The tardigrade, better known as a water bear or moss piglet, is the real star in the race of survival. This eight-legged, water-dwelling animal, which is a closer relative to nematodes (roundworms) than they are to their look-alikes, the arthropods (insects, myriapods, arachnids and crustaceans), has gone unseen for a significant part of our existence. Astonishingly, there are over 1200 different species of tardigrades, and it is believed that less than 10% of the species have been identified.
How big are tardigrades?
The biggest adults reach a body length of only around 1.5 mm, while the smallest is less than 0.1 mm. Newly hatched tardigrades may be even smaller than 0.05 mm. You would need microscopic vision to see these impeccable wonders of nature. Tardigrades are nearly translucent and they average about half a millimeter (500 micrometers) in length, about the size of the period at the end of this sentence. In the right light you can actually see them with the naked eye. But researchers who work with tardigrades see them as they appear through a dissecting microscope of 20- to 30-power magnification—as charismatic miniature animals.
What do tardigrades eat?
They feed on the fluids of plant and animal cells, piercing their cell walls and membranes, with the help of primitive piercing mouthparts, called stylets. Some species of water bears are known to engulf entire live organisms, such as rotifers. Tardigrades even eat other tardigrades! Yes, cannibalism is very much alive and well among certain species of tardigrades. When they have enough food and water to support their bodily functions, they live out the natural course of their lives. They are probably the most paradoxical animals on this planet. These organisms are impossibly cute, like tiny blimps bumbling around harmlessly on their stubby little legs, but they also enjoy a legendary reputation as the toughest and most indestructible creatures on Earth.
Tiny but Robust
Tardigrades have been reported to survive the piping hot sulphur springs amidst volcanic mountains; they have survived atop the Himalayas, beneath layers and layers of thick ice, remaining unscathed by the immense pressures and exceedingly low temperatures. Tardigrades are fully capable of surviving complete global mass extinctions caused by astrophysical events, such as gamma-ray bursts and giant meteorite impacts.
Tardigrades and Ant-Man
Pop culture seems to love tardigrades. And with good reason! Tardigrades have been a part of several sci-fi movies, shows, and documentaries. Science fiction loves good space monsters that are biologically dominant to their heroic counterparts. Tardigrades are no stranger to this culture; they have been featured in various movies like Star Trek: Discovery, Ant-Man, and Harbinger Down. However, was it just their terrifying and gruesome appearance that inspired the filmmakers to caste these critters, or were they well aware of their dominant stature amongst other mortal creatures? Either way, they weren’t wrong!
Tardigrades in Space
In 2007, a group of researchers pushed the resilience of these extraordinary animals to the extreme, in conditions that would be impossible for any other known living creature to survive. A sample of dehydrated tardigrades was exposed to outer space for ten whole days. The vacuum of space, which results in severe dehydration and cosmic radiation, was not a problem for water bears. When those same specimens were returned to earth and rehydrated, 68% of those shielded from the radiation survived; even a handful of those without radiation protection not only came back to life but also produced viable offspring.
The adaptation that allows tardigrades to live through such barbarous conditions is their ability to enter a dehydrated state, one that closely resembles death, and this is what makes all the difference. When encountered with environmental stresses, a tardigrade will moult its skin, and inside the shed skin, it curls up into a dry, lifeless ball called a tun. This reduces its metabolic activity immensely, to around 0.01% of its normal levels, but still protects their DNA. To do this, their body produces trehalose, a natural protective sugar that forms a gel-like medium that suspends and preserves the cell organelles and membranes. In the tun, a tardigrade can survive for decades or more. Once immersed in water, their bodies return to a normal metabolic state over the course of a few hours. In one such incident, a group of dehydrated tardigrades was reportedly taken from a museum sample of 100-year-old dried moss and was brought back to life by simple rehydration. That being said, the longer a tardigrade persists in a dehydrated state, the lower are its chances of being successfully revived.
Whether it is the scorching heat of thermal vents, the icy cold of subglacial lakes, or even the vacuum of space, these hard little critters will survive it all. Boil them, mash them, stick ’em in a stew… they are versatile and probably the most durable creatures on the planet. Having survived five great mass extinctions over a period of half a billion years, they might be the strangest combination of cute and terrifying that anyone has ever experienced while looking through a microscope. Extreme conditions that terrify us and would most certainly kill almost any life form, barely even faze them. These tiny tots have expanded the notion of habitable environments, and understanding their indestructibility will have profound implications for both earthbound medicines and survival on other worlds.