(Eutardigrade)

Smooth-bodied Tardigrades

Справжні тихоходи

Eutardigrada are a class of tardigrades (Tardigrada) without lateral appendages. Primarily freshwater bound, some species have secondarily gained the ability to live in marine environments (Halobiotus). By cryptobiosis many species are able to live temporarily in very dry environments. More than 700 species have been described.

The order Apochela consists of only one family, Milnesiidae, with two genera: Milnesium and Limmenius. Milnesium tardigradum can be found worldwide and is one of the biggest species among tardigrades (up to 1.4 mm); similar-looking species have been found in Cretaceous amber. The mouth of this predator has a wide opening, so the animal can eat rotifers and larger protists. Other eutardigrades belong to the order Parachela.

Morphology

They have a more or less cylindrical, short, thick body with a somewhat flattened ventral side and no obvious segmentation. It is believed to consist of five segments. The first segment, which merges with the head lobe, lacks appendages; the next four bear a pair of legs each. Three pairs are located laterally, while the fourth pair is situated at the posterior end of the body. The legs are short and unsegmented, appearing as knob-like outgrowths of the body with movable claws at their tips. Most tardigrades are nearly colorless and even transparent, though some may be yellow-green or plum-green, violet, or reddish.

A distinctive feature of the body structure of tardigrades is the constant cellular composition of certain tissues and organs. In particular, the body covering, muscles, and midgut in most species consist of a fixed number of cells that remains unchanged throughout the animal’s entire life.

Anatomy

Externally, the body of tardigrades is covered with a cuticle that differs from the cuticle of arthropods in both its fine structure and chemical composition. It consists of an epicuticle and a procuticle. The surface of the epicuticle lacks a cement layer; instead, there is a mucus layer composed of acidic mucopolysaccharides. A wax layer is located between the epi- and procuticle. The procuticle does not contain pore canals. The cuticle of tardigrades does not contain chitin. The cuticle is generally thin, sometimes becoming thickened to form segmental plates, and in some species it bears various spine-, tubercle-, or wing-like projections and similar structures. Beneath the cuticle lies an epithelium, which is never ciliated.

The body cavity is represented by a mixocoel filled with hemolymph, in which there are cells containing reserve nutrient substances.

In tardigrades, the muscles are smooth and are represented by separate longitudinal and transverse bundles on the dorsal, ventral, and lateral sides; in addition, there is a system of muscles associated with the limbs. Owing to the action of the trunk muscles, tardigrades are able to bend their bodies, with the hemolymph acting as an antagonist.

The mouth is located on the ventral side of the anterior end of the body. In the oral cavity there is a pair of sharp, forward-directed stylets, which tardigrades use to pierce mainly chlorophyll-containing cells of mosses and algae, more rarely small nematodes, rotifers, other tardigrades, and so on, and then suck out their contents. The ducts of the salivary glands open into the pharynx, which has a suctional enlargement. The pharynx leads into a long, sac-like midgut, which passes into the hindgut. Before the anal opening, the hindgut expands to form a cloaca.

At the boundary between the midgut and the hindgut there are three blind-ended outgrowths: a short one lying dorsally on the intestine and two longer ones on its sides. These are regarded as excretory organs similar to the Malpighian tubules of arthropods. In addition, they probably participate in osmoregulation. An indirect confirmation of this is the absence of Malpighian tubules in marine species; in such cases, the intestine performs the excretory function.

There are no circulatory organs. The blood is represented by colorless hemolymph.

There are no respiratory organs—gas exchange occurs over the entire body surface.

The nervous system consists of a four-lobed supraesophageal ganglion, circumpharyngeal connectives, and a ventral nerve cord with five ganglia. Nerves extend from the supraesophageal and ventral ganglia to small ganglia located near the bases of the legs, the stomach, and other organs.

The sense organs are poorly developed; in the anterior part of the body there is a pair of ocelli composed of several sensory cells surrounded by pigment cups. In marine species, there are also several pairs of sensory appendages in this region, innervated from the supraesophageal ganglion.

Smooth-bodied Tardigrades are dioecious. The gonads are unpaired, sac-like, and located above the intestine; the genital ducts (oviduct or vas deferens) open into the cloaca, and the gametes are released through the anal opening.

