Scientific Classification

Kingdom: Animalia
Phylum: Tardigrada, Micro-animals

Range and Population

About 1,150 known species form the phylum Tardigrada, a part of the superphylum Ecdysozoa.


Found everywhere, from mountaintops to the deep sea and mud volcanoes


A few researchers believe some species of tardigrades might even be able to survive desiccation for up to a century. Yet the average lifespan of a (continuously hydrated) tardigrade is rarely longer than a few months.


0.1 mm to 1.5 mm


Photo courtesy of hastingsreserve.org

The Basics: 

Tardigrades have 8 legs, are normally found in freshwater, and are segmented.  They were discovered in the 1773 and named water bears. They’re also known as water bears or moss piglets. The name tardigrades appeared three years later and it means “slow steppers”. It refers to their slow movement.

They’re most famous for being able to tolerate ridiculously extreme environments. They’re found in almost all environments on earth. These environments include deep waters, jungles, rivers, and even mud volcanoes. There’s probably a tardigrade within a stone’s throw of you right now.


There are over 1,000 species of tardigrades distributed among three classes and seven orders.The three classes are eutardigrada, heterotardrigrads, and mesotardigrada. The reproductive system is one of the main ways to tell them apart. Heterotardigrada have gonoducts that open to the outside through a pre-anal gonopore, rather than opening into the rectum as in the eutardigrada. This means eutardigrada has one single orifice for reproduction and digestion, and heterotardigrada have separate orifices. The third class, mesotardigrada, has only been described by one specimen in 1937. The specimen then was lost in an Earthquake and no other specimens have ever been found. So, it’s likely, or even probably, that this class doesn’t even exist.


They walk in a way that resembles the gait of a bear, hence their original name and current nickname, water bears. They range in size from 0.1 mm to 1.5 mm. Their body has a head, three segments with a pair of legs on each, and a caudal segment (a tail) that has another pair of legs. The legs have no articulations but have four claws at the end made of chitin—the same material crab shells are made of. The 6 front legs point downwards and are used to grab things and to move, and the legs on the caudal segment point backwards and are used to grab onto the floor or substrate while the water bear waits for food to come to them. In addition to this, tardigrades can do gas exchange through their whole body. They do not have a nose or any other respiratory organs.

  • Adult tardigrades have a set amount of cells. All members of the same species have the exact same number. This is called eutely, and occurs mostly on microscopic organisms.
    • Baby tardigrade eggs hatch within 2 weeks. Babies are born with the full number of cells. Instead of cell division, they grow when their cells get bigger. 

Their mouth has stylets, which are basically little sharp teeth used to pierce plants or small invertebrates. When pierced, they leak fluids. Tardigrades feed on those fluids by sucking them in using specialized sucking muscles in their pharynx.The stylets are replaced when they molt.

Tardigrades have anal ducts, but some species only poop when they molt. It’s like their version of pooping their pants and throwing the underwear in the garbage.

Most species have compound cup eyes. They also have little hairs along their body to feel vibrations around them. They lay eggs and they do it when they molt. So, the female will molt and leave the eggs in the old cuticle (their old “skin”). The male will then fertilize them, and in some cases move them and attach them to the substrate.

Photo courtesy of American Scientist


  • They can be found everywhere but the easiest way to find them is by putting moss in water, and then examining the water. 
  • Some species are herbivores, others eat bacteria, and others are carnivorous (and can eat other tardigrades)

Extreme Environments

Tardigrades can tolerate environments that would kill other animals in seconds. This is true mostly for terrestrial tardigrades. Marine and aquatic tardigrades are not as resilient because their environments are more stable. However, in extreme conditions terrestrial tardigrades can mummify themselves into little shriveled glass tardies called tuns. As tuns, they can stand things such as:

Extreme temperatures

  • Some species can survive at almost absolute zero (-273 C), and others can live through periods at temperatures over 150C.
  • Of course, they can’t tolerate those temperatures forever. The longer they stay at those temps they more likely they are to die. Still, they can live for a few days at -200 C and up to 30 years at -20.
  • Cold is dangerous because it damages cells. As water gets colder it expands, that’s why ice floats. As it expands it creates great pressure on the cells, which are mostly full of water. (Cells are basically water bags, that’s where the thing about us being 50-60% water comes from). When the water inside the cell expands so much that the cells explode it’s obviously bad news. That’s what causes frostbite.
  • To deal with this they enter a state called cryobiosis, where their metabolism gets suspended.
  • To protect themselves from expanding water inside the cell they release cryoprotectants, which make the ice crystallize in a way that’s less damaging to the cell, and that preserves some of the elasticity of the cell.

Extreme pressures

  • Some species can withstand pressures 6 times higher than those encountered at the bottom of Mariana Trench.
  • Others can survive in a vacuum. Some have survived in outer space, where they’re exposed to both a vacuum and solar UV radiation.
    • This happened in 2007 and was an experiment performed by Italian scientists from the European Space Agency.


  • Even their DNA is strong. It can tolerate radiation because they have a protein that protects. This protein is call dsup (damage suppressor). It’s not complete protection but they reduce x-ray radiation damage by 40%
  • They can tolerate 1,000 as much radiation as most animals. You would die if you receive doses of about 5 Gy (gray). Tardigrades can survive up to 5000 Gy. They become sterile after 1000 Gy, but still. Crazy stuff.
  • As a reference, a victim who was a mile away from the hiroshima bomb explosion received about 9 Gy.

Air deprivation

  • They swell and float around until they’re in a spot where they can breathe again.


  • They can go down to 3% of their normal water content, and shrivelling to a third of their size. This shrivelled tardigrade is called a tun.
  • This is called anhydrobiosis.


  • Some species can go without food for over 30 years. At this point they dry out and become dormant, then they can rehydrate, eat something, and go have babies.
    • 30 years is rather rare, but many species can do this for at least 5 years.
    • This process is called anhydrobiosis (meaning extreme desiccation, with water going down to 3% of their usual rate) and is a form of crytobiosis (a state where metabolic functions stop)
  • They have been found everywhere from the deep sea to the himalayas.
  • They have survived five mass extinction events and have the capacity to survive many more.
  • Note that some people called them extremophiles, but they’re not. Extremophiles are animals that are adapted to live in extreme environments. Tardigrades are just able to tolerate those environments.
Photo courtesy of American Scientist

How to Discover Tardigrades

  • New tardigrades are being discovered all the time. This is how you could discover one and name it after your dog or your favourite brand of mayonnaise.
  • Collect a clump of moss or lichen (dry or wet) and place in a shallow dish, such as a Petri dish.
  • Soak in water (preferably rainwater or distilled water) for 3-24 hours.
  • Remove and discard excess water from the dish.
  • Shake or squeeze the moss/lichen clumps over another transparent dish to collect trapped water.
  • Starting on a low objective lens, examine the water using a stereo microscope.
  • Use a micropipette to transfer tardigrades to a slide, which can be observed with a higher power under a compound microscope.