Tardigrades, often called “water bears” or “moss piglets,” are microscopic invertebrates known for their remarkable resilience. These tiny creatures, less than a millimeter in length, are found across the globe. Their ability to survive in a wide array of environments, from common damp spots to Earth’s most challenging conditions, fascinates scientists.
Ubiquitous Terrestrial and Aquatic Habitats
Tardigrades are widely distributed across terrestrial environments, frequently residing in the thin film of water that surrounds mosses, lichens, and leaf litter. These microhabitats provide moisture for tardigrades to remain active, as they require water around their bodies for gas exchange and to prevent drying. They are also found in soil and sand dunes.
Beyond land, tardigrades inhabit aquatic environments. Numerous species thrive in freshwater systems such as lakes and rivers, often found in bottom sediment. Approximately 150 marine species have been documented, living in coastal sediments and among seaweeds. Tardigrades have even been discovered in subterranean caves and groundwater.
Thriving in Extreme Environments
Tardigrades can endure conditions lethal to most other life forms. In arid deserts, they survive prolonged desiccation by entering a dormant state. They can withstand temperatures as low as -200°C (-328°F) and as high as 151°C (304°F), allowing them to exist in frozen polar regions and hot springs.
Their tolerance extends to extreme pressures, surviving conditions up to six times the pressure found in the deepest parts of the ocean, enabling their presence in deep-sea sediments. Tardigrades also survive in the vacuum of outer space, enduring extreme cold and radiation levels thousands of times greater than a human could withstand. They have even survived exposure to boiling alcohol and certain noxious chemicals.
Biological Adaptations for Habitat Versatility
The ability of tardigrades to survive diverse and challenging habitats stems from cryptobiosis, a reversible ametabolic state. This process allows their metabolic activities to come to a standstill, effectively pausing life functions until favorable conditions return. Several forms of cryptobiosis exist, each triggered by specific environmental stressors.
Anhydrobiosis, initiated by a lack of water, is a form where tardigrades curl into a compact shape called a “tun.” During this state, their metabolism can drop to less than 0.01% of normal, and they can lose almost all body water. This process involves the synthesis of protective sugars like trehalose and specialized proteins such as CAHS D, which replace lost water within cells and protect cellular structures.
Other forms of cryptobiosis include cryobiosis, which enables survival through freezing temperatures. Osmobiosis is triggered by increased solute concentrations, such as high salinity, causing them to contract into a tun to resist osmotic stress. Anoxybiosis allows them to endure environments with a lack of oxygen.