Reproduction

Smooth-bodied Tardigrades can reproduce by parthenogenesis. In populations of some species, males occur much less frequently than females, and in other species they are completely unknown. In experiments, several parthenogenetic generations of Hypsibius dujardini and Milnesium tardigradum were obtained, which developed and lived normally.

Fertilization, both internal and external, occurs in various ways. Many species of tardigrades lay fertilized eggs within the molted cuticle and remain with it for a long time, dragging it along together with the eggs. More rarely, eggs are laid freely on the substrate, either singly or in small clusters.

Tardigrade eggs are round and have a tough shell, often with various projections that are species-specific. The number of eggs laid depends not only on the species but also on the physiological condition of the female, particularly her degree of satiety.

Development is direct, without a larval stage, and growth is accompanied by molting. Since many organs in tardigrades consist of a fixed number of cells, growth mainly occurs through an increase in cell volume rather than in cell number.

Ecology

Tardigrades inhabit an extremely diverse range of aquatic and terrestrial habitats. Marine species mostly live in the coastal zone on soil and algae. Among them are known commensals that settle in the mantle cavity of mollusks, on copepods and isopods, etc., as well as one parasitic species, Tetrakentron synaptae, which lives on the tentacles of the sea cucumber Leptosynapta galliennei and feeds on the contents of the host’s living cells. Freshwater tardigrades are found in ponds, lakes, reservoirs, swamps, rivers, and similar habitats. However, most representatives of the class occur on land. Tardigrades living in extreme conditions are particularly interesting. For example, Hypsibius klebelsbergi lives only in glacial water (a glacier is a mass of ice on land that gradually moves under its own weight), which accumulates in small depressions or cracks in the ice, at temperatures of 0–1.5 °C. Its antipodes are two species: Thermozodium esakii, which inhabits algae in hot springs at +40 °C, and Hypsibius oberbaeuseri, which lives in moss at the edges of such springs.

Most known tardigrade species inhabit a wide variety of porous substrates in different terrestrial habitats: leaf litter and coniferous litter in forests, soil, lichens, and mosses, including those growing on rocks, trees, roof gutters, and similar locations. Some substrates are permanently submerged in water, but most experience alternating periods of drying and wetting. For active life, tardigrades require a water film or droplets that surround their bodies completely.

When the substrate dries, tardigrades do not die but enter a state of hidden life (cryptobiosis). During this process, the animal reduces in volume, retracts its limbs, and the elastic areas of the cuticle contract, forming characteristic folds, so that the tardigrade takes on the appearance of a microscopic barrel. A crucial condition for maintaining viability is the slow drying of the substrate and the gradual transition into cryptobiosis. Once the animals are re-immersed in water, they revive quite rapidly. The time required for the restoration of activity depends on the duration of desiccation. For example, Macrobiotus coronifer revives after 25 minutes following nine months of desiccation, after 35 minutes following 15 months, and after one day following 22 months.

Experimental studies have shown that tardigrades in the tun (barrel) stage can withstand extreme conditions that they would never encounter in nature. For example, Macrobiotus tuns retained viability for 20 months at temperatures from −190 °C to −200 °C and for 8 hours at −272 °C, as well as during brief heating up to +100 °C. Tardigrades were able to survive even when kept for several months in a hydrogen-rich atmosphere, which is known to be unsuitable for life.

Order: Apochela

Family: Milnesiidae

Bergtrollus: Bergtrollus dzimbowski.Milnesioides: Milnesioides exsertum.Limmenius: Limmenius porcellus.

Milnesium: Milnesium alabamae, Milnesium almatyense, Milnesium alpigenum, Milnesium antarcticum, Milnesium argentinum, Milnesium asiaticum, Milnesium barbadosense, Milnesium beasleyi, Milnesium beatae, Milnesium berladnicorum, Milnesium bohleberi, Milnesium brachyungue, Milnesium cassandrae, Milnesium burgessi, Milnesium dornensis, Milnesium dujiangensis, Milnesium eurystomum, Milnesium fridae, Milnesium granulatum, Milnesium jacobi, Milnesium katarzynae, Milnesium kogui, Milnesium krzysztofi, Milnesium lagniappe, Milnesium longiungue, Milnesium minutum, Milnesium quadrifidum, Milnesium rastrum, Milnesium reductum, Milnesium reticulatum, Milnesium sandrae, Milnesium shilohae, Milnesium swansoni, Milnesium tardigradum, Milnesium tetralamellatum, Milnesium tumanovi, Milnesium validum, Milnesium variefidum, Milnesium vorax, Milnesium zsalakoae.

