Tardigrada

(Water bears)

Phylum Tardigrada

Number of families 20

Thumbnail description
Microscopic, aquatic, multicellular, segmental animals with four pairs of legs

Evolution and systematics

The phylum Tardigrada belongs to the Panarthropoda group, together with the onychophorans (velvet worms) and arthropods, and comprises almost 1,000 described species. However, taxonomists expect that at least 10,000 species exist. The phylum is divided into three classes: Heterotardigrada, Eutardigrada, and Mesotardigrada. The latter was established on the basis of a single species, Thermozodium esakii, found in a hot sulfur spring in Nagasaki, Japan. However, the species has not been recorded since the end of World War II. The Heterotardigrada consists of two orders, Arthrotardigrada and Echiniscoidea, and is characterized by the presence of cephalic appendages, so-called cirri and clavae, that function as mechano- and chemoreceptors, respectively. The arthrotardigrades are marine forms that usually have median cirrus and telescopic legs, with or without toes, while the echiniscids are terrestrial armored or marine unarmored forms. The echiniscids have no median cirrus and the legs lack toes. All heterotardigrades have a separate gonopore and anus.

Eutardigrada consists of two orders, Parachela and Apochela, and is characterized by the absence or reduction of external sensory structures. The cuticle is unarmored and the legs have no toes. The so-called double claws of eutardigrades are differentiated into a primary and a secondary branch. Gametes, excretory products, and feces are released through a cloaca. True hetero- and eutardigrades are found in Cretaceous amber from Canada and the United States, and an aberrant tardigrade has recently been recorded in Siberian limestone from the Middle Cambrian.

Physical characteristics

Tardigrades are bilaterally symmetrical and vary in shape from cylindrical to extremely dorso-ventrally flattened. The majority of tardigrade species are white to translucent, but some terrestrial forms may exhibit strong colors such as yellow, orange, green, or red to olive-black. There are five distinct body segments, including a cephalic segment and four trunk segments, each bearing a pair of segmented legs with oblique- or cross-striated muscles. The terrestrial and limnic forms have reduced the segmentation in their stumpy legs that bear two to four claws, while the marine forms may have telescopic retractable legs, with up to 13 claws or four toes with complex claws. Other marine tardigrades have four to six toes with rod-shaped adhesive discs or round suction discs also inserted on the foot via toes.

The cuticle of tardigrades is very complex. Both the dorsal and ventral body cuticle may have segmental plates with different spines and appendages. The cuticle is frequently molted in juveniles and adults, much like in arthropods. In the beginning of the molting cycle, the tardigrade enters the simplex stage, which includes sheeting of stylets, stylet supports, buccal tubes, and pharyngeal cuticular rods, the socalled placoids. When the new cuticle is formed, the cuticle of the digestive system and toes/claws is also re-synthesized. The stylet apparatus is re-synthesized by two stylet glands ("salivary glands"), and cuticular claws and toes are formed in special claw glands in the legs.

The digestive system consists of three principal parts: the foregut (ectodermal origin), the midgut (mesodermal origin),

and the hindgut (ectodermal origin). The foregut is a very complex feeding structure that consists of a mouth cavity, a stylet apparatus, a buccal tube, and a tri-radiate pharynx with placoids. The two stylets and the stylet supporters are probably homologous with mouth limbs in arthropods.

The nervous system consists of a three-lobed brain, a subpharyngeal ganglion, and four ventral trunk ganglia. Paired eyespots may be present inside the forebrain. Tardigrades lack respiratory organs, and gas exchange takes place through the epidermis. All tardigrades lack excretory protoor metanephridia, which are common in many other invertebrates. Instead, eutardigrades have three Malpighian tubules at the junction between the mid- and hindgut. The Malpighian tubules may have both an excretory and osmoregulatory function. Heterotardigrades lacks these tubules, but some have segmental organs (coxal glands) that may have an excretory function.

The embryology of the tardigrades is still highly debated. Two theories exist: one theory postulates that the tardigrades have radial cleavage and an enterocoelic mode of coelom formation, whereas another theory suggests that the tardigrades have a modified spiral cleavage and schizocoelic mode of coelom formation. Schizocoely is also found among arthropods, which supports the close relationship between arthropods and tardigrades. The dispute about the tardigrade cleavage type has arisen because of disagreements among scientists about the observed cleavage pattern and cell fates. However, new and improved cell lineage studies appear to support the presence of radial cleavage.

