The tardigrade, often called a water bear or moss piglet, is a microscopic invertebrate belonging to the phylum Tardigrada. These tiny relatives of arthropods are famous for their ability to endure environmental conditions that would be fatal to nearly all other known life forms.
Physical Characteristics and Habitat
Water bears are microscopic, typically ranging from 0.05 to 1.2 millimeters long. They have plump, segmented bodies with a distinct head and four pairs of stubby legs. Each leg is tipped with four to eight claws, which they use to move in a slow gait reminiscent of a bear. Their bodies are composed of around 1,000 cells and have a fluid-filled cavity that functions like blood.
Tardigrades inhabit environments across the globe, from mountaintops to the deep sea. They are considered aquatic because they require a film of water around their bodies for gas exchange. They are most commonly found in the water on mosses and lichens, but also live in soil, sand dunes, and leaf litter—anywhere with intermittent moisture.
Extreme Survival Capabilities
Water bears can withstand a range of environmental extremes, including the vacuum of outer space. Their temperature tolerance is vast; they have been revived after exposure to temperatures just above absolute zero (–272.95 degrees Celsius) and have endured heat up to 150 degrees Celsius. Tardigrades can also withstand radiation levels hundreds of times greater than the dose that would kill a human and survive the immense pressures of the deep ocean.
The Process of Cryptobiosis
Water bears survive extreme conditions through cryptobiosis, a state of suspended animation where metabolic activity nearly stops. When facing environmental stress like dehydration, a tardigrade retracts its legs and curls into a desiccated ball called a “tun.” It expels up to 97% of its body’s water, and its metabolism can slow to less than 0.01% of its normal rate.
During cryptobiosis, special molecules protect the tardigrade’s cells. It produces unique proteins that form a protective, gel-like network as cells lose water. This is supplemented by a sugar, trehalose, which replaces water and protects cell membranes from damage. When water becomes available again, the tun rehydrates, and the animal returns to its active state.