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.
Perception Channels: visual ; tactile ; chemical
Although there are no known predators specific to Milnesium tardigradum, predation has been observed on other tardigrade species by nematodes, snails, and larger tardigrades.
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.
Range length: 0.2 to 0.7 mm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: sexes alike
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.
Range lifespan
Status: captivity: 14 to 58 days.
Average lifespan
Status: wild: 6 years.
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.
Range elevation: 2250 (high) m.
Habitat Regions: temperate ; tropical ; polar ; terrestrial ; saltwater or marine
Terrestrial Biomes: tundra ; taiga ; desert or dune ; forest ; rainforest ; scrub forest ; mountains
Aquatic Biomes: lakes and ponds; rivers and streams; temporary pools
Other Habitat Features: riparian
Milnesium tardigradum is a cosmopolitan, carnivorous eutardigrade species found throughout Europe, North America, Central, East and Southeast Asia, Oceania, and Antarctica.
Biogeographic Regions: nearctic (Native ); palearctic (Native ); oriental (Native ); australian (Native ); antarctica (Native ); oceanic islands (Native )
Other Geographic Terms: cosmopolitan
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.
Animal Foods: terrestrial worms; aquatic or marine worms
Plant Foods: algae
Other Foods: microbes
Primary Diet: omnivore
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.
Commensal/Parasitic Species:
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.
Positive Impacts: source of medicine or drug ; research and education
There are no known adverse effects of Milnesium tardigradum on humans.
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.
Milnesium tardigradum is an abundant and ubiquitous species that has no special conservation status.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
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.
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.
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.
Key Reproductive Features: iteroparous ; gonochoric/gonochoristic/dioecious (sexes separate); parthenogenic ; sexual ; asexual ; fertilization (External ); oviparous
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.
Parental Investment: precocial ; female parental care ; pre-hatching/birth (Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female)
“Milnesium tardigradum DOYÈRE, 1840 (fig. 33)
Length 310-880 µm. Specimens from King George Island are typical (comp. RAMAZZOTTI, 1972; PILATO, 1973), those from Antarctic Continent belong to the form (or geographic race—according to MARCUS, 1936) having variable middle branch of claw. That branch is single in typical specimens (fig. 33e), in mentioned form it is divided into 2-5 teeth, mostly 3-4 (fig. 33a-d); one specimen even had it reduced (fig. 33a). A number of teeth is different within this species, often even on one leg (fig. 33a-c). The form is widely distributed; from Antarctic Continent recorded by SUDZUKI (1964).
Localities. Typical specimens: King George Island—13(1), 19(4). The form: Enderby Land—2(9).
A cosmopolitan species.”
(Dastych, 1984: 432)
Milnesium tardigradum is a cosmopolitan species of tardigrade that can be found in a diverse range of environments.[1] It has also been found in the sea around Antarctica.[2] M. tardigradum was described by Louis Michel François Doyère in 1840.[3][4] It contains unidentified osmolytes that could potentially provide important information in the process of cryptobiosis.[5]
M. tardigradum has a symmetrical body with a total of eight legs; it uses claws—a distinctive feature for this tardigrade species. The total length of the body varies, with some measuring up to 0.7 mm in length.[6]
M. tardigradum have been found to possess a high level of radioresistance.[7] In 2007, individuals of two tardigrade species, Richtersius coronifer and M. tardigradum, were subject to the radiation, near-vacuum, and near-absolute zero conditions of outer space as part of the European Space Agency's Biopan-6 experiment. Three specimens of M. tardigradum survived.[8] The M. tardigradum are able to cope with high amounts of environmental stress by initiating cryptobiosis. During this state, the internal organic clock of M. tardigradum halts, thus the cryptobiotic state does not contribute to the aging process.[9]
M. tardigradum is an omnivorous predator. It typically feeds on other small organisms, such as algae, rotifers, and nematodes. There have also been recorded cases of M. tardigradum feeding on other smaller tardigrades.[6]
M. tardigradum has been phylogenetically linked to arthropods. Although the extent of the relationship is still debated, evidence suggests that tardigrades and arthropods have a close evolutionary history.[9] Recent research has shown a shortage in a particular subset of genes also found in nematodes, another member of the Ecdysozoa superphylum.[10]
The biogeographical distribution of M. tardigradum is large. The species occupies mostly aquatic environments such as marine, coastal, and terrestrial areas. The full distribution of M. tardigradum is difficult to analyze due to the difficulty in taxonomy and the lack of sufficient data.[9]
M. tardigradum reproduces both sexually and through parthenogenesis. The mating behavior of tardigrades is difficult to reproduce under artificial conditions, hence the frequency and time of reproduction is not fully understood. If and when a mating season exists for M. tardigradum is unknown.[6]
Females lay up to 12 eggs, which hatch after several days (around five to sixteen). The development of the newly hatched larvae is marked by various molting stages, rather than metamorphosis. The time frame of these molting stages vary from each tardigrade as it is dependent on the nutrition of the specific individual.[6] Once the molting stages are complete, the larva tardigrade attempts to find an ideal location to initiate ecdysis. Some eggs may be left in the discarded exuvia.[11] Tardigrades have been shown to respond to different temperature changes at different developmental stages. Specifically, the younger the egg, the less likely it is to survive extreme environments. However, not too long after development, tardigrades demonstrate a remarkable ability to withstand these conditions. To survive such conditions, tardigrades need time to develop important cellular structures and repair mechanisms.[12]
Milnesium tardigradum is a cosmopolitan species of tardigrade that can be found in a diverse range of environments. It has also been found in the sea around Antarctica. M. tardigradum was described by Louis Michel François Doyère in 1840. It contains unidentified osmolytes that could potentially provide important information in the process of cryptobiosis.
