has been documented for M. tardigradum
  • Range number of offspring
    1 to 12
  • Range gestation period
    5 to 16 days
  • Range age at sexual or reproductive maturity (female)
    8 to 12 days

Egg clutches are deposited in the space between a female’s old and new cuticle. When the mother has completed molting and departs, the old cuticle remains to protect and feed the developing offspring. (Suzuki, 2003)

Lifespan/Longevity

The only study performed on the life history of Milnesium tardigradum (in captivity) reported that the most long-lived individual entered its final anhydrobiotic state at 58 days, while some individuals died before their first molting. The average lifespan of all the individuals raised in captivity was 40 days. It is unknown what the lifespan is in the wild, but scientists speculate that several periods of anhydrobiosis could lead to a theoretical lifespan of over six years. (Suzuki, 2003)

  • Range lifespan
    Status: captivity
    14 to 58 days
  • Average lifespan
    Status: wild
    6 years

Behavior

The name tardigrade means “slow walker”, which describes these organisms' movement patterns. Because M. tardigradum lives in fast-changing microhabitats and actively needs to hunt for prey, locomotion is essential to its survival. However, it is not possible to observe the organism’s motion in its natural environment, and studies of locomotion have only been conducted in artificial environments that closely mimic natural habitats. The maximal rate of movement for this species was found to be 100 µm/s. Although M. tardigradum has a low movement speed on average, it is capable of short bursts of speed, most likely in response to environmental stimuli. (Hengherr, et al., 2010; Shcherbakob, et al., 2010)

Milnesium tardigradum is also resistant to extreme conditions and as a result, can colonize harsh habitats. Because these habitats are constantly undergoing seasonal changes, M. tardigradum can undergo prolonged periods of dormancy in a state of cryptobiosis as an adaptation to desiccation, freezing, ionizing radiation, and osmotic and anoxic stress. During these periods, the organism suspends all metabolic activity and enters an ametabolic stage where it forms a ‘tun’ state and can survive until environmental conditions become favorable. (Hengherr, et al., 2010; Shcherbakob, et al., 2010)

Home Range

No definitive home range information is currently available for M. tardigradum. Due to its small size, slow rate of movement, and environmental requirements, an individual's home range is unlikely to be any larger than the patch of algae or moss it inhabits. (Hallas, 1978; Shcherbakob, et al., 2010; Suzuki, 2003)

Communication and Perception

The methods by which tardigrades might communicate with one another are currently unknown. Milnesium tardigradum has a posterior eye that is composed of a cup-like pigment cell with microvilli, one to two ciliary cells, and four to five epithelial cells, suggesting an epidermal origin of the eye. There are many factors that may influence the ability of M. tardigradum to respond to light such as the light intensity and the organism’s age (because reactions may be subject to change throughout its lifetime). Additionally, there are six distinctive chemosensory lobes around the mouth that also exhibit movement. (Greven, 2007)

Food Habits

Milnesium tardigradum is an omnivorous predator that actively hunts its prey, feeding on rotifers, nematodes, and algae. This species has also been recorded feeding on smaller tardigrade species in the genera Diphascon and Hypsibius, as evidenced by the remains of claws and buccal apparatus found in the guts of M. tardigradum. The mouths of these predatory tardigrades are armed with characteristic calcium carbonate stylets, which pierce algae or smaller invertebrates, releasing the cellular contents for the tardigrades to eat. One study recorded an adult M. tardigradum consuming as many as 13 rotifers over an interval of 17 minutes. The same study found that larvae often took over fifteen minutes to consume the contents of a single rotifer through the lorica. However, the tardigrades were able to swallow the prey whole after they had passed their third instar stage. (Nelson, 2002; Suzuki, 2003)

  • Animal Foods
  • terrestrial worms
  • aquatic or marine worms
  • Plant Foods
  • algae
  • Other Foods
  • microbes

Predation

Although there are no known predators specific to Milnesium tardigradum, predation has been observed on other tardigrade species by nematodes, snails, and larger tardigrades. (Fox and García-Moll, 1962; Pohlad and Bernard, 1978; Sánchez-Moreno, et al., 2008)