Tardigrades are known to survive long periods of drying or freezing by cryptobiosis – a stage of latent life (ametabolic stage). In fact, it is only tidal tardigrades and the tardigrades that inhabit the interstitial water of mosses and lichens that are capable of cryptobiosis. There are four types of cryptobiosis: anhydrobiosis (dehydration), cryobiosis (very low temperature), osmobiosis (water potential and strong variations in salinity), and anoxybiosis (lack of oxygen). Anhydrobiosis and cryobiosis are well investigated. In these forms of cryptobiosis, the tardigrades can survive from a few months to several years. Neither Osmobiosis nor anoxybiosis are accepted by all scientists as true forms of cryptobiosis, but this is perhaps only a question of insufficient investigations. Usually, the terrestrial species will die after only short time (a day) in water without oxygen, but the tidal tardigrade Echiniscoides sigismundi has survived up to six months in seawater without oxygen. This species is currently the only known species that has the capacities to enter all four types of cryptobiosis.

Distribution

Some tardigrades have great migratory capacities, and eutardigrade species such as Macrobiotus hufelandi and Milnesium tardigradum may be true cosmopolitan species. The eggs of Macrobiotus species have been found in air-plankton collected by airplanes at several thousand feet. In Greenland, heterotardigrades have been found in rainwater samples after the powerful foehn storms. It is known that Echiniscus testudo is capable of migrating with the winds from one continent to another in its anhydrobiotic stage. However, the species has never been found in Australia, so it is not a true cosmopolitan. Dispersal in marine tardigrades is lesser known, but the tidal tardigrade, Echiniscoides sigismundi, may spread with the empty exuvia of the barnacles inside of which it lives. More than a hundred eggs of this tardigrade were found on one exuvium from the barnacle, Semibalanus balanoides. Ships may also help to disperse marine tardigrades. One example is the subspecies of Echiniscoides sigismundi in Australia. Along the coast of eastern Australia and in the Coral Sea, the subspecies Echiniscoides sigismundi polynesiensis is found, but in Nielsen Park in Sidney Harbor, the nominate form (Echiniscoides sigismundi sigismundi) was found on barnacles. The nominate form was described from Northern Europe and the subspecies, E. sigismundi polynesiensis, from the island of Tiahura in the Pacific Ocean. The only explanation for the presence of the nominate form in Sidney Harbor must be that ships from Europe have carried it from Europe to Australia.

Many species of tardigrades are not cosmopolitan. Species found in hot or warm springs may be endemic. The mesotardigrade, Thermozodium esakii, discovered in a hot sulfur spring in Nagasaki, was only found on one occasion, and may be extinct now. The very aberrant eutardigrade, Eohypsibius nadjae, was described from a cold mud volcano from west Greenland. Later, it was found in cold springs in the Faroe Islands and in the northern part of Italy. This unique species may be an Arctic relict that survived in cold springs. Several genera of Heterotardigrada show the old Gondwana (South America, Southern Africa, India, Antarctica, Australia, and New Zealand) distribution.

Habitat

Tardigrades are found in all different kinds of habitats, from the highest elevations in the Himalayas to the deepest trenches in the deep sea, and from hot, radioactive springs to the ice cathedrals inside the Greenland ice cap. Many of the so-called terrestrial species are semi-aquatic, because all tardigrades need a water film to be active. The arthrotardigrades are found in true marine habitats from the tidal beaches to the deep-sea mud. The terrestrial species live in mosses and lichens and tolerate desiccation for up to nine years.

Both the heterotardigrades and eutardigrades have independently invaded the terrestrial environment. One genus of eutardigrades, Halobiotus, has secondarily invaded the marine environment again. The species, H. crispae, is a strange tardigrade that cyclically changes form through the year—a transformation that usually is referred to as cyclomorphosis. The dark winter form is cyst-like and has a double cuticle, but it can still move around if it is not completely frozen. It may survive freezing for up to six months per year. The early spring form tolerates freshwater, has thin stylets, and lacks true placoids in the pharynx. The summer form is only active when the salinity is more than 30 parts per thousand, and it has a normal single layer cuticle and robust stylets with macroplacoids in the pharynx. In this stage, the gonads mature for reproduction.

