Tardigrades, often called water bears or moss piglets, are microscopic animals recognized for their resilience. These eight-legged invertebrates inhabit diverse environments, from the deepest oceans to the highest mountains. Their fame stems from an ability to withstand conditions lethal to nearly any other life form. This survival mechanism complicates the question of their lifespan, which is divided between active life and suspended animation.
The Active State Lifespan
When conditions are favorable, tardigrades exist in an active state. During this time, they are hydrated, feeding on plant cells or smaller invertebrates, growing, and reproducing. In this metabolically active phase, a tardigrade will live for a period ranging from a few months to two years, depending on the species. This active period represents the tardigrade’s natural lifespan, and for aquatic species with a limited ability to enter suspended animation, their total lifespan mirrors this duration.
Survival Through Suspended Animation
The tardigrade’s defining feature is its capacity for cryptobiosis, a state of suspended animation it enters to survive lethal environmental changes. When faced with conditions like extreme dryness or low oxygen, it retracts its legs, curls into a desiccated, motionless ball called a “tun,” and reduces its water content to as little as one percent of normal.
In this tun state, its metabolism slows to less than 0.01% of its normal rate, pausing all biological processes, including aging. This allows the tardigrade to endure a wide array of extreme conditions, including the vacuum of space, temperatures near absolute zero (-273.16°C), and extreme heat and pressure.
This adaptation is a survival mechanism, not a form of active living, allowing the tardigrade to persist until its environment becomes hospitable again. Once exposed to water, a tardigrade can rehydrate and return to its active state, often within minutes or hours.
Total Lifespan and The Limits of Immortality
A tardigrade’s total potential lifespan is the sum of its time in the active state plus the duration of all periods in cryptobiosis. Experiments have demonstrated this longevity; tardigrades were reanimated from dried moss stored in a museum for around 100 years, and another study revived specimens after thirty years in cryptobiosis.
This has led to the misconception that tardigrades are biologically immortal, but they are not. They age and die during their active periods. The time spent in cryptobiosis stops their biological clock, a phenomenon called the “Sleeping Beauty” model, but it does not reverse aging that has already occurred. When a tardigrade emerges from the tun state, its life resumes from the point it was paused.
Studies show that tardigrades entering cryptobiosis can live much longer overall than those that remain continuously active. Their lifespan isn’t endless, but it can be stretched across decades or potentially longer, spent waiting for favorable conditions to return.