Ecosystem Roles

This species acts as a predator of very small invertebrates and microbes in the microhabitats that it utilizes, and may be prey for larger invertebrate species. Milnesium tardigradum is also a host for the parasite Sorochytrium milnesiophthora, a primitive fungus in the division chytridiomycota. Thia parasite infects the tardigrade by attaching to its cuticle. It then becomes enclosed in a cyst, and creates an appressorium, a flattened, thickened tip of a hyphal branch through which it penetrates the host. (Dewel, et al., 1985)

Commensal/Parasitic Species
  • Sorochytrium milnesiophthora (Class Blastocladiomycetes, Phylum Blastocladiomycota)

Economic Importance for Humans: Positive

Milnesium tardigradum has a minimal economic impact on humans. However, the organism’s ability to undergo cryptobiosis has sparked an interest in the medical field. It has allowed for tests of the preservation of cells and organs, leading to a renewal of interest in the species at the proteomic and genomic levels. (Pilato and Maria, 2001)

Economic Importance for Humans: Negative

There are no known adverse effects of Milnesium tardigradum on humans.

Conservation Status

Milnesium tardigradum is an abundant and ubiquitous species that has no special conservation status. (Förster, et al., 2009)

Contributors

Ami Dhaduk (author), The College of New Jersey, David Kam (author), The College of New Jersey, Keith Pecor (editor), The College of New Jersey, Jeremy Wright (editor), University of Michigan-Ann Arbor.

Glossary

Antarctica

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

Australian

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

World Map

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.

World Map

Palearctic

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

World Map

asexual

reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents

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.

chemical

uses smells or other chemicals to communicate

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.

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.

drug

a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

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.

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.

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).

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.

native range

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

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.

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.

World Map

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

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.

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.

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.

sedentary

remains in the same area

sexual

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

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.

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.

visual

uses sight to communicate

young precocial

young are relatively well-developed when born

References

Beasley, C. 1967. Tardigrades from Kansas. Transactions of the Kansas Academy of Science, 70/4: 464-470.

Beasley, C., W. Miller. 2007. Tardigrada of Xinjiang Uygur Autonomous Region, China. Proceedings of the Tenth International Symposium on Tardigrada, J. Limnol., 66/Suppl. 1: 49-55.

Dewel, R., J. Joines, J. Bond. 1985. A new chytridiomycete parasitizing the tardigrade Milnesium tardigradum. Canadian Journal of Botany, 63: 1525-1534. Accessed March 21, 2012 at http://www.nrcresearchpress.com/doi/abs/10.1139/b85-211.

Faurby, S., K. Jönsson, L. Rebecchi, P. Funch. 2008. Variation in anhydrobiotic survival of two eutardigrade morphospecies: a story of cryptic species and their dispersal. Journal of Zoology, 275/2: 139-145.

Fox, I., I. García-Moll. 1962. Echiniscus molluscorum, new tardigrade from the feces of the land snail, Bulimulus exilis (Gmelin) in Puerto Rico (Tardigrada: Scutechiniscidae). Journal of Parasitology, 48: 177-181. Accessed March 21, 2012 at http://www.jstor.org/discover/10.2307/3275559?uid=36003&uid=3739808&uid=2129&uid=2&uid=70&uid=3&uid=67&uid=36002&uid=62&uid=3739256&sid=21100676537031.

Förster, F., C. Liang, A. Shkumatov, D. Biesser, J. Engelmann, M. Schnölzer, M. Frohme, T. Müller, R. Schill, T. Dandekar. 2009. Tardigrade workbench: comparing stress-related proteins, sequence-similar and functional protein clusters as well as RNA elements in tardigrades. BMC Genomics, 10: 469. Accessed March 21, 2012 at http://www.biomedcentral.com/content/pdf/1471-2164-10-469.pdf..

