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Tardigrades or Water Bears: Strange and Resilient Animals

Linda Crampton is a writer and experienced science teacher with an honors degree in biology. She enjoys writing about science and nature.

A lateral or side view of a tardigrade as seen under a light microscope

A lateral or side view of a tardigrade as seen under a light microscope

Surprising Animals With Remarkable Abilities

Tardigrades are interesting and impressively resilient animals. They are also known as water bears and as moss piglets. They survive in a range of extreme environmental conditions that would kill most other organisms, including very high and low temperatures, strong radiation, intense pressure, a vacuum, and dehydration. The animals are still surprising us. Studying their biology is fascinating and might be helpful for humans.

Tardigrades are tiny creatures but are visible to the unaided eye in suitable lighting conditions. At the moment, around 1,400 species of tardigrades have been identified. More may exist. In nature, the animals are found in fresh or salt water and in areas that tend to hold moisture on land. These areas include places with moss, lichens, leaf litter, or soil. The animals can survive in an inactive form if their environment dries up. They are widespread and can be found in many countries.

A Very Interesting Invertebrate

Tardigrades are invertebrates and belong to the phylum Tardigrada. The phylum contains three classes: Eutardigrada, Heterotardigrada, and—somewhat controversially— Mesotardigrada. The last class contains only one species at the moment. The members of the first class generally have a smooth outer covering, or cuticle, and the members of the second class have an armored one containing plates. The scanning electron micrograph below shows the plates of one species. The old cuticle is shed and replaced by a new one as the animal grows.

Despite their small size, the animals contain organs and structures associated with other invertebrates. Though a tardigrade may be visible without a microscope, the instrument is needed in order to see details of the animal’s body. It's a multicellular creature that is sometimes referred to as a "micro-animal."

They (tardigrades) were first named tardigrada in Italian from the Latin meaning “slow walker.”

— William Randoph Miller, via American Scientist

Physical Appearance of a Tardigrade

Some details of a tardigrade's body can be seen under a light microscope. People with a home or school microscope might be able to get a good view of a living animal. Tardigrades may be discovered in samples obtained from the environment, such as moss. Some science supply companies sell the animals.

A Transparent of an Opaque Animal?

One point that may puzzle some people is why a tardigrade looks translucent or transparent in real life or under a light microscope but opaque under a scanning electron microscope. The difference is due to the nature of the magnifying devices.

The tardigrade's body transmits light shone on it, so it looks transparent under a compound or light microscope. When a scanning electron microscope (SEM) is used, a concentrated beam of electrons is directed at the animal in a vacuum instead of light beams. The electrons strike the surface of the specimen and may travel a short distance into it.

Scanning Electron Microscope Images

In a scanning electron microscope, a beam of electrons is admitted by a device called an “electron gun”. The beam is focused so that it strikes the object. As a result of the strike, the object releases some of the electrons that were fired at it, some of its own electrons, and x rays. These are gathered by a detector and used to create a grayscale image on a computer screen.

Colour is a property of light, which isn’t used in an electron microscope. The advantage of the scanning electron microscope is that objects are magnified to a much greater extent than in a light microscope, and new details of the object can be seen.

Misleading Photos

A scanning electron microscope can be a wonderful device for showing details of an animal's surface, as in the photo below. It doesn't show the animal as we would see it under light illumination, however. Unfortunately, many illustrations of tardigrades show a picture resembling the opaque SEM view without indicating that the appearance of the animal is due to a particular investigative technique.

A scanning electron micrograph of a female armored tardigrade named Echiniscus succineus

A scanning electron micrograph of a female armored tardigrade named Echiniscus succineus

The full author description of the above image is Gąsiorek P, Vončina K (2019) New Echiniscidae (Heterotardigrada) from Amber Mountain (Northern Madagascar). Evolutionary Systematics 3(1): 29-39

External Features of the Species

Scientists are discovering that the external and internal body features are similar but not completely identical in the different classes of tardigrades. In addition, the terminology for some of the body parts of the animals varies. I've described some general features of the phylum below.

The body of a tardigrade is long, segmented, and often plump. It has eight short legs that end in curved claws. Three pairs of legs are located under the animal's body. The fourth pair extends from the end of the tardigrade. The first three pairs of legs are used to move the animal over the solid components of its environment. The back legs are used to grasp hold of objects.

