Tardigrades are microscopic invertebrates that have captured the public imagination due to their extreme biological resilience. These tiny, segmented creatures are famous for their ability to survive environmental conditions that would instantly kill nearly every other form of life on Earth. Understanding this unique animal involves exploring its physical dimensions, its global distribution, and the biological mechanism that grants it such extraordinary durability, including its unique method of suspended animation.
Defining the Tardigrade
Tardigrades belong to their own phylum, Tardigrada, which places them as distinct from insects, worms, or other familiar invertebrates. They are commonly known as “water bears” or “moss piglets,” monikers inspired by their plump appearance and their characteristic slow, clumsy gait. The scientific name Tardigrada translates from Latin to “slow walker,” a nod to their deliberate movement.
The body of a tardigrade is segmented, typically consisting of a head and four body segments, each bearing a pair of stout, unjointed legs, resulting in four pairs of legs. Each pair usually ends in four to eight claws or suction disks. Unlike larger animals, tardigrades lack specialized circulatory or respiratory systems, instead relying on the diffusion of oxygen and nutrients through their body fluid (hemolymph) and their outer cuticle.
The Range of Tardigrade Size
Most adult specimens fall within a size range of 0.3 to 0.5 millimeters (300 to 500 micrometers). This common size is roughly equivalent to the width of a standard paperclip wire or the size of the period at the end of this sentence. The average diameter of a human hair, by comparison, is only about 75 micrometers, meaning a typical tardigrade is several times wider than a single strand of hair.
While they are broadly considered micro-animals, the size range across the phylum is wider than the average. Some of the smallest newly hatched juveniles may measure as little as 0.05 millimeters. Conversely, the largest known species can reach an impressive length of up to 1.5 millimeters, making these specific individuals just visible to the naked eye under ideal conditions.
Because of their translucent bodies and diminutive size, viewing them requires magnification, typically at least 20 to 30 times power, which is easily achieved with a low-power dissecting microscope. Their relatively large size and distinct morphology make them favorites for collection and observation by amateur scientists.
Ubiquity: Where Water Bears Thrive
Tardigrades are considered cosmopolitan, meaning they are found on every continent, from the highest mountain peaks to the deepest abyssal trenches in the ocean. This sheer breadth of distribution highlights their adaptability, but the common factor in all their habitats is the presence of liquid water. They are fundamentally aquatic animals that require a film of water around their bodies to remain active, grow, and reproduce.
The most famous and easiest location to find them is in the thin film of water that coats terrestrial environments like mosses and lichens. These spongy materials act as reservoirs, providing the moist habitat that tardigrades need for their active life. They are also frequently collected from leaf litter, soil, freshwater sediments, and marine environments.
The eggs and the durable “tun” state are small and light enough to be carried long distances by wind or water, contributing to their global ubiquity. This passive transport mechanism allows them to colonize new, isolated patches of moist habitat across the planet.
Survival Secrets: The State of Cryptobiosis
The reputation of the tardigrade stems from its ability to enter a state of suspended animation called cryptobiosis, a reversible condition where metabolism is nearly stopped. The most studied form of this survival tactic is anhydrobiosis, which is initiated by desiccation, or the complete loss of environmental water. To survive this extreme drying, the tardigrade retracts its legs and head and curls into a compact, barrel-shaped structure known as a “tun.”
In this dried, shrunken form, the animal loses over 95% of its body water, and its metabolic rate slows to less than 0.01% of its normal level. The tun state is protected internally by unique biological molecules, including CAHS proteins and the sugar trehalose. These protectants stabilize the cellular membranes and internal structures, preventing damage as the water is removed.
This state allows the tardigrade to withstand extremes that are otherwise fatal to active life, including the vacuum of space, high doses of radiation, and temperatures ranging from just above absolute zero (around -272°C) to well over the boiling point of water (150°C). A tardigrade can remain in this desiccated tun state for years or even decades, reviving to an active life within hours of rehydration.