Order: Parachela

Family: Eohypsibiidae

Austeruseus: Austeruseus balduri, Austeruseus faeroensis, Austeruseus rokuri.

Bertolanius: Bertolanius birnae, Bertolanius mahunkai, Bertolanius markevichi, Bertolanius nebulosus, Bertolanius portucalensis, Bertolanius smreczynskii, Bertolanius volubilis, Bertolanius weglarskae.

Eohypsibius: Eohypsibius nadjae, Eohypsibius terrestris.

Family: Calohypsibiidae

Calohypsibius: Calohypsibius maliki, Calohypsibius ornatus, Calohypsibius placophorus, Calohypsibius schusteri, Calohypsibius verrucosus.

Haplohexapodibius: Haplohexapodibius seductor.

Haplomacrobiotus: Haplomacrobiotus hermosillensis, Haplomacrobiotus utahensis.

Hexapodibius: Hexapodibius bindae, Hexapodibius boothi, Hexapodibius christenberryae, Hexapodibius micronyx, Hexapodibius pseudomicronyx, Hexapodibius reginae.

Parhexapodibius: Parhexapodibius bactrianus, Parhexapodibius castrii, Parhexapodibius lagrecai, Parhexapodibius pilatoi, Parhexapodibius ramazzottii.

Family: Hypsibiidae

Bindius: Bindius triquetrus.

Diphascon: Diphascon arduifrons, Diphascon behanae, Diphascon belgicae, Diphascon carolae, Diphascon clavatum, Diphascon gordonense, Diphascon greveni, Diphascon maucci, Diphascon modestum, Diphascon montigenum, Diphascon onorei, Diphascon prorsirostre, Diphascon scoticum, Diphascon tricuspidatum, Diphascon triodon, Diphascon aculeatum, Diphascon alpinum, Diphascon australianum, Diphascon bidropion, Diphascon birklehofi, Diphascon bisbullatum, Diphascon boreale, Diphascon brevipes, Diphascon bullatum, Diphascon burti, Diphascon chilenense, Diphascon claxtonae, Diphascon dastychi, Diphascon dolmiticum, Diphascon elongatum, Diphascon faialense, Diphascon gerdae, Diphascon granifer, Diphascon halapiense, Diphascon higginsi, Diphascon humicus, Diphascon hydrophilum, Diphascon iharosi, Diphascon iltisi, Diphascon langhovdense, Diphascon latipes, Diphascon mirabilis, Diphascon mitrense, Diphascon nelsonae, Diphascon nobilei, Diphascon nodulosum, Diphascon nonbullatum, Diphascon oculatum, Diphascon ongulense, Diphascon opisthoglyptum, Diphascon patanei, Diphascon pingue, Diphascon pinguiforme, Diphascon platyungue, Diphascon polare, Diphascon puniceum, Diphascon ramazzottii, Diphascon recamieri, Diphascon rugocaudatum, Diphascon rugosum, Diphascon sanae, Diphascon secchii, Diphascon serratum, iphascon sexbullatum, Diphascon stappersi, Diphascon tenue, Diphascon trachydorsatum, Diphascon victoriae, Diphascon zaniewi, Diphascon bicorne, Diphascon coniferens, Diphascon marcuzzii, Diphascon mariae, Diphascon punctatum, Diphascon rivulare, Diphascon speciosum.

Paradiphascon: Paradiphascon manningi.

Acutuncus: Acutuncus antarcticus.

Borealibius: Borealibius zetlandicus.

Hypsibius: Hypsibius allisoni, Hypsibius antonovae, Hypsibius arcticus, Hypsibius biscuitiformis, Hypsibius calcaratus, Hypsibius camelopardalis, Hypsibius choucoutiensis, Hypsibius conifer, Hypsibius convergens, Hypsibius dujardini, Hypsibius fuhrmanni, Hypsibius giusepperamazzotti, Hypsibius heardensis, Hypsibius hypostomus, Hypsibius iskandarovi, Hypsibius janetscheki, Hypsibius klebelsbergi, Hypsibius kunmingensis, Hypsibius macrocalcaratus, Hypsibius maculatus, Hypsibius marcelli, Hypsibius microps, Hypsibius montanus, Hypsibius morikawai, Hypsibius multituberculatus, Hypsibius novaezeelandiae, Hypsibius pachyunguis, Hypsibius pallidus, Hypsibius pallidoides, Hypsibius pedrottii, Hypsibius pradellii, Hypsibius ragonesei, Hypsibius roanensis, Hypsibius runae, Hypsibius scaber, Hypsibius scabropygus, Hypsibius septulatus, Hypsibius seychellensis, Hypsibius shaanxiensis, Hypsibius stiliferus, Hypsibius thaleri.