A tardigrade has a circular mouth. Its eye spots enable it to distinguish light from dark but don't allow it to see an image. The animal may have a distinct color due to pigments in the cuticle or to the contents of the digestive tract. It may be almost colorless, yellow, orange, brown, green, red, or multicolored.

Internal Anatomy and Physiology

Feeding and Digestion

A tardigrade's mouth contains two sharp, rod-like structures called stylets. These are used to pierce plant and animal tissue and suck up the contents. Nematodes (roundworms) seem to be a favorite prey of some tardigrades.

A sucking pharynx pulls the food into the digestive tract from the stylets. The pharynx leads to the esophagus. The remaining sections of the digestive tract are often referred to as the stomach, the intestine, and the rectum. Sometimes the word "gut" is used instead. Indigestible food is eliminated from the body via an opening at the end of the rectum. Food is digested, or broken up, as it travels along the gut, and nutrients are absorbed into the body.


Malpighian tubules are connected to the digestive tract. They are thought to act as excretory organs, but the details of their behavior haven't been discovered yet. Excretion is the removal of metabolic waste products from an animal. Our kidneys do this job in us.

Circulation and Respiration

A tardigrade contains unpigmented hemolymph instead of blood. It doesn't have a heart or blood vessels. The hemolymph is spread through the body. The animal is small enough to enable the fluid to circulate during body movement.

Tardigrades lack a special respiratory system. Oxygen diffuses into the hemolymph through the cuticle, and carbon dioxide moves in the opposite direction.

Nervous System and Muscles

The animal has a brain in its head, which is connected to a ventral (lower body) double nerve cord. The "brain" has a simpler structure from ours. It's made of fused ganglia. A ganglion is a concentration of neuron cell bodies. The cell body of a neuron (the biological name for a nerve cell) contains the nucleus. Ganglia are also located along the double nerve cord.

A tardigrade doesn't have bones, but it does have muscles. These are attached to the inside of the cuticle and enable the animal to move effectively.

Reproduction Facts

The single reproductive organ of a tardigrade is located above the digestive tract. The genders are often separate, but some species contain hermaphrodites (animals with both male and female organs). The ovary produces eggs. The testis produces sperm. The hermaphrodites produce both eggs and sperm. It's known that self-fertilization occurs in these animals, but it's unknown whether cross-fertilization does.

Reproduction in tardigrades is still being studied. Several methods have been discovered. Some females shed their old cuticle (the exuvia) with the eggs inside. The male then fertilizes the eggs. In some species, however, the sperm fertilizes the eggs while the exuvia is still on the female's body. In other species, the female lays her eggs in the environment and then the male fertilizes them. In still other species, parthenogenesis occurs (creation of a new individual from an unfertilized egg). More research is needed to understand the details of the various reproduction methods.

Lifespan of the Animals

Tardigrades aren't immortal. As is the case for other animals, they eventually die. "Eventually" may be a very long time, depending on an animal's experiences during its life. The lifespan of tardigrades when they don't face environmental stress seems to be quite short. It's believed to be less than a year and may be only a few months. On the other hand, under certain conditions, they may live for decades in an inactive state known as a tun.

The maximum time that the animals can exist in the tun form is still unknown. A few animals have survived for thirty years in this state. Some researchers have speculated that the animals may be able to survive as a tun for a hundred years, though the evidence for this idea seems to be weak at the moment.

The creature loses up to 97 percent of its body moisture and shrivels into a structure about one-third its original size, called a tun.

— William Randoph Miller, via American Scientist

Cryptobiosis in Tardigrades

Cryptobiosis is very important for enabling tardigrades to survive a stress. It's a state in which no metabolic processes can be observed in an animal. The state resembles death, yet the animal is still alive. The body processes that lead to cryptobiosis are reversible, and metabolism begins again when conditions are satisfactory.

Three types of cryptobiosis that have been observed in tardigrades have been given special names.

  • Anhydrobiosis is caused by loss of water.
  • Cryobioisis is caused by freezing conditions.
  • Osmobiosis is caused by extreme salinity.

