Water Bears: Tardigrada

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WATER BEARS: Tardigrada



Water bears are mostly microscopic, measuring 0.00787 to 0.0472 inches (0.2 to 1.2 millimeters) in length. They have bilateral symmetry (bye-LAT-er-uhl SIH-muh-tree) and can only be divided into similar halves along one plane. The outside of the body, or cuticle (KYU-tih-kuhl), may have platelike scales, spines, and other appendages. Most species are whitish or clear, but some land-dwelling, or terrestrial (te-REH-stree-uhl), species are yellow, orange, green, red, or greenish black.

Their bodies are made up of five indistinct segments, including the head and four trunk segments. Each trunk segment bears a pair of stumpy, segmented legs. The first three pairs are used for walking, while the last pair is used for clinging. Terrestrial and freshwater species have legs with fewer segments that end in two or four claws. Most marine water bears have telescoping legs that can be withdrawn inside their bodies. Depending on the species, these legs have up to 13 claws or four toelike structures with varying numbers of claws. In some species, the toes are tipped with suction cups or sticky, rod-shaped discs, like the feet of a gecko.

Water bears feed through a mouth with bristlelike jaws called stylets (STAI-lehts). The stylets are used to puncture cell walls so fluids are sucked into the mouth. Because they are small and live in moist or wet environments, water bears do not have a circulatory or respiratory system. They breathe directly through the body wall. Some species have kidneylike organs that remove waste and regulate body salts, while others rely on special glands located at the bases of the legs.

Both young and adult water bears shed the cuticle, or molt, just like insects, spiders, and other arthropods. The mouthparts and surrounding structures, as well as the toes and claws, are produced by special glands after the new cuticle has formed.


Water bears are found on all continents and in all oceans.


Water bears require water and are found in a wide variety of marine, freshwater, and terrestrial habitats. Some species live in hot, radioactive springs, while other live in ice caves formed by glaciers and other sheets of ice. Many marine species are found along beaches. Those living on land are found in primitive plants and plantlike organisms growing on rocks, logs, and soil, such as mosses, liverworts, and lichens.


Water bears eat plants, microscopic animals, and bacteria. Terrestrial species either suck juices from mosses and lichens or eat bacteria growing on these organisms. Some marine species are parasites (PAIR-uh-sites) and live and feed on other animals, such as sea cucumbers and barnacles. A few species rely on bacteria living inside special organs in their heads to provide nutrition in the form of proteins and sugars.


Although many species of water bears are slow and lumbering like bears, species that prey on other microscopic animals are fairly quick and can move faster than the human eye can follow.

Water bears can survive extreme conditions for months, even years, by shutting down all life processes, a phenomenon known as cryptobiosis (KRIP-toe-bye-OH-sihs). Cryptobiosis is triggered mainly by lack of water, or dehydration (dih-high-DRAY-shun), and very low temperatures. Water bears in cryptobiosis as a result of dehydration are said to be in the tun (tuhn) stage.

Most water bears require males and females to reproduce. Some males place sperm directly into special sperm-storing organs on the female's body. Some females have special structures that are inserted into the male, allowing them to grab sperm inside the male's body. A few species are parthenogenic (PAR-thih-no-JEH-nik) and capable of producing young without mating. In some species, individuals have reproductive organs of both males and females. Most water bears are thought to lay eggs. Young water bears resemble the adults, but may have fewer claws and other structures.


The cryptobiotic abilities of water bears have long been of interest to scientists. Several species in the tun stage have been exposed to cosmic radiation, vacuum, and temperatures close to absolute zero and have survived, clearing the way to use them as experimental animals in space. Scientists are also analyzing the proteins and sugars produced by water bears during the tun stage to protect their delicate tissues under extreme conditions. These and other studies may help to explain how life began and developed on Earth.


Water bears are the only animals capable of stopping all life functions and still be alive. They survive harsh conditions in the barrel-like tun stage. Tun is another name for a wine barrel or cask. Water bear tuns can be picked up by winds and carried hundred of miles. Tuns have been revived from moss specimens collected more than 100 years ago. They can survive 20 days at temperatures of -328°F (-200°C).


No water bears are considered endangered or threatened.


Physical characteristics: This species is a relatively large (up to 0.039 inches or 1 millimeter) water bear. They are usually yellow to orange with large black eyes. Each leg has two equally sized claws.

Geographic range: This species is found in the Arctic, Sweden, Turkey, Nepal, and Colombia.

(Specific distribution map not available.)

Habitat: Giant yellow water bears live on mosses in high mountain habitats up to 18,300 feet (5,600 meters) and in Arctic habitats.

Diet: This species is believed to suck fluids from the cells of mosses.

Behavior and reproduction: Giant yellow water bears can survive severe dehydration for up to 9 years. They are also capable of tolerating temperatures down to -320°F (-196°C) whether they are in the tun stage or not.

This species requires males and females to reproduce, but can also switch to parthenogenesis.

Giant yellow water bears and people: This species has possible uses as an experimental animal in outer space. They are capable of surviving high temperatures, vacuum, and cosmic radiation.

Conservation status: Giant yellow water bears are not considered endangered or threatened. ∎



Kinchin, Ian M. The Biology of Tardigrades. London: Portland, 1994.


Romano, F.A. "On Water Bears." Florida Entomologist 86 (2003): 134-137.

Wright, J. C., P. Westh, and H. Ramløv. "Cryptobiosis in Tardigrada." Biological Reviews of the Cambridge Philosophical Society 67 (1992): 1-29.

Web sites:

Hunting for Water Bears in the Backyard.http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artjun00/mmbearp.html (accessed on January 19, 2005).

The Incredible Water Bear. http://www.microscopy-uk.org.uk/mag/index-mag.html?http://www.microscopy-uk.org.uk/mag/artjun00/mmbearp.html (accessed on January 19, 2005).

Water Bears-Tardigrades.http://www.tardigrades.com (accessed on January 19, 2005).