The microscopic animals that survive outer space
Tardigrades, or water bears, can survive the vacuum of space, extreme radiation, and temperatures near absolute zero. They do this by entering a state called cryptobiosis, where they expel almost all water from their bodies and suspend their metabolism. They can remain in this dormant state for decades, reviving when rehydrated.
The Discovery and Anatomy of the Slow Stepper
In 1773, German zoologist Johann August Ephraim Goeze first described a peculiar, microscopic animal he called the 'little water bear.' A few years later, Italian biologist Lazzaro Spallanzani gave the group the name Tardigrada, meaning 'slow steppers,' describing their clumsy, lumbering gait. Despite their miniature scale—most adults measure between 0.3 and 0.5 millimeters in length, barely visible to the naked eye—tardigrades possess a surprisingly complex anatomical blueprint. They are multicellular, bilaterally symmetrical animals equipped with a complete digestive tract, a brain with interconnected nerve cords, and sensory bristles.
A typical tardigrade possesses a barrel-shaped body divided into a head and four distinct trunk segments. Each segment bears a pair of unjointed legs terminating in tiny claws or suction discs, which they use to cling to surfaces and navigate wet environments. As members of the superphylum Ecdysozoa, they share an evolutionary lineage with arthropods and nematodes. Like insects and crabs, tardigrades grow by periodically shedding their tough, protective outer cuticle. Many species are also eutelic, meaning individuals of the same species are composed of a fixed number of cells once they reach adulthood, with further growth occurring purely through cell enlargement rather than cell division.
Entering Cryptobiosis: The Tun State
The secret to the tardigrade's legendary resilience lies in a survival strategy known as cryptobiosis. Under normal conditions, tardigrades are delicate aquatic organisms that require a surrounding film of water to move, feed on plant cells or micro-invertebrates, and breathe through simple gas exchange across their bodies. However, when environmental moisture vanishes, the animal initiates a profound physiological transformation, contracting its limbs, expelling up to 97 percent of the water from its body, and curling into a tiny, shriveled husk known as a 'tun.'