Tardigrades ( / ˈ t ɑːr d ɪ ɡ r eɪ d z / ), also called water bears or moss piglets, are a group of tiny, eight-legged animals. A German scientist named Johann August Ephraim Goeze first described them in 1773, calling them Kleiner Wasserbär, which means "little water bear" in German. In 1776, an Italian scientist named Lazzaro Spallanzani gave them the scientific name Tardigrada, which means "slow walkers."
Tardigrades live in many places on Earth, including high mountains, the deep ocean, rainforests, and Antarctica. They are among the toughest animals on Earth, able to survive extreme conditions such as very hot or cold temperatures, high or low pressure, lack of air, radiation, drying out, and not eating for long periods. Tardigrades have even survived in space.
There are about 1,500 known types of tardigrades, which belong to a larger group called Ecdysozoa. The oldest known tardigrade fossil is from the Cambrian period, about 500 million years ago. These animals do not have some genes that help arthropods form body parts. Instead, most of their body is similar to the head of an arthropod.
Tardigrades are usually about 0.5 mm (0.02 in) long when fully grown. They are short and round, with four pairs of legs that end in claws or sticky pads. Tardigrades are often found in moss and lichen and can be easily seen with a simple microscope, making them useful for students and amateur scientists to study. Their slow, wobbly movement and ability to survive harsh conditions have made them famous in science fiction and popular culture, appearing in clothing, statues, toys, and crochet patterns.
Description
Tardigrades have a short, plump body with four pairs of hollow, unjointed legs. Most are between 0.05 and 0.5 mm (0.002 to 0.02 in) long, though some species can grow up to 1.3 mm (0.051 in). Their body cavity is called a haemocoel, an open system filled with colorless fluid. The body is covered by a cuticle, which is shed during molting. This cuticle contains hardened proteins and chitin but is not made of calcium. Each leg ends with one or more claws, which may be modified into sticky pads in some species. Marine tardigrades have telescopic legs. They do not have lungs, gills, or blood vessels, so they exchange gases through their cuticle and body cavity. Tardigrades have about 1,000 cells in their bodies.
The nervous system of tardigrades includes a pair of ventral nerve cords, each connected to a ganglion that serves a pair of legs. These nerve cords end near the mouth at a pair of subpharyngeal ganglia. These ganglia are linked by paired commissures to the cerebral ganglion, or "brain," located on the back of the head. The brain contains two eyespots and several sensory structures, including cirri and hollow, antenna-like clavae that may act as chemoreceptors.
The tardigrade Dactylobiotus dispar can be trained to curl into a defensive "tun" state when exposed to a blue light paired with a small electric shock. This shows that tardigrades can learn.
Although their bodies are flexible and filled with fluid, their movement does not rely mainly on hydrostatic pressure. Instead, muscles in their legs work in pairs to move them backward and forward. Some muscles also counteract the pressure of the haemocoel. Claws help prevent legs from slipping during movement and are used for gripping surfaces.
Tardigrades feed by sucking fluids from animal or plant cells or by consuming detritus. A pair of stylets made of aragonite pierce their prey, and the pharynx muscles pump the fluids into the gut. Salivary glands secrete digestive fluid into the mouth and produce new stylets during molting. Non-marine species have Malpighian tubules near the junction of the intestine and hindgut. Some species have excretory or other glands located near or at the base of their legs.
Some tardigrade species are hermaphroditic and self-fertilizing, but most have separate male and female individuals that mate in various ways. Females lay eggs, which vary in shape. For example, Austeruseus faeroensis eggs are spherical, 80 μm in diameter, with a knobby surface. Other species, like Hypsibius annulatus, have ovoid or spherical eggs with pyramidal or bottle-shaped surface patterns. Some species lack males, suggesting parthenogenesis is common.
Both sexes have a single gonad (ovary or testis) located above the intestine. In males, a pair of ducts leads from the testis to a single gonopore near the anus. In females, a single oviduct opens either just above the anus or directly into the rectum, forming a cloaca.
