Tardigrades and rotifers – life on the edge
Life in Antarctica is harsh! The organisms that survive in such an extreme environment often have unique adaptations. Some organisms survive by remaining active – some have physiological adaptations like feathers, blubber or ‘antifreeze proteins’ in their blood. Other organisms go dormant by suppressing their metabolism – sometimes for decades – returning to their usual state when conditions improve. Two microscopic creatures that use dormancy as a survival mechanism are tardigrades and rotifers.
Tardigrades and rotifers – translucent survivors
Tardigrade (left) and bdelloid rotifer (right) are tiny invertebrate species that use anhydrobiosis – a physiological adaptation – to survive the extremes of life in Antarctica.
Tardigrade micrograph by Philippe Garcelon, CC BY 2.0, and rotifer micrograph by Rkitco, CC BY-SA 3.0.
Survival strategies – cryptobiosis and anhydrobiosis
Cryptobiosis is a broad term for the dormant state in which an organism’s metabolic activity is reduced to an almost undetectable level in response to adverse environmental conditions. While in this state, the organism does not grow, reproduce or self-repair. Later, the organism resumes normal metabolism and life functions and is unharmed.
Anhydrobiosis is a type of cryptobiosis in which an organism enters a state of extreme dehydration – triggered by desiccation.
Tardigrades and anhydrobiosis
Tardigrades are tiny invertebrates that are well known as hardy survivors. They use anhydrobiosis – a reversable dormant state – to survive extreme conditions.
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Tardigrades
Tardigrades have the reputation as one of the most resilient animals on Earth. They are tiny – about 0.5 mm when fully grown. Tardigrades have eight legs, claws and sharp teeth. Their common name – water bears – comes from their resemblance to actual bears.
Tardigrade in its active state
This scanning electron microscope image shows a tardigrade in its hydrated, active state. Tardigrades have five body sections encased in a sturdy but flexible cuticle. When the tardigrade is in an inactive tun state, it resembles a small oval ball.
Tardigrades are aquatic animals that live just about everywhere – on land and in freshwater and marine environments. On land, they live in the thin water film on the surface of mosses, lichens and leaf litter. They require water around their bodies for both gas exchange and to prevent anhydrobiosis. Some of their more extreme aquatic environments include hot springs, high altitudes such as the Himalayas and at extreme depths in the ocean.
Individual tardigrade species are sometimes limited to a single type of habitat. For example, Acutuncus antarcticus is endemic to Antarctica and offshore islands. This species uses cryptobiosis to survive both freezing and dry conditions.
In extreme conditions, tardigrades enter tun state – they contract their bodies, withdraw their legs and quickly lose water – reducing their body volume by as much as 85–90%! In terrestrial ecosystems, tardigrade enter tun state within 30 minutes. However, marine tidal species can enter this state in as little as a few seconds. When conditions become favourable again, they quickly revive.
In addition to extreme temperatures and desiccation, tardigrade can survive:
short periods of time without oxygen
the pressure of the deep ocean and the vacuum of space
1,000 times more radiation than other animals
impacts of nearly 900 m/sec.
Tardigrades have had several space missions. They were the first animals to survive exposure to outer space in 2007. They’ve travelled on board the Space Shuttle Endeavour and the International Space Station. A few thousand tardigrade even crash landed on the Moon in 2019! Although they are hardy, the Moon lacks liquid water and oxygen, so they are unlikely to be revived.
Bdelloid rotifers
Rotifers are microscopic animals common in freshwater environments all over the world. They are sometimes called wheel animals. The bdelloid species living in Antarctica have two rotating wheels of cilia on their heads. The cilia create currents in the water that direct food into their mouths.
Mount Erebus is a very dry environment. Bdelloid rotifers deal with this by rapidly dehydrating themselves and remaining dormant in this state until there’s enough water for them to rehydrate.
Rotifers are found in the moss beds at Tramway Ridge.
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Rotifers
Rotifers are tiny creatures also found in freshwater and some marine environments throughout the world. They are sometimes called wheel animals – due to wheel-like structures on their heads called corona. The corona have tufts of cilia that resemble rotating wheels – some species have more than one tuft. The cilia create currents in the water that direct food into their mouths.
Bdelloid rotifers are widespread in Antarctica with a high level of endemism. They also use anhydrobiosis as a survival strategy – sometimes remaining dormant for years. Bdelloid rotifers are asexual – reproducing by parthenogenesis – and use anhydrobiosis as a reproductive advantage. Parasites that threaten bdelloid rotifers are destroyed during desiccation, and while desiccated, rotifers can be blown away from their old habitat to one where the parasite may not live.
Extra-tough survival stories
There is debate whether tardigrades and rotifers should be called extremophiles because they survive rather than thrive in hostile conditions. Regardless, they have some incredible survival stories.
An Antarctic moss sample was gathered in 1983 and stored at -20°C after collection. Just over 30 years later, the sample was thawed and then soaked in water. Two Acutuncus antarcticus tardigrade in the moss were revived – and went on to lay eggs and reproduce.
Bdelloid rotifers have an even more incredible story. A Siberian permafrost sample, carbon dated to around 24,000 years old, was thawed and placed in a Petri dish. Once defrosted and rehydrated, researchers discovered rotifers in the Adineta genus. The ancient rotifers not only recovered but were able to reproduce.
Related content
These resources have additional information about life in Antarctica:
Activity idea
In the Animal and plant adaptations activity, students learn about animal and plant adaptations in Antarctic species and use these ideas to design their own unique animal or plant.
Useful links
The Science of Sea-Monkeys | Survival in extreme environments is a teaching activity on the New Zealand Association of Science Educators website. It uses brine shrimp (sea monkeys) to observe and explore cryptobiosis.
New species of tardigrades were discovered in glaciers in the Southern Alps in 2022.
This Live Science article has additional information about tardigrades.
This American Scientist article delves more deeply into tardigrade survival mechanisms, including diagrams and other helpful media.
There are thousands of species of rotifers. This Evident article has information on their general biology.
Watch Journey to the Microcosmos’s YouTube video, which delves more deeply into rotifer biology and reproduction.
