Tardigrades, commonly known as water bears or moss piglets, are microscopic, eight-legged invertebrates known for their extraordinary resilience. Ranging in size from 0.05 mm to 1.2 mm, these plump, segmented animals are found in nearly every corner of the planet, from the highest mountains to the deepest oceans. Their global ubiquity is a direct consequence of their absolute dependence on moisture. Habitats are defined less by geography and more by the physical presence of water.
The Microscopic World of Water Films
An active tardigrade is fundamentally an aquatic animal that requires a thin film of water surrounding its body for life processes. Since they lack specialized respiratory and circulatory organs, gases like oxygen must diffuse directly across the animal’s cuticle, a process that only efficiently occurs in a hydrated state. This microscopic water layer serves as their entire environment and facilitates movement.
This reliance dictates that even in terrestrial locations, tardigrades are confined to interstitial spaces between soil particles or plant crevices where moisture is trapped by surface tension. When this thin film of water evaporates, the tardigrade cannot move, respire, or feed, forcing it into a dormant state. The difference between a thriving habitat and a life-threatening drought is often a matter of millimeters.
Primary Terrestrial Locations: Moss and Lichen
The most common habitats for tardigrades are terrestrial plant communities like mosses and lichens, which are often found growing on rocks, tree bark, or rooftops. These bryophytes and symbiotic organisms are structurally ideal because they act like natural sponges, efficiently holding and retaining moisture for extended periods. The dense, porous structure creates a stable, humid microclimate that buffers the tardigrades against rapid desiccation.
Within these damp environments, tardigrades thrive by feeding on the fluids of plant cells, algae, bacteria, and sometimes smaller invertebrates like nematodes. A single square meter of damp moss or leaf litter can host incredible population densities, sometimes exceeding two million tardigrades.
Aquatic and Global Extremes
Many species of tardigrades live in permanently aquatic environments, including freshwater sediments and marine ecosystems. Approximately 150 species are marine, living in the sand, mud, or on seaweeds at various depths, including the deep sea. These aquatic species generally exist in stable environments and have not evolved the extreme survival mechanisms of their terrestrial counterparts.
The tardigrades’ reputation for toughness comes from their ability to survive in places where they cannot actively live, due to a process called cryptobiosis. By entering a dried-out, suspended animation state known as a “tun,” they can be found in extreme environments like deserts, volcanic mud, and high-altitude mountain ice.
In this desiccated form, their metabolism slows to less than 0.01% of normal, allowing them to withstand conditions such as the vacuum of space, intense radiation, and temperatures ranging from near absolute zero to above the boiling point of water. This dormant state allows them to persist globally until rehydrated, broadening their potential habitat to almost anywhere on Earth.