Four animals, no filler species
Species deep-dives with papers that exist.
v1 covers five — a lab model, an extremotolerant genome star, a carnivore that flew on FOTON-M3, an armored heterotardigrade, and the tun model. Citations are real titles. If a claim needs a DOI, follow the Sources hub rather than a fabricated link.
Hypsibiidae · model
Hypsibius exemplaris
For years the go-to lab water bear was called Hypsibius dujardini, especially the Z151 culture used by Bob Goldstein’s laboratory at UNC Chapel Hill. In 2018, Gąsiorek, Stec, Morek and Michalczyk published an integrative redescription in Zootaxa and named the cultured lineage Hypsibius exemplaris — “the exemplary Hypsibius.” NCBI later updated the Z151 assemblies to that name. The wild European H. dujardini of Doyère, 1840, is not automatically the same animal.
It is small, transparent, easy to rear on algae, and photogenic under DIC. It can enter anhydrobiosis, but it is a slow dehydrator: it needs preconditioning at high humidity before it forms a reliable tun. That is the opposite of Ramazzottius varieornatus, which can dry quickly. Comparative genomics by Yoshida, Koutsovoulos, Laetsch, Stevens and colleagues (2017, PLOS Biology, “Comparative genomics of the tardigrades Hypsibius dujardini and Ramazzottius varieornatus”) used that contrast as the point of the paper.
The genome is on the order of 100 Mb (assembly nHd_3.1 is about 104 Mb). Developmental biologists use it because embryos are visible and the animal is a compact panarthropod. It is the water bear most likely to be on a lab poster, a news thumbnail, or this site’s favicon crop.
Read: Gąsiorek et al. 2018 Zootaxa; Yoshida et al. 2017 PLOS Biology e2002266; the Goldstein lab at UNC Chapel Hill, which established it as a lab model. More in Sources.
Hypsibiidae · extremotolerant
Ramazzottius varieornatus
If H. exemplaris is the friendly lab rat, R. varieornatus is the animal people mean when they say “tardigrade superpowers.” The YOKOZUNA-1 strain, established by Daiki D. Horikawa and colleagues, survives rapid desiccation. Hashimoto, Kunieda and colleagues reported a high-quality genome in 2016 in Nature Communications: “Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein.”
That paper is the origin of the celebrity protein Dsup (Damage suppressor), a tardigrade-unique DNA-associating protein. In human cultured cells, Dsup reduced X-ray-induced DNA damage in their assay. Later work (including Chavez, et al., eLife 2019, “The tardigrade damage suppressor protein binds to nucleosomes…”) showed nucleosome binding and protection from hydroxyl radicals. A Dsup-like protein was subsequently recognized in H. exemplaris as well.
The same genomic era named tardigrade-specific heat-soluble proteins: CAHS, SAHS, MAHS, and mitochondrial LEA-like proteins. Yoshida et al. 2017 found that R. varieornatus does not mount a huge transcriptional response as it dries — protection looks more constitutive — while H. exemplaris does a lot of last-minute transcription. That is biology, not a superhero origin story.
Heat is a weaker suit. Neves, Møbjerg and colleagues (2020, Scientific Reports) showed active R. varieornatus are vulnerable near ordinary hot-day temperatures, while desiccated animals tolerate much more, with time as a limit. Tardigrades are not heat-proof bears.
Read: Hashimoto et al. 2016 Nat Commun 7:12808; Yoshida et al. 2017 PLOS Biology; Boothby et al. 2017 Molecular Cell on CAHS gels (desiccation tolerance proteins, tardigrade).
Milnesiidae · predator · spaceflight
Milnesium tardigradum
Doyère described Milnesium tardigradum in 1840. It is the tardigrade people sketch from memory: a slightly blunt, almost bulldog head, a telescopic mouth cone, and claws of the “Milnesium type” — two claws per leg, each with a long primary branch. It hunts rotifers and nematodes, not moss cells.
Taxonomy warning, because we will not pretend otherwise: M. tardigradum became a species complex. Michalczyk, Kaczmarek and colleagues redescribed it and split lookalikes; many older “M. tardigradum” records are now other Milnesium species. When a space paper or a textbook says M. tardigradum, read the strain and the date.
It is one of the two eutardigrades flown in the TARDIS experiment on ESA’s FOTON-M3 mission (September 2007), with Richtersius coronifer. Jönsson, Rabbow, Schill, Harms-Ringdahl and Rettberg reported the result in Current Biology (2008): “Tardigrades survive exposure to space in low Earth orbit.” Vacuum alone was not a massacre. Vacuum plus full solar UV was. A few M. tardigradum still recovered after the harshest UV-all treatment. That is the paper behind the sentence “tardigrades survived space.”
SEM plates of active, tun, and embryonic M. tardigradum from a 2012 PLOS ONE proteomics paper (Schokraie / Schill and colleagues) are on Commons and used below — tuns look like barrels with claws tucked in.
Read: Jönsson et al. 2008 Curr Biol 18:R729–R731; Doyère 1840 on tardigrade organization; Michalczyk et al. on the Milnesium redescription. Space narrative continues on Space.
Echiniscidae · heterotardigrade · armored
Echiniscus testudo — the armored one
The other three animals on this page are eutardigrades. Echiniscus testudo is this site’s first heterotardigrade, and the name-bearing species for the class Heterotardigrada. Doyère described it in 1840 as Emydium testudo. Its armored dorsal plates are what set it apart.
Gąsiorek, Stec, Morek and Michalczyk published an integrative redescription in 2017 (Zoologischer Anzeiger 270:107–122). The neotype came from moss on a grave in Montmartre Cemetery, Paris. That work showed the three-cirri and four-cirri forms, trifilis and quadrifilis, are one species. It made E. glaber and E. filamentosus junior synonyms and raised E. mongoliensis to a species.