Behavior

The name Tardigrada means "slow walker" and was given to the first described species of eutardigrades by the Italian scientist Spallanzani in 1776. He described their lumbering gait and in the capacity to form tuns in the cryptobiotic state, but already in 1773, the German priest Goeze had called a tardigrade Kleiner Wasser Bär (little water bear). The fascinating appearance of a moss-living eutardigrade, with its slow and bear-like gait, gives the observer associations to a miniature teddy bear. However, many tardigrades are not slow walkers at all. The carnivorous eutardigrade, Milnesium tardigradum, is the tiger among the water bears. When it attacks nematodes or rotifers, it moves very quickly and several of the eight legs do not touch the substrate during the jump. The fastest mowing tardigrades are found among the very specialized arthrotardigrades. In the genus Batillipes, the claws have been modified to form suction discs. The moving behavior of the species, Batillipes noerrevangi from Denmark, has been video-recorded. This species uses its toe discs for suction, as in the suction discs of geckoes. When it moves from a sand grain under the microscope to the glass slide, it moves faster than the human eye can follow.

Feeding ecology and diet

Tardigrades may be carnivorous, herbivorous, or bacteriovorous. Furthermore, a few marine tardigrade species are parasites on other marine invertebrates. Tetrakentron synaptae is found on the holothurian, Leptosynapta galliennei, where it punctures the epidermal cells of the holothurian and sucks out the cell contents. This species is the only tardigrade that has true adaptations for parasitism. It is dorso-ventrally flattened and all the sensory structures are reduced. As well, the claws are armed with three large hooks that are used to penetrate the epidermis of the holothurian. Females in particular are less mobile and are located in small depressions in the tegument of the holothurian. Another parasitic species is Echiniscoides hoepneri that lives on the embryos of the barnacle, Semibalanus balanoides.

The feeding ecology of the many marine species is not fully understood, but it is known that several species do not eat at all for long periods. Some species have symbiotic bacteria in special head vesicles. The genus Wingstrandarctus that lives in coral sand has three head vesicles containing thiobacteria (sulfur bacteria). The bacteria may give the tardigrade dissolved organic matter (DOM) products such as amino acids and glucose. In the deep-sea family Coronarctidae, the mid-gut may be filled with a white amorphous content that is very similar to gut contents found in bacteriovorous tardigrades.

The feeding ecology of terrestrial and freshwater tardigrades is much better known. The heterotardigrades of the family Echiniscidae seems to be adapted to suck out the cell contents of mosses. Many species have very long stylets to penetrate the thick cellulose walls of mosses. Large eutardigrade species such as Milnesium tardigradum, Macrobiotus richtersi, and Amphibolus nebulosus are carnivores and eat nematodes, rotifers, and other tardigrades. Smaller bryophilous species of eutardigrades may not always suck out the moss cells, but instead may eat the epiphytic diatoms and bacteria that live on the moss. Small eutardigrades living in soil or in the rhizoids of mosses have very thin and narrow buccal tubes. In the genus Diphascon, the buccal tube is flexible with spiral rings like a vacuum cleaner tube. This genus is also found in cryoconite ("star dust") on the Greenland ice cap. The species, Diphascon recamieri, has a rusty colored mid-gut and probably feeds on iron bacteria in the cryoconite.

Reproductive biology

The male reproductive system in all tardigrades seems to be relatively simple. The single testis is located dorsally and the two seminal vesicles open latero-ventrally via two seminal ducts in an oval gonopore papilla (in heterotardigrades) or in the cloaca (in eutardigrades). Penile structures have never been found in any male tardigrade, and it is still uncertain how the sperm transfer occurs. However, it seems that some females have structures that can be inserted into the male so that the female can actively grab the sperm. This method of sperm transfer is very unusual in the animal kingdom.

The female reproductive system in heterotardigrades consists of a single ovary; there is a single oviduct opening in a six-lobed rosette gonopore system anterior to the anus. In eutardigrades, the oviduct opens into the cloaca. Two cuticular seminal receptacles are present in many arthrotardigrades, and a single internal receptacle is present in several eutardigrades. The internal receptacle opens into the hindgut and it lacks a cuticle covering. The seminal receptacles are not homologous in heterotardigrades and eutardigrades. In Milnesium tardigradum, a ventral fourth Malpighian tubule has been described. However, this structure is actually a single seminal receptacle.