Males may deposit sperm into the cloaca, penetrate the female's cuticle to place sperm directly into the body cavity, or insert sperm under the cuticle. In species like H. annulatus, the male places sperm under the female's cuticle, and the female lays eggs into the shed cuticle, where they are fertilized. Some aquatic tardigrades engage in courtship, with males using their cirri to stimulate females to lay eggs, followed by external fertilization.
Up to 30 eggs may be laid, depending on the species. Terrestrial eggs have drought-resistant shells. Aquatic species either attach their eggs to a surface or leave them in the shed cuticle. Eggs hatch within 14 days, and the hatchlings use their stylets to break through their eggshells.
Ecology and life history
Tardigrades are found in many places around the world, living in different environments on land, in freshwater, and in the ocean. Their eggs and tough life stages, such as cysts and tuns, are small and strong enough to travel long distances. These stages can move on the feet of other animals or be carried by the wind.
Some tardigrade species live in specific areas and are limited to certain types of habitats, such as mountains. Others have wide ranges. For example, Echiniscus lineatus is found in tropical regions worldwide. Halobiotus lives only in cold Holarctic seas. Species like Borealibius and Echiniscus lapponicus are found in both polar regions and on tall mountains. This pattern may result from wind transport or from ancient geographic ranges when the climate was colder. A small number of tardigrade species are found globally.
Most tardigrades live in damp places, such as on lichens, liverworts, and mosses, as well as in soil and leaf litter. In freshwater and the ocean, they live on or inside the seafloor, between particles, or near seaweeds. Some live in specialized environments, such as hot springs or as parasites or commensals of marine invertebrates. In soil, there can be up to 300,000 tardigrades per square meter; on mosses, they can reach over 2 million per square meter.
Tardigrades have many tiny microbes living inside them, including bacteria like Flavobacterium, Ferruginibacter, and Polaromonas, which are common in glacial environments. Many tardigrades are predators, such as Milnesium lagniappe, which eats other tardigrades like Macrobiotus acadianus. They also hunt nematodes and are eaten by soil arthropods, including mites, spiders, and cantharid beetle larvae.
Except for 62 species that live only in freshwater, all non-marine tardigrades are found on land. Since most marine tardigrades belong to Heterotardigrada, the oldest group, the phylum likely originated in the ocean.
Environmental tolerance
Tardigrades are not considered extremophiles because they are not adapted to live in extreme environments, only to survive in them. This means they are more likely to die if they stay in these environments for a long time, unlike true extremophiles that thrive there.
Tardigrades can stop their metabolism and enter a state called cryptobiosis. When water is not available, such as when the moss or pond they live in dries up, they curl into a desiccated cyst called a "tun." In this state, they do not need food or water for many years. They also become very resistant to extreme conditions, such as very low temperatures (as low as −272 °C or −458 °F), very high temperatures (up to +149 °C or 300 °F for short periods), lack of oxygen, vacuum, radiation, and high pressure.
Marine tardigrades, like Halobiotus crispae, change each year between an active summer form and a hibernating winter form that can survive freezing and low salt levels. However, reproduction only happens in the summer form.
Tardigrades can survive impacts at speeds up to about 900 meters per second (3,000 feet per second) and sudden pressures up to about 1.14 gigapascals (165,000 psi).
Tardigrades have survived in space. In 2007, dehydrated tardigrades and eggs were sent on the FOTON-M3 mission. They were exposed to vacuum, vacuum and UV light, or vacuum and full solar UV for 10 days. After returning to Earth, they were rehydrated. Vacuum alone did not harm them, but vacuum combined with UV light killed many. Only a few survived, and their egg-laying was reduced. Exposure to full solar UV was even more deadly, with only three tardigrades surviving.
In 2011, tardigrades were sent to the International Space Station STS-134, showing they can survive microgravity and cosmic radiation. This makes them good models for scientific study.
In 2019, a capsule containing tardigrades in a dormant state was on the Israeli lunar lander Beresheet, which crashed on the Moon.