Astatumen: Astatumen bartosi, Astatumen tamaensis, Astatumen tamurai, Astatumen trinacriae.

Itaquascon: Itaquascon biserovi, Itaquascon cambewarrense, Itaquascon enckelli, Itaquascon globuliferum, Itaquascon mongolicus, Itaquascon pawlowskii, Itaquascon pisoniae, Itaquascon placophorum, Itaquascon simplex, Itaquascon umbellinae, Itaquascon unguiculum.

Mesocrista: Mesocrista marcusi, Mesocrista spitzbergensis.

Parascon: Parascon nichollsae, Parascon schusteri.

Platicrista: Platicrista affine, Platicrista angustata, Platicrista cheleusis, Platicrista horribilis, Platicrista itaquasconoide, Platicrista ramsayi.

Family: Microhypsibiidae

Fractonotus: Fractonotus caelatus.

Microhypsibius: Microhypsibius bertolanii, Microhypsibius japonicus, Microhypsibius minimus, Microhypsibius truncatus.

Family: Ramazzottidae

Hebesuncus: Hebesuncus conjungens, Hebesuncus ryani, Hebesuncus schusteri.

Ramazzottius: Ramazzottius affinis, Ramazzottius agannae, Ramazzottius andreevi, Ramazzottius anomalus, Ramazzottius baumanni, Ramazzottius belubellus, Ramazzottius bunikowskae, Ramazzottius cataphractus, Ramazzottius caucasicus, Ramazzottius edmondabouti, Ramazzottius horningi, Ramazzottius ljudmilae, Ramazzottius montivagus, Ramazzottius nivalis, Ramazzottius novemcinctus, Ramazzottius oberhaeuseri, Ramazzottius rupeus, Ramazzottius saltensis, Ramazzottius semisculptus, Ramazzottius subanomalus, Ramazzottius szeptycki, Ramazzottius theroni, Ramazzottius thulini, Ramazzottius tribulosus, Ramazzottius valaamis, Ramazzottius varieornatus.

Family: Isohypsibiidae

Apodibius: Apodibius confusus, Apodibius nuntius, Apodibius richardi.

Doryphoribius: Doryphoribius amazzonicus, Doryphoribius bertolanii, Doryphoribius bindae, Doryphoribius dawkinsi, Doryphoribius doryphorus, Doryphoribius dupliglobulatus, Doryphoribius evelinae, Doryphoribius flavus, Doryphoribius gibber, Doryphoribius huangguoshuensis, Doryphoribius koreanus, Doryphoribius korganovae, Doryphoribius longistipes, Doryphoribius macrodon, Doryphoribius maranguensis, Doryphoribius mariae, Doryphoribius mexicanus, Doryphoribius minimus, Doryphoribius neglectus, Doryphoribius picoensis, Doryphoribius pilatoi, Doryphoribius polynettae, Doryphoribius qinlingense, Doryphoribius quadrituberculatus, Doryphoribius smokiensis, Doryphoribius solidunguis, Doryphoribius taiwanus, Doryphoribius tergumrudis, Doryphoribius tessellatus, Doryphoribius turkmenicus, Doryphoribius vietnamensis, Doryphoribius zappalai, Doryphoribius zyxiglobus.

Eremobiotus: Eremobiotus alicatai, Eremobiotus ovezovae.

Halobiotus: Halobiotus arcturulius, Halobiotus crispae, Halobiotus stenostomus.