All tardigrades have been considered egg-laying, but there exists a single unpublished record from an arthrotardigrade (Styraconyx sp.) collected in the deep sea that shows a larva coming out of the gonopore. If this is true, other deep-sea tardigrades may be viviparous as well.

Eggshell morphology has great taxonomic importance in Eutardigrada. The egg of Macrobiotus hufelandi was the first to be observed in a scanning electron microscope, and the details of the egg sculpture have fascinated scientists ever since. All echiniscids form cysts before they lay eggs. When a female hatches from the cyst, she lays the unsculptured eggs in the old exuvium.

Although information on the mating behavior in Arthrotardigrada is extremely scarce, it has been observed that males and females in the species Parastygarctus sterreri mate venter to venter when the male ejects the sperms into the external seminal receptacles of the female.

In eutardigrades, the male clings with its first leg pair to the anterior part of the female. The claws on the first leg pair of the males may be strongly modified, as in Milnesium tardigradum. Internal fertilization is common in several eutardigrade species, and these species always lay free eggs. However, some species lay their eggs in the old exuvium right after molting, so that the male can afterward spread the sperm into the exuvium.

Most tardigrades are dioecious, but hermaphroditism also occurs. Hermaphroditism is especially common in many genera of limno-terrestrial eutardigrades, whereas it has only been recorded in a single arthrotardigrade species, Orzeliscus sp. Besides usual sexual reproduction, many species are capable of reproducing by parthenogenesis (reproduction without male fertilization). In some species, males have never been observed and all reproduction is solely by parthenogenesis. However, it has been shown that some apparently parthenogenetic species sometimes have populations with males. It is not known how the production of males is triggered in these populations.

It seems likely that the evolution of parthenogenesis in tardigrades is linked to the evolution of cryptobiosis. This postulate is supported by the strong correlation between the presence of parthenogenetic and cryptobiotic capacities. The parthenogenetic capacities in tardigrades have evolved independently in the echiniscid heterotardigrades and the parachelate eutardigrades.

Conservation status

No species are listed by the IUCN.

Significance to humans

The phenomenon of cryptobiosis has fascinated humans since it was discovered in tardigrades by Spallanzani in 1776. Today, researchers think that tardigrades could be used as test animals for traveling into outer space. Experiments in which tardigrades in anhydrobiosis (tun stage) are exposed to cosmic radiation, vacuum, and temperatures close to absolute zero have been very successful, and ongoing experiments with ionosphere balloons have shown that both the species Echiniscus testudo and Richtersius coronifer may be the right test animals for true outer space experiments in space shuttles. The pharmaceutical industry has been very interested in role of the sugar trehalose that tardigrades produce prior to anhydrobiosis and cryobiosis stages. Trehalose appears to protect the cellular membranes of tardigrades against damage from freezing and dehydration. Trehalose may be used in organ transplantation to avoid freeze damage. Recently, it has become clear that the phenomenon of cryptobiosis is much more complex than first thought. New results show that it is not only trehalose that is responsible for survival during cryobiosis and anhydrobiosis. In the species Richtersius coronifer, a very large protein (ice-nucleating agent) seems to protect the cellular structures from fast freezing in active animals. The phenomenon of cryptobiosis is a fascinating biological puzzle. By solving the puzzle of how a tardigrade can go into a reversible death (ametabolic stage) for many years, and after a few minutes of rehydration can climb around again, it might explain how life developed on earth.