In each type of cryptobiosis, the tardigrade shrivels up to form a tun. The process is shown in a sped-up view in the video below. In real life, the process is much slower. In experiments, researchers have discovered that once it has become a tun, the animal is resistant to additional stresses, including intense radiation, extreme increase and decrease in temperature, very high pressure, and a vacuum.

Strong radiation is known to damage DNA, which is the genetic material of tardigrades and us. The return to normality in tardigrades after exposure to experimental radiation suggests that the animals have potent DNA repair mechanisms.

The species that live in permanently wet environments such as large bodies of water don't appear to be especially resilient to dehydration. The situation is different for the species that live in an environment that may periodically dry out. They have the amazing ability to lose most of their body water, shrivel up, and still remain alive.

Researchers have observed two additional responses to stress in tardigrades. Anoxybiosis involves the reversible swelling of the body and turgidity after lack of oxygen. Encystment involves formation of extra cuticle layers followed by dormancy.

Surviving Extreme Temperature and Pressure

It's important to note that not all tardigrade species may be able to survive the stresses described below. In addition, other conditions may need to exist in the animals in order for them to be able to stay alive, such as being in the tun state. A further point to note is that only some of the animals in an experimental group survived. Nevertheless, the results of the experiments are amazing.

According to Dr. William Randolph Miller, a biologist and tardigrade researcher at Baker University in Kansas, at least some species of tardigrades or at least some animals in specific species have survived the following extreme conditions in experiments.

  • -272.95 degrees Celsius for twenty hours (Absolute zero is -273.15 degrees Celsius)
  • 150 degrees Celsius (for an unspecified time)
  • -200 degrees Celsius for twenty months
  • 40,000 kilopascals of pressure and 6,000 atmospheres (for an unspecified time in each case)
  • an "excessive" concentration of carbon monoxide, carbon dioxide, sulfur dioxide, and nitrogen
  • a complete vacuum

Many questions exist in relation to how the animals withstand the stresses listed above. For example, at low temperatures, how do their bodies prevent the formation of ice crystals that would damage cells? It's suspected that their cells contain chemicals that act as cryoprotectants and prevent ice formation.

The paramecium in the video above consists of a single cell, unlike the multicellular tardigrade.

Tardigrades in Space

A Deliberate Experiment

In 2007, a spaceship containing tardigrades was launched. While in space, the box containing the animals was opened, exposing 3,000 animals to the outer environment for twelve days under varying conditions. At the end of this time period, the box was closed and the spaceship returned to Earth.

The researchers found that the animals exposed to a vacuum but not solar radiation survived and seemed to develop no problems as a result of their experience. Exposure to a vacuum plus solar radiation was more challenging for the animals. Many died, but some individuals did survive the stresses.

Some tardigrade species have survived exposure to ultraviolet light on Earth, as described below. Tardigrades have also been exposed to intense x-rays on Earth and survived.

An Accidental Experiment

In April 2019, a spacecraft crashed into the moon. The craft contained tardigrades in a dehydrated form. Researchers say that it should be no problem for any animals that survived the crash to stay alive on the moon, as long as they were in the tun state when they arrived. (Animals that aren't in this state can be killed easily.) The scientists also say that it's highly unlikely that the tuns on the moon will be rehydrated, however, due to the lack of water.

Though the moon does have a thin atmosphere that contains some water, there's not enough present for rehydrating tardigrades. There may well be life on the moon now due to the presence of living tardigrades, but it's almost certainly dormant life, scientists say. Even if the animals were able to rehydrate, they wouldn't be able to find food.

Fluorescent Tardigrades

Scientists recently discovered an interesting mechanism by which one tardigrade species may survive UV radiation exposure. Some researchers in India found a new species of tardigrade growing in moss on a concrete wall. The animal is currently known as Paramicrobiotus BLR. The researchers decided to see how resistant it was to radiation. They found that it survived a fifteen-minute exposure to "germicidal" levels of ultraviolet radiation for thirty days. The exposure killed a tardigrade named Hypsibius exemplaris within twenty-four hours.

The researchers also made another discovery. While they were studying the two tardigrade species under ultraviolet light, they noticed that the test tubes of Paramicrobiotus were glowing due to fluorescence and the Hypsibius tubes weren't. They suspected that the substance or substances producing the fluorescence might be responsible for the protection from ultraviolet radiation.