Isohypsibius: Isohypsibius altai, Isohypsibius annulatus, Isohypsibius arbiter, Isohypsibius archangajensis, Isohypsibius arcuatus, Isohypsibius asper, Isohypsibius austriacus, Isohypsibius baicalensis, Isohypsibius baldii, Isohypsibius baldiioides, Isohypsibius barbarae, Isohypsibius bartosi, Isohypsibius basalovoi, Isohypsibius belliformis, Isohypsibius bellus, Isohypsibius borkini, Isohypsibius brevispinosus, Isohypsibius brevitubulatus, Isohypsibius brulloi, Isohypsibius bulbifer, Isohypsibius cameruni, Isohypsibius campbellensis, Isohypsibius canadensis, Isohypsibius ceciliae, Isohypsibius changbaiensis, Isohypsibius chiarae, Isohypsibius costatus, Isohypsibius cyrilli, Isohypsibius damxungensis, Isohypsibius dastychi, Isohypsibius deconincki, Isohypsibius deflexus, Isohypsibius dudichi, Isohypsibius duranteae, Isohypsibius effusus, Isohypsibius elegans, Isohypsibius eplenyiensis, Isohypsibius franzi, Isohypsibius fuscus, Isohypsibius gilvus, Isohypsibius glaber, Isohypsibius glazovi, Isohypsibius gracilis, Isohypsibius granditintinus, Isohypsibius granulifer, Isohypsibius gyulai, Isohypsibius hadzii, Isohypsibius helenae, Isohypsibius hydrogogianus, Isohypsibius hypostomoides, Isohypsibius improvisus, Isohypsibius indicus, Isohypsibius irregibilis, Isohypsibius jakieli, Isohypsibius jingshanensis, Isohypsibius jinhouensis. Isohypsibius josephi, Isohypsibius kenodontis, Isohypsibius kotovae, Isohypsibius kristenseni, Isohypsibius ladogensis, Isohypsibius laevis, Isohypsibius latiunguis, Isohypsibius leithaicus, Isohypsibius liae, Isohypsibius lineatus, Isohypsibius longiunguis, Isohypsibius lunulatus, Isohypsibius macrodactylus, Isohypsibius malawiensis, Isohypsibius mammillosus, Isohypsibius marcellinoi, Isohypsibius marii, Isohypsibius mihelcici, Isohypsibius monoicus, Isohypsibius monstruosus, Isohypsibius montanus, Isohypsibius myrops, Isohypsibius neoundulatus, Isohypsibius nipponicus, Isohypsibius nodosus, Isohypsibius novaeguineae, Isohypsibius palmai, Isohypsibius panovi, Isohypsibius papillifer, Isohypsibius pappi, Isohypsibius pauper, Isohypsibius pilatoi, Isohypsibius pratensis, Isohypsibius prosostomus, Isohypsibius pseudoundulatus, Isohypsibius pulcher, Isohypsibius pushkini, Isohypsibius qinlingensis, Isohypsibius rahmi, Isohypsibius reticulatus, Isohypsibius roberti, Isohypsibius ronsisvallei, Isohypsibius rudescui, Isohypsibius rugosus, Isohypsibius sabellai, Isohypsibius sattleri, Isohypsibius schaudinni, Isohypsibius sculptus, Isohypsibius sellnicki, Isohypsibius septentrionalis, Isohypsibius silvicola, Isohypsibius sismicus, Isohypsibius solidus, Isohypsibius taibaiensis, Isohypsibius tetradactyloides, Isohypsibius theresiae, Isohypsibius torulosus, Isohypsibius truncorum, Isohypsibius tuberculatus, Isohypsibius tuberculoides, Isohypsibius tubereticulatus, Isohypsibius tucumanensis, Isohypsibius undulatus, Isohypsibius vejdovskyi, Isohypsibius verae, Isohypsibius verrucosus, Isohypsibius wilsoni, Isohypsibius woodsae, Isohypsibius yunnanensis.

Mixibius: Mixibius fueginus, Mixibius ninguidus, Mixibius ornatus, Mixibius pilatoi, Mixibius saracenus, Mixibius schnurae, Mixibius sutirae, Mixibius tibetanus.

Pseudobiotus: Pseudobiotus hirsutellus, Pseudobiotus kathmanae, Pseudobiotus longiunguis, Pseudobiotus matici, Pseudobiotus megalonyx, Pseudobiotus spinifer, Pseudobiotus vladimiri .

Ramajendas: Ramajendas frigidus, Ramajendas heatwolei, Ramajendas renaudi.

Thalerius: Thalerius konradi.

Thulinius: Thulinius augusti, Thulinius itoi, Thulinius ruffoi, Thulinius saltursus, Thulinius stephaniae.

Family: Macrobiotidae

Adorybiotus: Adorybiotus granulatus.