Species accounts

List of Species

Large carnivorous water bear

Milnesium tardigradum

order

Apochela

family

Milnesiidae

taxonomy

Milnesium tardigradum Doyére, 1840, close to Paris, France.

other common names

None known.

physical characteristics

Measure 0.0197–0.0236 in (500–600 µm), females sometimes up to 0.0394 in (1,000 µm), males much smaller. Body is elongated to torpedo shaped; color varies from colorless to reddish or brownish and black eyes are posterior on each side of the pear-shaped pharynx; head region displays a number of unique characteristics. Terminal mouth opening is surrounded by six robust, triangular lamellae that serve as closing apparatus for mouth opening when it is not feeding; has six oral and two lateral papillae, which may have chemoreceptory function. Double claws unusual because primary and the secondary branches are completely separate; primary branch is very long and flexible, while secondary branch is a short and robust claw, usually with three spurs. Buccal tube is wide and short, and armed with short convex stylets and stylet supports.

distribution

Commonly cosmopolitan; from dry tropical deserts to polar freshwaters; M. tardigradum is actually a complex of several species; is also one of the tardigrade species that are present in the fossil records. A eutardigrade found in Cretaceous amber from United States, described as M. swolenskyi, is very similar to M. tardigradum, meaning they probably lived together with the dinosaurs. (Specific distribution map not available.)

habitat

Particularly common in drier temperate terrestrial habitats such as mosses and lichens; one of the first described species from mosses on roofs and in gutters of houses; species recorded from supralitoral lichens of the genus Ramalina and from ornitho-coprophilous lichens (grow in bird feces) such as the yellow Xanthoria elegans. In these lichens, it is colored reddish to brownish.

behavior

A fast "runner," moving in a very characteristic way, and does not use its fourth pair of legs. Almost stands up, like a mini carnivore dinosaur, when it attacks prey.

feeding ecology and diet

Exclusively carnivorous, feeding on nematodes, rotifers, and other smaller eutardigrades. Large nematodes are attacked at the middle of their trunks and pierced by the two stylets; cell contents of nematode are sucked out by strongly muscular pharynx. Smaller nematodes swallowed like spaghetti. The mid-gut can be filled with jaws from rotifers; genus Philodina may be a favorite diet. Bucco-pharyngeal apparatus of smaller tardigrades also found in the mid-gut of the species.

reproductive biology

Fertilization is internal; female has a single seminal vesicle, which has been mistaken for a fourth malpighian tubule. Males are much smaller than females; male claws on the first pair of legs are strongly modified; secondary branch of the double claw is a rough and robust hook; assumed that males use claws to attach to female during mating; up to 18 smooth-shelled eggs deposited in the cast exuvium. The size of the eggs ranges from 0.00276 to 0.00453 in (70–115 µm). Newly hatched juveniles resemble miniature adults.

conservation status

Not listed by the IUCN.

significance to humans

Has been used for pest control of nematodes in soil, but without very good results.

Balloon water bear

Tanarctus bubulubus

order

Arthrotardigrada

family

Halechiniscidae

taxonomy

Tanarctus bubulubus Jørgensen and Kristensen, 2001, Faroe Bank, North Atlantic.

other common names

English: Balloon animal.

physical characteristics

Small species measuring 0.00346–0.00425 in (88–108 µm). Cephalic sensory structures consist of very long primary clavae (longer than the body), two lens-shaped buccal clavae, two long internal and external cirri, two short lateral cirri, and a single, median cirrus. Adults have telescopic legs with lance-like tibia, conic tarsus, and four toes with internal claws with small dorsal spurs; 18–20 balloon-like appendages are attached on the fourth leg pair. Balloons vary greatly in shape, and regulate the buoyancy of animal when it adheres to substrate. Females have two cuticular seminal receptacles that open lateral to the gonopore.

distribution

Faroe Bank and Bill Bailey Bank, North Atlantic. (Specific distribution map not available.)

habitat

Subtidal and lives interstitially in clean shell gravel. Is very rare and has only been found in water depths of 308–656 ft (94–200 m).

behavior

Often attached to empty shells of tintinids (ciliates), but one specimen has been seen floating in the water column upside-down with the 18 balloons extended. Dorsal side of tardigrade is totally covered with empty pieces of coccoliths (calcareous algae); coccolith layer probably protects them from predators.

feeding ecology and diet

Four-lobed mid-gut is usually filled with a white amorphous content, similar to content in bacterivorous tardigrades. Stylets and buccal tube are very thin and narrow, indicating that it pierces bacteria and sucks out cell contents with its trilobed pharynx, which is armed with fused calcium carbonate-encrusted placoids. In one locality, the sediment was filled with methano-bacteria, and it has been suggested that species feeds on this type of bacteria.

