Tardigrades are micro-animals also called water bears or moss piglets, often found in water drops on moss. Known for resilience, they can also happily survive in extreme environments deadly for most life forms.
These tiny invertebrates are beloved by biologists and science students for their easy availability and ability to return to a state of life after being fully dehydrated and rehydrated. It's only one of their talents.
Tardigrades are ancient creatures with history dating back over 500 million years, to the Cambrian period. They predate the dinosaurs and survive environmental upheaval and multiple mass extinctions.
Cosmopolitan dwellers, tardigrades are found almost everywhere on Earth. They exist in icy peaks and deep ocean trenches, tropical rainforests and suburban back yards.
They're most common in moist environments like moss, lichen, soil and leaf litter, where they can easily access water and nutrients. They're often present in the moisture film on plant leaves.
The plump, segmented body of a tardigrade is usually less than a millimeter long. The animals belong to phylum Tardigrada, or "slow stepper," describing their characteristic lumbering gaits.
Under a microscope, they resemble eight-legged bears with little claws. Named Kleiner Wasserbär (Little Water Bear) by their discoverer, they have an innate charm contributing to their prevalence in popular culture.
Their body structure features a tough cuticle to protect them like armor. When the going gets tough, Tardigrada enter a state of cryptobiosis. They essentially pause their metabolism.
Basic Biology and Life Cycle
A tardigrade has a simple body plan. Its a mouth is equipped with piercing stylets used to suck fluids from plant cells, algae, or even small invertebrates. Its digestive system consists of intestine and anus.
It breathes through its skin. Tardigrades lack circulatory or respiratory systems. Reproduction in tardigrades can be sexual or asexual, depending on the species and environmental conditions.
The life cycle of tardigrades consists of four stages: egg, larva, juvenile, and adult. Females lay eggs, which are fertilized or develop without fertilization.
The eggs are deposited in a protective casing, shielding them from environmental stresses while they develop. On hatching, the larvae look like miniature adults and molt several times before reaching maturity.
Reproductive methods vary among species. While many use sexual reproduction, some can reproduce asexually through parthenogenesis, allowing them to thrive even in isolation.
The life cycle of a tardigrade ranges from a few months to over a year.
Secrets to Survival: Tun State
Tardigrades enter a state of suspended animation called the "tun" state. When faced with conditions like dehydration, extreme temperatures, radiation, or lack of oxygen, the tardigrade retracts its head and legs.
It shrivels up into a tiny, dehydrated ball, and reduces its metabolic activity to almost zero.
In this state, tardigrades can withstand:
Extreme Temperatures: From near absolute zero (-273°C) to over 150°C (302°F).
Extreme Pressure: Over 6 times the pressure found in the deepest ocean trenches.
Radiation: Hundreds of times the lethal dose for humans.
Dehydration: Years without water.
Vacuum of Space: Exposure to the harsh conditions of outer space.
During the European Space Agency’s FOTON-M3 mission in 2007, tardigrades are exposed to the stresses of outer space for ten days and most return unharmed. They soon produce viable embryos.
They can also repair their own DNA after radiation damage. Exact mechanisms behind their survival abilities are largely unknown.
They include proteins to protect DNA and cell structures. Synthesis of trehalose, a sugar which prevents crystallization during dehydration, is also considered a factor.
Tardigrades in Popular Culture
The impressive resilience of tardigrades has made them a popular subject in science fiction and popular culture. They appear across media, including documentaries, cartoons and video games.
Ant-Man: In the Marvel Cinematic Universe, Ant-Man uses a size-altering disc to grow a tardigrade to giant size, which he then rides.
Star Trek: Discovery: The series features a tardigrade as a key component of a new starship propulsion system.
"Adventure Time" features a character named Tardigrade, who embodies the creature’s tenacity and adaptability in the broad scope of overcoming life's challenges.
Astrobiology
Scientists enthuse about tardigrades in astrobiology. The resilience of tardigrades tweaks hope for the potential of life beyond Earth.
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