Biserovus: Biserovus bindae.

Calcarobiotus: Calcarobiotus digeronimoi, Calcarobiotus filmeri, Calcarobiotus gildae, Calcarobiotus hainanensis, Calcarobiotus imperialis, Calcarobiotus longinoi, Calcarobiotus occultus, Calcarobiotus parvicalcar, Calcarobiotus polygonatus, Calcarobiotus tetrannulatus.

Famelobiotus: Famelobiotus scalicii.

Insuetifurca: Insuetifurca arrowsmithi, Insuetifurca austronipponica, Insuetifurca fujiense, Insuetifurca xiae.

Macrobiotus: Macrobiotus acadianus, Macrobiotus almadai, Macrobiotus alvaroi, Macrobiotus anderssoni, Macrobiotus andinus, Macrobiotus anemone, Macrobiotus annewintersae, Macrobiotus ariekammensis, Macrobiotus azzunae, Macrobiotus basiatus, Macrobiotus birendrai, Macrobiotus biserovi, Macrobiotus caelestis, Macrobiotus caelicola, Macrobiotus canaricus, Macrobiotus caymanensis, Macrobiotus crenulatus, Macrobiotus crustulus, Macrobiotus dariae, Macrobiotus deceptor, Macrobiotus denticulus, Macrobiotus diversus, Macrobiotus dolosus, Macrobiotus drakensbergi, Macrobiotus dulciporus, Macrobiotus echinogenitus, Macrobiotus engbergi, Macrobiotus fontourai, Macrobiotus furcatus, Macrobiotus glebkai, Macrobiotus grandis, Macrobiotus gretae, Macrobiotus halophilus, Macrobiotus hannae, Macrobiotus horningi, Macrobiotus hufelandi, Macrobiotus humilis, Macrobiotus hupingensis, Macrobiotus hyperboreus, Macrobiotus iharosi, Macrobiotus insularis, Macrobiotus joannae, Macrobiotus julianae, Macrobiotus kamilae, Macrobiotus kazmierskii, Macrobiotus kirghizicus, Macrobiotus kosmali, Macrobiotus kristenseni, Macrobiotus kurasi, Macrobiotus lazzaroi, Macrobiotus lissostomus, Macrobiotus longipes, Macrobiotus macrocalix, Macrobiotus maculatus, Macrobiotus madegassus, Macrobiotus mandalaae, Macrobiotus margoae, Macrobiotus marlenae, Macrobiotus martini, Macrobiotus mileri, Macrobiotus modestus, Macrobiotus muralis, Macrobiotus naginae, Macrobiotus naskreckii, Macrobiotus nebrodensis, Macrobiotus nelsonae, Macrobiotus noemiae, Macrobiotus noongaris, Macrobiotus norvegicus, Macrobiotus occidentalis, Macrobiotus occidentalis, Macrobiotus ocotensis, Macrobiotus ovovittatus, Macrobiotus pallarii, Macrobiotus papei, Macrobiotus patagonicus, Macrobiotus paulinae, Macrobiotus persimilis, Macrobiotus personatus, Macrobiotus peteri, Macrobiotus pisacensis, Macrobiotus polonicus, Macrobiotus polyopus, Macrobiotus polypiformis, Macrobiotus porifini, Macrobiotus primitivae, Macrobiotus psephus, Macrobiotus pseudofurcatus, Macrobiotus pseudohufelandi, Macrobiotus pseudopallarii, Macrobiotus punctillus, Macrobiotus radiatus, Macrobiotus ragonesei, Macrobiotus ramoli, Macrobiotus rawsoni, Macrobiotus rebecchii, Macrobiotus recens, Macrobiotus richtersi, Macrobiotus ripperi, Macrobiotus rybaki, Macrobiotus sandrae, Macrobiotus santoroi, Macrobiotus sapiens, Macrobiotus scoticus, Macrobiotus semmelweisi, Macrobiotus serratus, Macrobiotus seychellensis, Macrobiotus shonaicus, Macrobiotus siderophilus, Macrobiotus sottilei, Macrobiotus terminalis, Macrobiotus trunovae, Macrobiotus vanescens, Macrobiotus vladimiri, Macrobiotus wandae, Macrobiotus wuyishanensis, Macrobiotus yunshanensis, Macrobiotus tonollii.