reproductive biology

Has internal fertilization; two seminal receptacles are often filled with thread-like spermatozoa. One egg can fill one-third of entire female. Two-clawed newly hatched larva lacks balloons, but already is length of 0.00303 in (77µm) when 18 balloons are present.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Turtle water bear

Echiniscus testudo

order

Echiniscoidea

family

Echiniscidae

taxonomy

Emydium testudo Doyére, 1840, close to Paris, France.

other common names

None known.

physical characteristics

Measures up to 0.0142 in (360 µm); brownish red, or reddish to yellow, with red eyespots. dorsal side covered with segmental sclerotized dorsal plates ornamented with rounded irregular pores. The cephalic sensory structures consist of two pairs of clavae, and three pairs of cirri; median cirrus is lacking. Lateral appendages on the trunk, also named cirri, are present. These filaments, called cirri a to e, probably have an adhesive function rather than a sensorial one; cirrus d is always lacking, and has instead two dorsal spikes. Four claws are robust and the internal claw has a miniscule basal spur.

distribution

Common; recorded from most of Europe, Canada, Greenland, South America, India, Turkey, and Afghanistan. (Specific distribution map not available.)

habitat

Lives in mosses and lichens, and appears to prefer insolated localities that often dry out. Common in urban areas on tile roofs with old moss populations.

behavior

Has a slow bear-like gait.

feeding ecology and diet

Lives in green part of mosses. Has been postulated that it sucks out the moss cells, but in fact there are no observations of feeding behavior. The yellow color in coelomocytes and mid-gut cells is formed by carotene (same dye as in carrots); may come from the diet of mosses or lichens, but unfortunately the association between the diet and the color of cells has never been proven with labeled isotopes.

reproductive biology

Males have never been observed; always suggested that species reproduces solely by parthenogenesis. However, small males have been found in other species of genus Echiniscus. If males present, they must be very rare. The two to five reddish eggs are laid in the old exuvium. Newly hatched juveniles have only two claws on each leg, and they have usually fewer trunk cirri than the adult.

conservation status

Not listed by the IUCN.

significance to humans

None known, but species is sometimes found in house dust.

Tidal water bear

Echiniscoides sigismundi sigismundi

order

Echiniscoidea

family

Echiniscoididae

taxonomy

Echiniscus sigismundi Schultze, 1865, Helgoland, Germany.

other common names

None known.

physical characteristics

Adults measure 0.00618–0.0134 in (157–340 µm); lack dorsal plates, and the dorsal cuticle varies from smooth to a delicate mammilate sculpture. Large black eyes are always present; mouth opening is subterminal. Stylets are very long with large furcae; stylet supports lacking. Three straight placoids are encrusted with calcium carbonate. All sensory structures are reduced in length. Median cirrus is lacking, minuscule internal and external cirri are located around mouth cone. Key characteristic of genus echiniscoides is presence of numerous claws on the legs. In adults, the number of claws varies from 7–13; fourth pairs of legs have usually one claw less than other legs. All claws smooth and lack spurs.

distribution

Commonly cosmopolitan in the tidal zone; one record from soil samples at altitude of 3,280 ft (1,000 m) in the former Belgian Congo. Many subspecies described throughout the world.

habitat

Marine, intertidal species. Lives on Enteromorpha algae or as symbiont on barnacles. Always restricted to upper tidal zone and is capable of tolerating desiccation (anhydrobiosis).

behavior

During low tide, animals are gregarious. More than 100 individuals have been observed in the sutures of barnacles; animal capable of tolerating all kinds of physiological stress; can survive in distilled as well as saturated seawater (osmobiosis). In polar regions, it may be frozen twice a day (low tide), or stay frozen for up to six months in areas without tides.

feeding ecology and diet

Herbivorous and pierces unicellular algae and cyanobacteria. Six-lobed mid-gut is usually green right after molting and turns black just before new molt; animal can only defecate during the molt, and it takes place inside the old exuvium.

reproductive biology

Female lacks seminal vesicles; fertilization is external. Up to 12 mature eggs ready for ovoposition have been observed in single ovary. Several females lay free eggs in big clusters; afterward, males fertilize them. Newly hatched juveniles always have fewer claws than the adults.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Giant yellow water bear

Richtersius coronifer

order

Parachela

family

Macrobiotidae

taxonomy

Macrobiotus coronifer Richters, 1903, or Adorybiotus coronifer Richters, 1903, Spitsbergen Island.

