Tardigrades are also called moss piglets and water bears

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Anatomy: parts of the body and how they fit and work together......more

Gut: the intestines and stomach....more

Organism: a living thing that can be small like bacteria or large like an elephant.

Oxygen: an element that makes up about 21% of air and that many organisms need to live.

What Makes a Tardigrade?

Under a microscope, a drop of water collected from moss or leaves on the forest floor might look like an alien world. You might think, “how can I find a tardigrade in this soup of floating, spinning, and wriggling critters?” Luckily, tardigrades are one-of-a-kind. If you can spot a few important features, you can find your own tardigrades too!

tardigrade on the left rotifer on the right

Tardigrades (left) can look a lot like rotifers (right) if you're not paying attention. However, rotifers scoot along more like inch-worms. Also, all tardigrades have eight legs. The two back ones on this one are hard to see. Image of tardigrade by Dmitry Brant, image of rotifer by Specious Reasons.

Tardigrades on the Prowl

aphid piercing leaf

Tardigrades aren't the only animal with stylets. This aphid uses a stylet in its straw-shaped mouth to pierce into leaves and drink sap. Image by Caramosca.

One trait all tardigrades share is their eight stubby legs. Tardigrades have three legs on each side of the body, and two on the back. The legs often have long, bear-like claws on them. You’d think with eight legs, tardigrades should move pretty fast, but they don’t. Instead, they are slow and clumsy. They hook their claws onto objects like plants or sand, as they move along.

Though they might look silly through a microscope, you would not be laughing if you were tinier than a tardigrade. Tardigrades have round mouths with muscles in their heads that work like cheeks to suck up food. Instead of teeth, they have two terrifying spears. They use these spears, called stylets, to pierce their food and drink the insides like juice. Some tardigrades are plant eaters, but others eat smaller creatures and single-celled organisms. Some even eat other tardigrades.

Tardigrade Anatomy: Same But Different

If you watch a tardigrade wobble around on its little clawed legs and poke things with its pointy face, it might remind you of a tiny hamster. If you looked inside its body, you’d notice one big difference: no tiny bones. Instead, tardigrades have a special “skeleton” that works like a rubber glove filled with water. The liquid that fills tardigrades is called hemolymph, which is full of nutrients, much like our blood.

tardigrade anatomy muscles digestive system nervous system brain

A Tardigrade's anatomy is a lot like ours - they have muscles, brains, eyes, and stomachs. Image by Ioulia Bespalova. Click for more detail.

CuticleA strong but flexible outer covering, similar to skin.
ClawA sharp, curved, pointed structure on a finger, toe, or foot.
Claw GlandA group of cells that release material to grow claws.
MuscleTardigrades have muscles that attach to their cuticle (they don't have bones). The body muscles move the tardigrade’s legs and body around.
MouthAn opening in the face where food is taken into the body.
StyletA sharp mouth structure that pierces plant and body tissue so the contents can be sucked out.
Buccal TubeA tube that connects the mouth to the pharynx.
PharynxA muscular organ that works with the buccal tube to help create suction for feeding.
Salivary GlandA group of cells that can create different chemicals and fluids for the mouth. Some scientists think the salivary glands release materials to grow the stylets.
GutThe place where most food digestion happens, connected to the pharynx.
Malpighian TubuleWhere scientists think tardigrade pee is made. These three tubes connect to the gut and remove water, chemicals, and waste from the body fluid - a lot like our kidneys do
BrainA major organ in the nervous system, responsible for controlling many body functions.
Simple EyeA small eye that is made of just a few cells. The cells include sensory cells and a “pigment cup” which has dark-colored granules in it that work like sunglasses. The simple eye is used to sense light.
Nerve CordA major bundle of nerve fibers that connects nerves throughout the body. It controls movement and other bodily functions.
Ovary/Testis/ OvotestisThe female reproductive organ is the ovary and it makes eggs. The male reproductive organ is the testis, and it makes sperm. Some kinds of tardigrades are both male and female at the same time (hermaphroditic), because they have an ovotestis, which makes both sperm and eggs.
EggsFemale reproductive cells. In asexual tardigrades, these develop into baby tardigrades. In sexual tardigrades, they develop after being fertilized by sperm.
SpermathecaAn organ that stores sperm in the female body after mating.
CloacaThe opening through which waste from the gut is removed from the body, and through which sperm and eggs can enter and exit.

Many tardigrades have long hairs on their bodies that sense the environment, like a cat’s whiskers. They also have simple eyes that notice if it’s light or dark out, but they can’t see color. Just like us, they have guts that hold and digest food, muscles for moving, and brains for thinking. Unlike us, they don’t have lungs, and instead soak up oxygen straight from water.

Additional images via Wikimedia Commons and Flickr. Bear image via Flickr by Tom Pickering.

View Citation

You may need to edit author's name to meet the style formats, which are in most cases "Last name, First name."

Bibliographic details:

  • Article: What Makes a Tardigrade?
  • Author(s): Ioulia Bespalova
  • Publisher: Arizona State University School of Life Sciences Ask A Biologist
  • Site name: ASU - Ask A Biologist
  • Date published: February 18, 2019
  • Date accessed: July 15, 2024
  • Link: https://askabiologist.asu.edu/tardigrade-anatomy

APA Style

Ioulia Bespalova. (2019, February 18). What Makes a Tardigrade?. ASU - Ask A Biologist. Retrieved July 15, 2024 from https://askabiologist.asu.edu/tardigrade-anatomy

American Psychological Association. For more info, see http://owl.english.purdue.edu/owl/resource/560/10/

Chicago Manual of Style

Ioulia Bespalova. "What Makes a Tardigrade?". ASU - Ask A Biologist. 18 February, 2019. https://askabiologist.asu.edu/tardigrade-anatomy

MLA 2017 Style

Ioulia Bespalova. "What Makes a Tardigrade?". ASU - Ask A Biologist. 18 Feb 2019. ASU - Ask A Biologist, Web. 15 Jul 2024. https://askabiologist.asu.edu/tardigrade-anatomy

Modern Language Association, 7th Ed. For more info, see http://owl.english.purdue.edu/owl/resource/747/08/
Bored Bear

Tardigrades are sometimes called water bears because of their claws, bulky body, and snout-like mouth. What do you think, do tardigrades look like this bear to you?

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