Tardigrades can survive long periods without water, but this is not due to a sugar called trehalose. Instead, they produce special proteins that protect their cells. Three of these proteins are unique to tardigrades and may form a glass-like structure that protects their cells during drying. In Hypsibius exemplaris, 1,422 genes are activated during desiccation, with 406 being specific to tardigrades.
Tardigrades have a cold shock protein that may help protect their cells after freezing.
Tardigrade DNA is protected from radiation by a protein called Dsup. This protein binds to DNA and helps repair damage caused by radiation.
Some of these proteins are useful in medical research. For example, Dsup may help protect cells from damage, and other proteins may help cells survive drying or prevent cell death.
Taxonomic history
In 1773, Johann August Ephraim Goeze gave the tardigrade the name Kleiner Wasserbär, which means "little water-bear" in German. Today, Germans often call them Bärtierchen, meaning "little bear-animal." The name "water bear" comes from the way they move, which looks similar to how a bear walks. The name Tardigradum means "slow walker" and was given by Lazzaro Spallanzani in 1776. In 1834, C.A.S. Schulze provided the first formal description of a tardigrade called Macrobiotus hufelandi in a work with the subtitle "a new animal from the crustacean class, capable of reviving after long periods of no air and dryness." Soon after, L.M.F. Doyère described other species, including Echiniscus testudo, Milnesium tardigradum, Hypsibius dujardini, and Ramazzottius oberhaeuseri in 1840. These four species are now the first species used to classify larger groups of tardigrades. Hartmut Greven, a scientist who studies animals, wrote that "All later researchers agree that Doyère's 1842 dissertation Memoire sur les Tardigrades is an important achievement in the study of tardigrades."
From 1900 to 1913, Ferdinand Richters studied the classification of tardigrades, focusing on species from Nordic, Arctic, marine, and South American regions. He described many species during this time and in
Evolution
Tardigrade fossils are rare. The only known examples come from mid-Cambrian deposits in Siberia (part of the Orsten fauna) and a few found in amber from the Cretaceous period in North America and the Neogene period in the Dominican Republic. The Siberian fossils differ from modern tardigrades in several ways. They have three pairs of legs instead of four, simpler head structures, and lack posterior head appendages. However, they share a similar columnar cuticle construction with living tardigrades. Scientists believe these fossils represent early ancestors of modern tardigrades.
Evidence suggests that tardigrades became smaller over time from a larger ancestor, likely a lobopodian, possibly similar to the mid-Cambrian Aysheaia, which is closely related to the tardigrade lineage. Another theory suggests that tactopoda evolved from a group including dinocaridids and Opabinia. A 30-million-year-old fossil called Sialomorpha, found in Dominican amber, shows some similarities to tardigrades but is not one. A 2023 study concluded that luolishaniids, a group of Cambrian lobopodians, might be the closest known relatives of tardigrades.
The oldest remains of modern tardigrades are from Milnesium swolenskyi, a species in the living genus Milnesium, found in Late Cretaceous (Turonian) New Jersey amber, about 90 million years old. Another fossil, Beorn leggi, belongs to the family Hypsibiidae and was found in Late Campanian (~72 million years old) Canadian amber. A related species, Aerobius dactylus, was also discovered in the same amber. The youngest known fossil tardigrade genus, Paradoryphoribius, was found in amber dated to about 16 million years ago.
Studies using both physical traits and genetic data have explored how tardigrades relate to other ecdysozoan groups. Alternative theories suggest tardigrades might be closely related to arthropods, onychophorans, or lobopodians. These relationships remain debated due to conflicting evidence.
Tardigrade genomes vary in size. Hypsibius exemplaris has a compact genome of 100 million base pairs and a generation time of about two weeks. It can be grown indefinitely and frozen. Ramazzottius varieornatus, a highly stress-tolerant species, has a genome about half that size, at 55 million base pairs. About 1.6% of its genes were acquired from other species, but this does not significantly affect its biology.