Minibiotus: Minibiotus acadianus, Minibiotus acontistus, Minibiotus aculeatus, Minibiotus africanus, Minibiotus allani, Minibiotus aquatilis, Minibiotus asteris, Minibiotus bisoctus, Minibiotus claxtonae, Minibiotus constellatus, Minibiotus continuus, Minibiotus crassidens, Minibiotus decrescens, Minibiotus diphasconides, Minibiotus eichhorni, Minibiotus ethelae, Minibiotus fallax, Minibiotus formosus, Minibiotus floriparus, Minibiotus furcatus, Minibiotus granatai, Minibiotus gumersindoi, Minibiotus harrylewisi, Minibiotus hispidus, Minibiotus hufelandioides, Minibiotus intermedius, Minibiotus jonesorum, Minibiotus julietae, Minibiotus keppelensis, Minibiotus maculartus, Minibiotus marcusi, Minibiotus milleri, Minibiotus orthofasciatus, Minibiotus pilatus, Minibiotus poricinctus, Minibiotus pustulatus, Minibiotus ramazzottii, Minibiotus scopulusMinibiotus, Minibiotus stuckenbergi, Minibiotus subintermedius, Minibiotus taiti, Minibiotus vinciguerrae, Minibiotus weglarskae, Minibiotus weinerorum, Minibiotus wuzhishanensis, Minibiotus xavieri.

Minilentus: Minilentus dubius.

Paramacrobiotus: Paramacrobiotus alekseevi, Paramacrobiotus areolatus, Paramacrobiotus beotiae, Paramacrobiotus centesimus, Paramacrobiotus chieregoi, Paramacrobiotus corgatensis, Paramacrobiotus crenatus, Paramacrobiotus csotiensis, Paramacrobiotus danielae, Paramacrobiotus derkai, Paramacrobiotus fairbanksi, Paramacrobiotus garynahi, Paramacrobiotus gerlachae, Paramacrobiotus huziori, Paramacrobiotus kenianus, Paramacrobiotus lorenae, Paramacrobiotus magdalenae, Paramacrobiotus palaui, Paramacrobiotus peteri, Paramacrobiotus richtersi, Paramacrobiotus rioplatensis, Paramacrobiotus savai, Paramacrobiotus tonollii, Paramacrobiotus vanescens, Paramacrobiotus walteri.

Pseudodiphascon: Pseudodiphascon inflexum.

Pseudohexapodibius: Pseudohexapodibius degenerans.

Richtersius: Richtersius coronifer.

Schusterius: Schusterius tridigitus.

Tenuibiotus: Tenuibiotus bondavallii, Tenuibiotus bozhkae, Tenuibiotus ciprianoi, Tenuibiotus danilovi, Tenuibiotus higginsi, Tenuibiotus hyperonyx, Tenuibiotus hystricogenitus, Tenuibiotus kozharai, Tenuibiotus mongolicus, Tenuibiotus tenuiformis, Tenuibiotus tenuis, Tenuibiotus voronkovi, Tenuibiotus willardi.

Xerobiotus: Xerobiotus euxinus, Xerobiotus pseudohufelandi, Xerobiotus xerophilus, Xerobiotus gretae.

Family: Murrayidae

Dactylobiotus: Dactylobiotus ambiguus, Dactylobiotus ampullaceus, Dactylobiotus aquatilis, Dactylobiotus caldarellai, Dactylobiotus dervizi, Dactylobiotus dispar, Dactylobiotus grandipes, Dactylobiotus haplonyx, Dactylobiotus henanensis, Dactylobiotus kansae, Dactylobiotus lombardoi, Dactylobiotus luci, Dactylobiotus macronyx, Dactylobiotus octavi, Dactylobiotus parthenogeneticus, Dactylobiotus selenicus, Dactylobiotus ovimutans.

Macroversum: Macroversum mirum.

Murrayon: Murrayon dianeae, Murrayon hastatus, Murrayon hibernicus, Murrayon nocentiniae, Murrayon ovoglabellus, Murrayon pullari.

Family: Necopinatidae

Necopinatum: Necopinatum mirabile.

(Acutuncus antarcticus)

Body length: 373.8–452.9 µm.

It lives in Antarctica, and South Georgia Island and the South Sandwich Islands. It is found in terrestrial, marine and freshwater habitats, but are most commonly found in terrestrial habitats living in mosses, lichens, grasses, algae, soil and cyanophytan mats.