Greven, H. 2007. Comments on the eyes of tardigrades. Arthropod Structure & Development, 36: 401-407. Accessed March 21, 2012 at http://www.sciencedirect.com/science/article/pii/S1467803907000473.

Hallas, T. 1978. Habitat preference in terrestrial tardigrades. Annales Zoologici Fennici, 15: 66-68.

Hengherr, S., A. Reuner, F. Brümmer, R. Schill. 2010. Ice crystallization and freeze tolerance in embryonic stages of the tardigrade Milnesium tardigradum. Comparative Biochemistry and Physiology - Part A: Molecular & Integrative Physiology, 156: 151-155. Accessed March 21, 2012 at http://www.sciencedirect.com/science/article/pii/S1095643310000280.

Hohl, A., W. Miller, D. Nelson. 2001. The distribution of tardigrades upwind and downwind of a Missouri coal-burning power plant. Zoologischer Anzeiger - A Journal of Comparative Zoology, 240/3-4: 395-401.

Horikawa, D., S. Higashi. 2004. Desiccation tolerance of the tardigrade Milnesium tardigradum collected in Sapporo, Japan, and Bogor, Indonesia. Zoological Science, 21/8: 813-816. Accessed November 17, 2012 at http://www.bioone.org/doi/pdf/10.2108/zsj.21.813.

Mehlen, R. 1969. New tardigrada from Texas. American Midland Naturalist, 81/2: 395-404.

Miller, W., H. Heatwole, R. Pidgeon, G. Gardiner. 1994. Tardigrades of the Australian Antarctic Territories: the Larsemann Hills, East Antarctica. Transactions of the American Microscopical Society, 113/2: 142-160.

Nelson, D. 2002. Current status of the Tardigrada: evolution and ecology. Integrative and Comparative Biology, 42: 652–659. Accessed March 21, 2012 at http://icb.oxfordjournals.org/content/42/3/652.full.pdf.

Pechenik, J. 2010. Biology of the Invertebrates. New York: McGraw-Hill.

Pilato, G., G. Maria. 2001. Biogeography and limno-terrestrial tardigrades: are they truly incompatible binomials?. Zoologischer Anzeiger - A Journal of Comparative Zoology, 3-4: 511-516. Accessed March 21, 2012 at http://www.sciencedirect.com/science/article/pii/S0044523104700522.

Pohlad, B., E. Bernard. 1978. A new species of Entomophthorales parasitizing tardigrades. Mycologia, 70: 130-139. Accessed March 21, 2012 at http://www.jstor.org/discover/10.2307/3758693?uid=36003&uid=3739808&uid=2129&uid=2&uid=70&uid=3&uid=67&uid=36002&uid=62&uid=3739256&sid=21100676537031.

Shcherbakob, D., R. Schill, F. Brümmer. 2010. Movement behaviour and video tracking of Milnesium tardigradum Doyère, 1840 (Eutardigrada, Apochela). Contributions to Zoology, 79: 33-38. Accessed March 21, 2012 at http://dpc.uba.uva.nl/cgi/t/text/get-pdf?c=ctz;idno=7901a02..

Suzuki, A. 2003. Life history of Milnesium tardigradum Doyère (Tardigrada) under a rearing environment. Zoological Science, 20/1: 49-57. Accessed November 19, 2012 at http://www.ncbi.nlm.nih.gov/pubmed/12560601.

Sánchez-Moreno, S., H. Ferris, N. Guil. 2008. Role of tardigrades in the suppressive service of a soil food web. Agriculture, Ecosystems and Environment, 124: 187–192. Accessed March 21, 2012 at http://plpnemweb.ucdavis.edu/nemaplex/FerrisPublications/pdf%20files/156Sanchez-etal2008.pdf.

Tumanov, D. 2006. Five new species of the genus Milnesium (Tardigrada, Eutardigrada, Milnesiidae). Zootaxa, 1122: 1-23.

Wright, J. 1991. The significance of four xeric parameters in the ecology of terrestrial Tardigrada. Journal of Zoology, 224/1: 59-77.

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