The researchers ground up members of the first species and created a liquid. They then added the liquid to a dish contain the second species of tardigrade. The animals in the second species had significantly higher survival rates after UV exposure than they did without the presence of the liquid, though they didn't survive as long as the first species. At the moment, the researchers don't know the identity of the protective chemical or chemicals in Paramicrobiotus BLR.

A dorsal or top view of a tardigrade (unknown species)

A dorsal or top view of a tardigrade (unknown species)

Current Secrets and Future Discoveries

Tardigrades are fascinating and very unusual animals. They may have more secrets to reveal. Many species exist, and relatively few of them have been studied in detail. It's unlikely that every tardigrade species responds in the same way to the stresses that researchers have applied in experiments or even to conditions that they may encounter naturally. The observations obtained so far are tantalizing.

The animals may have a lot to teach us. Despite the claims by some publications, they aren't indestructible. Certain species do seem to be amazingly resilient to some extreme stresses, however. Some scientists suspect that studying the processes that occur in tardigrades as they respond to these stresses may provide benefits for us. Even if this isn't the case, learning more about the amazing abilities of the animals should be very interesting.


  • A description of tardigrades from American Scientist (Article written by Dr. William Randolph Miller)
  • Detecting objects with an electron microscope from Purdue University
  • Facts about the organs of the animals from "Ask a Biologist" at Arizona State University
  • Information about tardigrades (including information from scientists investigating the animals) from Popular Mechanics
  • Reproduction in the animals by Kenta Sugiura and Midori Matsumoto via the Invertebrate Reproduction and Development Journal
  • Tardigrades in space from the Current Biology journal
  • Resilient animals from Gizmodo
  • Cryptobiosis in tardigrades from BBC Earth
  • Tardigrades on the moon from
  • The fluorescent shield of one species from CNN

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2020 Linda Crampton


Linda Crampton (author) from British Columbia, Canada on October 23, 2020:

Thanks for the visit and the comment, Linda. I think you may be referring to the tun stage. Tardigrades lose water when they become a tun and then absorb water as they return to their normal state. They have to absorb the water from their environment in order to leave the tun stage.

Linda Crampton (author) from British Columbia, Canada on October 23, 2020:

Hi, Adrienne. I think the animals are worth studying, too. It would be wonderful if we could discover some information that helps us!

Linda Chechar from Arizona on October 23, 2020:

Weird that are species tardigrade created liquid! What tiny creatures!

Adrienne Farricelli on October 23, 2020:

Water bears! I recall studying them once in class, but forgot all about their existence until I stumbled on your article. They are truly worthy of studying considering that they are almost indestructible and can even survive in outer space! Maybe we need to learn something from them.

Linda Crampton (author) from British Columbia, Canada on October 23, 2020:

Thank you for the comment, Manatita. Tardigrades are unusual animals, but like you I think they're interesting. I hope researchers learn more about them soon.

manatita44 from london on October 23, 2020:

Very interesting and different. They are going to the moon as well. Maybe I'll join them. Lol.

Interesting video. Quite different from the caterpillar but some similarities there too.

Linda Crampton (author) from British Columbia, Canada on October 23, 2020:

Thanks for the interesting comment, Heidi. I think the ability of the animals to withstand such extreme conditions is impressive.

Linda Crampton (author) from British Columbia, Canada on October 23, 2020:

"Weirdly strange" is a great way to describe tardigrades! I appreciate your visit and comment, Viet.

Heidi Thorne from Chicago Area on October 23, 2020:

This is a fascinating species, especially as it relates to living in the harsh conditions of space and space travel. When they do find life on other seemingly barren planets or moons, I do think it will be something similar to tardigrades who can handle extreme environments.

Thanks for sharing more details on this little creature!

Viet Doan from Big Island, Hawaii on October 23, 2020:

So weirdly strange yet fascinating creatures! I love their stumpy little legs and claws! Thanks Linda, your article makes me want to rush outside and peer into the water of my backyard pond to find these bizarre animals. Oh wait, I need a microscope!

Linda Crampton (author) from British Columbia, Canada on October 23, 2020:

Thanks, Devika. I appreciate your comment very much.

Devika Primić from Dubrovnik, Croatia on October 23, 2020:

Incredible! A fascinating hub on tiny creatures and something I had no idea of. You share interesting and informative hubs. These topics are unique and on all of your hubs you thoroughly research every topic to share the best to us.

