Tardigrades (/ˈtɑːrdɪɡreɪdz/ ),[1] known colloquially as water bears or moss piglets,[2][3][4][5] are a phylum of eight-legged segmented micro-animals.[2][6] They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbär 'little water bear'.[7][8] In 1776, the Italian biologist Lazzaro Spallanzani named them Tardigrada (/tɑːrˈdɪɡrədə/ ), which means 'slow steppers'.[9][10]
They have been found in diverse regions of Earth's biosphere – mountaintops, the deep sea, tropical rainforests, and the Antarctic.[10] Tardigrades are among the most resilient animals known,[11][12] with individual species able to survive extreme conditions – such as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, radiation, dehydration, and starvation – that would quickly kill most other known forms of life.[13] Tardigrades have survived exposure to outer space.[14][15] There are about 1,300 known species[16] in the phylum Tardigrada, a part of the superphylum Ecdysozoa consisting of animals that grow by ecdysis (shedding an exoskeleton) such as arthropods and nematodes. The earliest known true members of the group are known from Cretaceous (145 to 66 million years ago) amber, found in North America, but are essentially modern forms. Their origin is therefore likely much earlier, as they diverged from their closest relatives in the Cambrian more than 500 million years ago.
Tardigrades are usually about 0.5 mm (0.020 in) long when fully grown.[2] They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or suction disks.[2][17] Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a low-power microscope, making them accessible to students and amateur scientists.[18]
Johann August Ephraim Goeze originally named the tardigrade Kleiner Wasserbär, meaning 'little water-bear' in German (today, they are often referred to in German as Bärtierchen 'little bear-animal'). The name water-bear comes from the way they walk, reminiscent of a bear's gait. The name Tardigradum means 'slow walker' and was given by Lazzaro Spallanzani in 1777.[10]
The largest adults may reach a body length of 1.5 mm (0.059 in), the smallest below 0.1 mm (0.0039 in). Newly hatched tardigrades may be smaller than 0.05 mm (0.0020 in). For comparison, grass pollen is typically 0.025–0.04 mm (0.00098–0.00157 in).
Tardigrades are often found on lichens and mosses— for example, by soaking a piece of moss in water.[19] Other environments in which they are found include dunes and coasts generally, soil, leaf litter, and marine or freshwater sediments, where they may occur quite frequently, up to 25,000 animals per litre (95,000 animals per gallon). One tardigrade, Echiniscoides wyethi,[20] may be found on barnacles.[21]
With the exception of 62 known species that live exclusively in freshwater, all non-marine tardigrades are found in terrestrial environments. Because the majority of the marine species belongs to Heterotardigrada, the most ancestral class, it confirms the phylum's marine origin.[22]
Tardigrades have barrel-shaped bodies with four pairs of stubby legs. Most range from 0.3 to 0.5 mm (0.012 to 0.020 in) in length, although the largest species may reach 1.2 mm (0.047 in).[10] The body consists of a head, three body segments each with a pair of legs, and a caudal segment with a fourth pair of legs. The legs are without joints, while the feet have four to eight claws each. The cuticle contains chitin and protein and is moulted periodically. The first three pairs of legs are directed downward along the sides and are the primary means of locomotion, while the fourth pair is directed backward on the last segment of the trunk and is used primarily for grasping the substrate.[23]
Tardigrades lack several Hox genes and a large intermediate region of the body axis. In insects, this corresponds to the entire thorax and the abdomen. Practically the whole body, except for the last pair of legs, is made up of just the segments that are homologous to the head region in arthropods.[24]
All adult tardigrades of the same species have the same number of cells (see eutely). Some species have as many as 40,000 cells in each adult, while others have far fewer.[25][26]
The body cavity consists of a haemocoel, but the only place where a true coelom can be found is around the gonad. No respiratory organs were found, with gas exchange able to occur across the entirety of the body. Some tardigrades have three tubular glands associated with the rectum; these may be excretory organs similar to the Malpighian tubules of arthropods, although the details remain unclear.[27] Also, nephridia are absent.[28]
The tubular mouth is armed with stylets, which are used to pierce the plant cells, algae, or small invertebrates on which the tardigrades feed, releasing the body fluids or cell contents. The mouth opens into a triradiate, muscular, sucking pharynx. The stylets are lost when the animal molts, and a new pair is secreted from a pair of glands that lie on either side of the mouth. The pharynx connects to a short esophagus, and then to an intestine that occupies much of the length of the body, which is the main site of digestion. The intestine opens, via a short rectum, to an anus located at the terminal end of the body. Some species only defecate when they molt, leaving the feces behind with the shed cuticle.[27]
The tardigrade nervous system consists primarily of the brain and four segmental ganglia associated with the four body segments.[29] The brain comprises about 1% of the total body volume.[30] The brain develops in a bilaterally symmetric pattern.[31] Tardigrades have a dorsal brain atop a paired ventral nervous system. The brain includes multiple lobes, mostly consisting of three bilaterally paired clusters of neurons.[32] The brain is attached to a large ganglion below the esophagus, from which a double ventral nerve cord runs the length of the body. The cord possesses one ganglion per segment, each of which produces lateral nerve fibres that run into the limbs. Many species possess a pair of rhabdomeric pigment-cup eyes, and numerous sensory bristles are on the head and body.[33]
Tardigrades all possess a buccopharyngeal apparatus (swallowing device made of muscles and spines that activates an inner jaw and begins digestion and movement along the throat and intestine[34]) which, along with the claws, is used to differentiate species.
Although some species are parthenogenic, both males and females are usually present, although females are frequently larger and more common. Both sexes have a single gonad located above the intestine. Two ducts run from the testes in males, opening through a single pore in front of the anus. In contrast, females have a single duct opening either just above the anus or directly into the rectum, which forms a cloaca.[27]
Tardigrades are oviparous, and fertilization is usually external. Mating occurs during the molt with the eggs being laid inside the shed cuticle of the female and then covered with sperm. A few species have internal fertilization, with mating occurring before the female fully sheds her cuticle. In most cases, the eggs are left inside the shed cuticle to develop, but some species attach them to a nearby substrate.[27]
The eggs hatch after no more than 14 days, with the young already possessing their full complement of adult cells. Growth to adult size occurs by enlargement of the individual cells (hypertrophy), rather than by cell division. Tardigrades may molt up to 12 times.[27]
Tardigrades tend to court before mating. Courtship is an early step in mating and was first observed in tardigrades in 1895. Research shows that up to nine males aggregate around a female to mate.[35]
Most tardigrades are phytophagous (plant eaters) or bacteriophagous (bacteria eaters), but some are carnivorous to the extent that they eat smaller species of tardigrades (for example, Milnesium tardigradum).[36][37] In addition, a few extant species, such as Tetrakentron synaptae, alongside the undescribed Cambrian “Orsten” tardigrade, are parasitic.[38][39]
Tardigrades share morphological characteristics with many species that differ largely by class. Biologists have a difficult time finding verification among tardigrade species because of this relationship.[clarification needed] These animals are most closely related to the early evolution of arthropods.[40] Tardigrade fossils go as far back as the Cretaceous period in North America. Tardigrades are considered cosmopolitan and can be located in regions all over the world. The eggs and cysts of tardigrades are so durable that they can be carried great distances on the feet of other animals.[17]
Tardigrades have survived all five recognized mass extinctions due to their plethora of survival characteristics, including the ability to survive conditions that would be fatal to almost all other animals (see the next section).
The lifespan of tardigrades ranges from three to four months for some species, up to two years for other species, not counting their time in dormant states.[41]
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