Genomic studies help scientists understand how tardigrade body segments evolved compared to other Panarthropoda. A 2023 review concluded that despite the diversity of body plans among Panarthropoda, tardigrade segments align best with a simple one-to-one match to head regions in arthropods. These studies may eventually explain how tardigrades became smaller from larger ecdysozoans.
Tardigrades lack some Hox genes found in arthropods and have fewer body segments. In insects, this corresponds to the thorax and abdomen. Most of a tardigrade’s body, except for the last pair of legs, consists of segments similar to the head region in arthropods. This suggests tardigrades evolved from an ancestor with a longer body and more segments.
In 2012, a study using molecular markers (ribosomal RNA) found that Heterotardigrada and Arthrotardigrada might be paraphyletic. In 2018, a report combining multiple studies concluded that Arthrotardigrada appear paraphyletic, but Heterotardigrada is a valid group. Lower-level classifications have been reorganized, but major groupings remain unchanged. In 2019, Noemi Guil and colleagues proposed elevating the order Apochela to a new class called Apotardigrada.
In culture and society
The first time tardigrades appeared in non-scientific writing was in the short story "Bathybia" by geologist and explorer Douglas Mawson. The story was published in the 1908 book Aurora Australis, printed in Antarctica. It describes an expedition to the South Pole where the team sees giant mushrooms and arthropods. The group watches a giant tardigrade fight a similarly large rotifer. Another giant water bear bites a man's toe, causing him to fall into a coma for half an hour due to the numbing effect of the bite. Finally, a four-foot-long tardigrade, waking from hibernation, scares the narrator from sleep. The narrator later realizes the events were a dream.
Tardigrades are often found in mosses and lichens on walls and rooftops. They can be easily collected and observed under a low-power microscope. If they are dry, they can be revived on a microscope slide by adding a small amount of water, making them accessible to students and amateur scientists. Current Biology noted their popularity because of their "clumsy crawling [which] is about as adorable as can be." Zoologists James F. Fleming and Kazuhuru Arakawa called them "a charismatic phylum." Tardigrades are known for surviving extreme conditions, such as being dried out. This ability was first demonstrated in the 18th century when scientist Spallanzani revived them from dry sediment in a gutter. In 2015, astrophysicist Neil deGrasse Tyson called Earth "the planet of the tardigrades." They were also nominated for the American Name Society's Name of the Year Award. Tardigrades appear on merchandise like clothing, earrings, soft toys, and keychains, including crochet patterns for people to make their own. Dutch artist Arno Coenen created statues for St. Eusebius' Church in Arnhem, including a tardigrade and a coronavirus.
In 2025, The Guardian named Milnesium tardigradum the winner of its "Invertebrate of the Year" competition, chosen from ten finalists. The article about the contest stated that the species "has endured all five previous planetary extinction events."
Tardigrades' ability to survive extreme conditions has made them appear in science fiction and pop culture. Musician Cosmo Sheldrake imagines himself as a tough tardigrade in his 2015 song "Tardigrade Song." He sings, "If I were a tardigrade… Pressure wouldn't squash me and fire couldn't burn… I can live life in vacuums for years with no drink (A ha)."
Biologists Mark Blaxter and Kazuharu Arakawa described tardigrades' presence in science fiction and fantasy as "rare but entertaining walk-on parts." They noted that in the 2015 sci-fi horror film Harbinger Down, the main characters face mutated tardigrades that became intelligent and deadly shapeshifters through Cold War experiments.
In the 2017 Star Trek: Discovery, the alien creature "Ripper" is a large but "generally recognizable" version of a terrestrial tardigrade. In the story, the Ripper "incorporates foreign DNA into its own genome via horizontal gene transfer" from its symbiotic fungi. Scholar Lisa Meinecke, in Fighting for the Future: Essays on Star Trek: Discovery, wrote that the Ripper shares some traits with real tardigrades, such as "its physical resilience to extreme environmental stresses." She added that while taking on fungal DNA is "based on real science," it also carries a "mystical idea" of species blending, which changes both organisms and connects all life.