other common names

None known.

physical characteristics

Large animals (up to 0.039 in [1 mm]), usually yellow to orange with large black eyes; buccal tube is rather narrow with hook-shaped appendices for the stylet muscle insertions; pharynx is ovoid with two short and square macroplacoids; body lacks sensory appendages. Legs with two equally sized claws; each double claw has an enormous crescentic marking, named a lunula, with 10–18 denticles. The large, yellow eggs (larger than 0.00787 in [200 µm]) are more or less pliable and ornamented with rough thorns.

distribution

Common species in dry mosses on carbonate bedrock. Has been recorded from various lowland localities in Europe (Sweden), Turkey, Colombia, and the Arctic. Also found in the dry and high parts of the Himalayas at 18,300 ft (5,600 m) elevation (Nepal, Kala Pathar, Khumbu Himal). (Specific distribution map not available.)

habitat

Bryophilic (moss-living) tardigrade and lives mainly in alpine or arctic environments. Avoids acid bedrock, but is common in dry mosses growing on limestone or basalt. Prefers mosses of the genera Grimia, Orthotrichum, and Tortula. On Öland (Sweden), there may be more than 1,000 individuals in one moss-cushion.

behavior

During dry spells, found in the anhydrobiotic tun stage. Capable of surviving severe desiccation and low temperatures down to −320°F (−196°C), both in anhydrobiotic and the hydrated states. During desiccation, accumulates trehalose. Molecules protect cell membrane against damage from dissociation. Can survive approximately nine years in dried conditions.

feeding ecology and diet

Herbivorous; it has been suggested that the two large stylets can penetrate the cell walls of the mosses and so animal can suck out cell contents with its strong pharyngeal apparatus. No direct observations made of feeding behavior, but the mid-gut can be filled with green to dark green chlorophyll-containing material.

reproductive biology

Indications that it is capable of switching between sexual and parthenogenetic reproduction.

conservation status

Not listed by the IUCN.

significance to humans

Used as a laboratory animal. Danish-Italian experiments with ionosphere balloons have shown that it can survive high temperatures, vacuum, and cosmic radiation. Will be used in further experiments in outer space.

Resources

Books

Bertolani, Roberto. Tardigradi. Guide per il Riconnoscimento delle Specie Animali delle Acque Interne Italiane. Verona, Italy: Consiglio Nazionale Delle Ricerche, 1982.

Dewel, Ruth A., Diane R. Nelson, and William C. Dewel. "Tardigrada. Vol. 12: Onychophora, Chilopoda and Lesser Protostomata." In Microscopic Anatomy of Invertebrates, edited by F. W. Harrison and M. E. Rice. New York: Wiley-Liss, 1993.

Greven, Hartmut. Die Bärtierschen. Die Neue Brehm-Bucherei, Volume 537. Wittenberg, Germany: A. Ziemsen Verlag, 1980.

Kinchin, Ian M. The Biology of Tardigrades. London: Portland, 1994.

Nelson, Diane R., and Sandra J. McInnes. "Tardigrada." In Freshwater Meiofauna: Biology and Ecology, edited by S. D. Rundle, A. L. Robertson and J. M. Schmid-Araya. Leiden, The Netherlands: Backhuys Publishers, 2002.

Periodicals

Kristensen, R. M. "The First Record of Cyclomorphosis in Tardigrada Based on a New Genus and Species from Arctic Meiobenthos." Zeitschrift für Zoologische Systematik und Evolutions-forschung 20 (1982): 249–270.

——. "An Introduction to Loricifera, Cycliophora, and Micrognathozoa." Integrative and Comparative Biology 42 (2002): 641–651.

Rebecchi, L., V. Rossi, T. Altiero, R. Bertolani, and P. Menozzi. "Reproductive Modes and Genetic Polymorphism in the Tardigrade Richtersius coronifer (Eutardigrada, Macrobiotidae)." Invertebrate Biology 22 (2003): 19–27.

Wright, J. C., P. Westh, and H. Ramløv. "Cryptobiosis in Tardigrada" Biological Reviews of the Cambridge Philosophical Society 67 (1992): 1–29.

Reinhardt Møbjerg Kristensen, PhD

Martin Vinther Sørensen, PhD