Linda Crampton (author) from British Columbia, Canada on October 23, 2020:

Thank you for the comment, Chitrangada. Nature can be amazing.

Chitrangada Sharan from New Delhi, India on October 23, 2020:

Such a tiny creature, but so many important details about it. It amazes me that there are so many little and big creatures in the universe, about which we don’t know about.

This was another interesting, and well written article. Thanks for the education.

Linda Crampton (author) from British Columbia, Canada on October 22, 2020:

Hi, Peggy. Even though tardigrades are tiny, they have many interesting features. I hope scientists learn more about them soon. Thank you for commenting.

Peggy Woods from Houston, Texas on October 22, 2020:

Studying creatures in nature is fascinating. Thanks for writing this article about tardigrades. I had never previously read anything about them. Under microscopes, they look fierce with those claws, and the armored one with the spikes looks particularly fierce. It appears that scientists can learn much more about their adaptability to survive in stressful environments.

Linda Crampton (author) from British Columbia, Canada on October 22, 2020:

Thanks for the comment, Fran. Tardigrades play an important role in their ecosystem, such as by the products that they make and use, the organisms that they eat, and the organisms that eat them.

Linda Crampton (author) from British Columbia, Canada on October 22, 2020:

I appreciate your comment very much, Ankita. Thank you for the visit.

Linda Crampton (author) from British Columbia, Canada on October 22, 2020:

Thank you, Maren. I enjoyed the man's commentary, too!

fran rooks from Toledo, Ohio on October 22, 2020:

Alicia, another very interesting article. Might I ask what is their purpose? They are so unusual.

Ankita B on October 22, 2020:

I have never heard of tardigrades before. They are truly fascinating creatures. Interesting and wonderfully written, as always.

Maren Elizabeth Morgan from Pennsylvania on October 22, 2020:

Fascinating. I enjoyed the sense of humor of the narrator for the "birth" video.

Linda Crampton (author) from British Columbia, Canada on October 22, 2020:

Thank you for the visit and the comment, Eman.

Linda Crampton (author) from British Columbia, Canada on October 22, 2020:

Thanks for the recommendation, Bill. I appreciate your comment very much.

Linda Crampton (author) from British Columbia, Canada on October 22, 2020:

Hi, Pamela. Thanks for the comment. The resilience of tardigrades is very impressive.

Linda Crampton (author) from British Columbia, Canada on October 22, 2020:

Hi, Eric. I think tardigrades are fascinating animals. It's interesting to think about the abilities of such a tiny creature.

Linda Crampton (author) from British Columbia, Canada on October 22, 2020:

Thanks for the visit, Liz. It is amazing that researchers have discovered so much about the animals when they are so small. I hope more discoveries are made soon.

Linda Crampton (author) from British Columbia, Canada on October 22, 2020:

Hi, Bill. It is amazing that so many life forms exist in our environment without our knowledge. Thanks for the comment.

Eman Abdallah Kamel from Egypt on October 22, 2020:

It is really an interesting and informative article about water bears.

Bill Holland from Olympia, WA on October 22, 2020:

I was watching "My Octopus Teacher" the other night on Netflix, and I thought of you. It's a brilliant documentary, and if you haven't seen it, please watch it. That movie, and your articles, are so important. Thank you for this, my friend. Excellent!

Pamela Oglesby from Sunny Florida on October 22, 2020:

I have never heard of these little creatures before and this was an interesting read. It is amazing that they took 3,000 of them to outer space and experimented really. For the most part they seem to survive almost any environmental situation.

Eric Dierker from Spring Valley, CA. U.S.A. on October 22, 2020:

Very interesting. So much to know about such a tiny creature. The tests in space and the heat span are amazing.

Liz Westwood from UK on October 22, 2020:

I had not come across tardigrades previously. Your article gives a thorough, interesting and well-presented account of them. I shall keep an eye out for these in future. I am amazed at how much information has been collated about such tiny creatures.

Bill De Giulio from Massachusetts on October 22, 2020:

How interesting, Linda. I had never heard of the the tardigrade. It’s fascinating the creatures that exist around us that we are not aware of. I find it amazing the extreme conditions that some species can survive. Thank you for the education.