It is parthenogenetic, herbivorous, and terrestrial, living in dry mosses and lichens. It is recorded on every continent except Antarctica and Australia, mostly Holarctic. The supporting source is Erdmann et al. 2021, PeerJ. That paper also tested E. testudo and Milnesium inceptum entering and leaving anhydrobiosis under hypomagnetic conditions.
Jørgensen, Møbjerg and Kristensen (2007, Journal of Limnology) sampled 13 localities on three continents. They found low sequence diversity, high haplotype diversity, and no isolation by distance. They concluded it is a young species with high dispersal potential, plausibly through airborne tuns.
Nobody keeps a pet E. testudo culture, because the species cannot be cultured yet. Murai et al. 2021 (BMC Genomics) is a multiomics study of a species that cannot be cultured in the lab yet. It suggests heterotardigrades have anhydrobiosis machinery partly shared with other tardigrades but distinct, possibly through convergent evolution of abundant heat-soluble proteins.
The famous roughly 20-year revival is an anecdotal record, as described by Roszkowska et al. 2023 (PLOS ONE). In their controlled tests, urban and natural populations survived anhydrobiosis for up to 120 days.
Read: Gąsiorek et al. 2017 Zoologischer Anzeiger (doi:10.1016/j.jcz.2017.09.006); Jørgensen et al. 2007 Journal of Limnology (doi:10.4081/jlimnol.2007.s1.77); Murai et al. 2021 BMC Genomics (doi:10.1186/s12864-021-08131-x); Roszkowska et al. 2023 PLOS ONE (doi:10.1371/journal.pone.0270386); Erdmann et al. 2021 PeerJ (doi:10.7717/peerj.10630); Kamilari et al. 2026 Molecular Ecology (doi:10.1111/mec.70471), which includes a new E. testudo transcriptome; Doyère 1840 (no DOI). The list lives on Sources.
Richtersiidae · tun model
Richtersius coronifer — the tun model
Ferdinand Richters described this animal in 1903 as Macrobiotus coronifer. It is now the type species of Richtersius Pilato & Binda, 1989. Guidetti et al. 2016 (Zoological Journal of the Linnean Society 178:834–845) moved it into a new family, Richtersiidae, within Macrobiotoidea.
Maucci & Ramazzotti (1981) designated a neotype from Bodø, Norway. Stec & Michalczyk 2020 (Zootaxa 4858(2)) showed that designation invalid under ICZN Art. 75.3.4 and not from the original type locality, and designated a new neotype from the original locus typicus in Svalbard, with SEM, DNA barcodes and a transcriptome.
It is probably more than one species. Stec, Krzywański, Arakawa & Michalczyk 2020 (Zoological Letters 6:2) redescribed a Spitsbergen population with four markers (18S, 28S, ITS-2, COI) plus a transcriptome. COI delimitation across Spitsbergen, Italy, Poland, Greece and GenBank records (China, Greenland, Italy, Mongolia) indicated seven distinct potential species in Richtersius besides the nominal one. A century of worldwide “R. coronifer” records is likely a species complex. That is why some papers write “Richtersius cf. coronifer” (cf. = “compare with”), for example Pedersen et al. 2021 in Astrobiology.
Halberg, Jørgensen & Møbjerg 2013 (PLOS ONE 8:e85091) measured the tun: ~87% body-volume reduction from active animal to tun. Mitochondrial energy production is needed to survive desiccation; uncoupling mitochondria abolished tun formation; inhibiting muscles impaired viable tuns. The tun is built by muscles, not just a passive shrivel.
Drying costs energy. Jönsson & Rebecchi 2002 (Journal of Experimental Zoology 293:578–584) found that storage cells shrank over a period of anhydrobiosis — first evidence energy is used in the process — and that large animals survived less well than medium-sized ones.
It is not unlimited. Czerneková & Jönsson 2016 (PLOS ONE 11:e0164062) found survival of up to 6 repeated desiccations, with survival falling each cycle. More animals failed to contract into a proper tun at the 5th and 6th; storage-cell numbers fell.
Jönsson, Harms-Ringdahl & Torudd 2005 (International Journal of Radiation Biology 81:649–656) irradiated animals at 0.5 and 1 kGy gamma. Survival of both desiccated and hydrated animals did not differ from controls; no eggs laid by irradiated animals hatched. The authors suggest tolerance is not from dry-state protectants and may rely on DNA repair.
It was one of the two desiccated species on TARDIS, FOTON-M3, 2007 (Jönsson et al. 2008, Current Biology). Space narrative continues on Space.
Read: Guidetti et al. 2016 Zoological Journal of the Linnean Society (doi:10.1111/zoj.12428); Stec & Michalczyk 2020 Zootaxa (doi:10.11646/zootaxa.4858.2.10); Stec et al. 2020 Zoological Letters (doi:10.1186/s40851-020-0154-y); Pedersen et al. 2021 Astrobiology (doi:10.1089/ast.2020.2371); Halberg et al. 2013 PLOS ONE (doi:10.1371/journal.pone.0085091); Jönsson & Rebecchi 2002 Journal of Experimental Zoology (doi:10.1002/jez.10186); Czerneková & Jönsson 2016 PLOS ONE (doi:10.1371/journal.pone.0164062); Jönsson et al. 2005 International Journal of Radiation Biology (doi:10.1080/09553000500368453); Jönsson et al. 2008 Curr Biol (doi:10.1016/j.cub.2008.06.048); Richters 1903; Pilato & Binda 1989; Maucci & Ramazzotti 1981 (no DOI). The list lives on Sources.
