Tardigrades (also known as water bears or moss piglets) are water-dwelling, segmented micro-animals, with eight legs. They were first described by the German pastor J.A.E. Goeze in 1773. The name Tardigrada (meaning "slow stepper") was given three years later by the Italian biologist Lazzaro Spallanzani.
Usually, tardigrades are about 0.5 mm (0.020 in) long when they are fully grown. They are short and plump with four pairs of legs, each with four to eight claws also known as "disks". The animals are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a very-low-power microscope, making them accessible to students and amateur scientists. Their mouths have sharp pointy objects, called stylets. They use their stylets to cut into moss leaves or algae, their main foods. Then they suck the juices from the plant. Tardigrades live worldwide in moist land habitats, along rocky shorelines, and on the bottoms of streams, lakes, and oceans.
There are more female Water bears than males. The females are also slightly larger. After mating, females lay eggs in a shedexoskeleton. Water bears molt (shed their exoskeletons) just like insects and crayfish. Young Water Bears must molt several times before they are adults. Water bears can be different colors, depending on the species, including gray, bluish, yellowish-brown, reddish, and brown. About 400 species of tardigrades are known and they range in length from 0.1 to 0.5 mm (0.004 to 0.02 in).
The most intriguing aspect of tardigrade biology is their ability to go into anabiosis. When this happens, the tardigrade retracts its legs, loses water, and forms a thick, double-walled cuticle around itself. Its life processes drop to a barely detectable level. In cryptobiosis (an extreme form of anabiosis), the metabolism is not detectable at all. In this phase, which may last for decades, the animal is called a tun. Tuns have survived experimental immersion in liquid helium at -272° C (-458° F), heating to temperatures of 149° C (300° F), exposure to ionizing radiation and toxic chemicals such as ether, and long periods without oxygen. They can withstand pressures about six times stronger than pressures found in the deepest ocean trenches, ionizing radiation at doses hundreds of times higher than the lethal dose for a person, and the vacuum of outer space. They are theoretically capable of surviving a space journey without protection. When life-supporting conditions resume, the tardigrade can return to normal activities within a few hours. (Adapted from Encarta encyclopedia and Wikipedia)