Why these micro-organisms are the real superheroes of our planet?
If you have seen the movie Ant-Man and the Wasp (send me some royalties for the mention Marvel) you would have seen Hank Pim shrinking down to a micro size before entering the quantum realm. At this micro size, he ran into some pretty peculiar pint-size creatures. They had eight legs with gnarly claws like Freddy Kruger. They had a monstrous mouth equipped with an extendable sucking pharynx popping out like a chest-buster in Alien. And they slowly but assuredly approached Pim’s vessel like he was the final girl in a slasher movie. But they were also somehow pretty cute, with their chubby limbs pottering through their medium, like if Winnie the Pooh and a Demigorgan had a love child. Despite their appearance in a superhero flick, I would argue that these minibeasts are much more deserving of the superhero moniker. So, I am going to describe and decipher the life of these Tardigrades and then explain exactly how deserving of an Avenger call up these creatures are.
Tardigrada is a phylum of very small bilaterally symmetrical invertebrates in the same grouping of Ecdysozoa as insects, arachnids, crustaceans and nematodes, among other inverts that routinely perform ecdysis (shedding of the exoskeleton in order to grow). There are about 1,300 known species and grow to an average size of 0.5 to 1mm in size. That is about the size of a full stop, so not entirely invisible to the naked eye but you definitely need a microscope to see the finer details of these animals.
A tardigrade body is divided into five segments: a well-defined head and four body segments each with a pair of legs. This body is encased in a rugged cuticle that must be shed as it grows. The hindmost legs are attached backwards, just to make this animal even more peculiar. These four pairs of lobopod short stubby legs are all equipped with ferocious-looking claws and sucking discs. These claws are designed to grasp onto tiny plants as their stocky legs propel them through their aquatic environment. Tardigrade in Latin translates to “slow-walking”, a very apt name for these pottering peculiarities.
They have a sucking pharynx and a tubular mouth equipped with piercing stylets. These stylets pierce the cells of algae, lichens and moss and allow the sucking out of the internal fluids. Some Tardigrades also eat other animal microbes, and some have even adapted a cannibalistic nature. Despite their diminutive stature, these tiny animals have very similar physiology to that of larger animals, as they possess an alimentary canal, digestive system, well-developed muscles and a dorsal brain atop a paired ventral nervous system. They have an open hemocoel-type fluid-filled body cavity that touches every cell in the body allowing for efficient nutrition and gas exchange. This means that there is no need for a circulatory or respiratory system.
Tardigrades have been given the nickname ‘water bears’, probably because of their Winnie-Gorgan origins (Demi-pooh doesn’t work as well). But also because of their thick limbed appearance and their aquatic lifestyle. They are found in nearly every aquatic habitat, filling a large range of niches in marine, freshwater and terrestrial habitats. They are usually benthic, crawling through the substrate or on plants on the floor of their environment. Terrestrial Tardigrades are limnoterrestrial, meaning that they are aquatic animals living within a film of water in a terrestrial habitat, this includes living in moss, lichen, leaf litter and soil. In fact, up to 2 million tardigrades can be found on a one metre square area of moss. They also live in some of the Earth’s harshest regions, places where superheroes might go to train for the epic end of movie battle. These habitats include:
- In the highest mountains, such as 5546m high in the Himalayas
- In hot springs
- Temporary ponds that dry up seasonally
- At the bottom of the ocean, for example, ?4,690 m in the Indian Ocean
- In freezing waters under ice, such as in benthic algal mats in maritime Antarctic lakes
- Cryoconite holes in glaciers (water-filled depressions on the surface of a glacier)
Because they live in such rapidly changing and/or extreme habitats, it has led to incredible survival adaptations and developing a Superman level resilience. The most famous case of the Tardigrades toughness is in 2007 when the European Space Agency carried out the TARDIS project. Thousands of Tardigrades were attached to a satellite and sent straight into space, for a period of 12 days. When they made their eventual return to Earth, not only had a lot of them survived but a lot of females had even laid eggs, with these young then successfully hatching and surviving. Being able to survive these conditions is unthinkable for all other life on Earth, let alone being able to reproduce.
The superbeing capabilities of Tardigrades to survive have been rigorously tested in a laboratory setting to see just how durable these micro-metazoans are. They have been subjected to temperatures as low as 0.005 Kelvins, functional absolute zero (-272.95°C) for a period of 20 hours. After this time, they were warmed and rehydrated, and they managed to come back to life. To add to their resilience to extreme cold, Tardigrades have been stored at -200°C for a period of 20 months, managing to survive. After being exposed to 150°C of extreme heat, more than 40,000 kilopascals of pressure and suffocating gases such as CO2and SO2, still managing to survive. So, what allows these tiny creatures to be so extremely tough? Mutated genes in a military lab experiment (Deadpool comes to mind here)? Or do they come from another planet where they are subjected to extremes unfathomable to human scientists’ imaginations?
In the case of studied terrestrial species of Tardigrades, they have several states of being. They can be in the active state, when Tardigrades eat, grow, fight, reproduce and every other normal routine of their life. This state is seen when conditions for these creatures are perfect and a layer of water is present. All the other states can be classified as cryptobiosis. This is the suspension of metabolism in response to extreme conditions.
- Anoxybiosis: this state occurs in situations of low oxygen levels. Tardigrades are very sensitive to O2content in their environment, during prolonged periods of asphyxia osmoregulatory controls that regulate body water fail. In this case, Tardigrades will puff up and float around as an expanded version of itself until a suitable oxygen level returns to the environment.
- Anhydrobiosis: this is a state of extreme dehydration where metabolic activity is paralyzed as there is no liquid water present. When living on moss and lichen, these sponge-like habitats can dry out very slowly. As the surroundings desiccate, so do the tardigrades with them. They lose between 97-99% of their body water content, shrivelling to about a third of their normal size. This is known as a tun, and in this state, the animal can survive almost anything, apart from an infinity stone enhanced Thanos snap of the fingers of course.
- Cryobiosis: this state allows Tardigrades to live in freezing conditions. Usually, the water in the cells of organisms crystallizes and breaks the structures of the cells killing the creature. Tardigrades use proteins to avoid this cell breakage.
- Osmobiosis: this state comes into play in cases of salinity changes. They transform into their tun state until conditions are more favourable.
Through these several states of resistance, Tardigrades can survive desiccation, high and low temperatures, low oxygen, high salinity, high pressure and exposure to the stresses and radiation of outer space! Even Wolverine’s remarkable recovery rate could not compete with that of these micro-sized monsters. Tardigrades may be a miniature mash-up of lots of different attributes seen in horror and hero movies but I think it is pretty clear that the next multi-million dollar Hollywood blockbuster should be centred around these full stop sized super beast.
Diane R. Nelson, Current Status of the Tardigrada: Evolution and Ecology, Integrative and Comparative Biology, Volume 42, Issue 3, July 2002, Pages 652–659. https://academic.oup.com/icb/article/42/3/652/724023
American Scientist https://www.americanscientist.org/article/tardigrades
Persson, D., et al. 2011. Extreme stress tolerance in tardigrades: Surviving space conditions in low earth orbit. Journal of Zoological Systematics and Evolutionary Research 49(suppl 1):90–97 https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1439-0469.2010.00605.x
Tardigrades: Animals with Superpowers, All You Need Is Biology. https://allyouneedisbiology.wordpress.com/tag/anoxybiosis/