Despite being the dominant species on the planet, humans cannot dream of competing with the various strengths of these minuscule creatures. Expose them to a hundred times the radiation that could kill us, crush them under the weight of miles-high water, chuck them into the void of space, or starve them for years and they could still survive. Meet tardigrades, also known as the water bears, space bears, or moss piglets, the tiny creatures that have fascinated many generations of zoologists and scientists from various other fields.
The tardigrades are micro-animals first described in 1773 by the German zoologist Johann August Ephraim Goeze. There are over 1,500 different species of tardigrades and their earliest fossils date as far back as 530 million years ago, in the Cambrian period.
Goeze originally named the tardigrades kleiner Wasserbär, meaning the “little water bear” in German, because of the resemblance between its walk and a bear’s gait. In 1776, Italian biologist Lazzaro Spallanzani gave them the name Tardigradum, meaning “slow walker.” While the newly hatched tardigrades are smaller than 0.05 mm, the average length of a fully grown tardigrade is around 0.5 mm. The biggest of them could reach up to 1.5 mm and the smallest being less than 0.1 mm. They have segmented, barrel-shaped bodies with four pairs of stubby legs. Though there are both males and females, some of the species are parthenogenic, that is they can reproduce without fertilization.
The most common places to easily find the tardigrades are on the lichens and mosses. They live in a wide variety of environments including fresh and marine water sediments at a frequency of up to 25,000 of them per liter.
They can be found everywhere in the world from the top of the Himalayas at a height of 6,000 meters (20,000 feet) to the bottom of deep oceans below 4,000 meters (13,000 feet), and from deserts to the frozen polar regions. They are even on the lichens and mosses in your backyard and can be easily found by soaking a piece of moss in the water. Desert dunes, soil, beaches, hot springs, solid ice, lakes, ponds, meadows, and even stone walls and roofs are a few other places where they live.
Tardigrades are one of the most resilient animals known to humans. They can withstand temperatures from −272 °C (−458 °F) to 150 °C (300 °F). They can survive supernovae, gamma-ray bursts, and asteroid impacts. They can go without food or water for 30 years and revive themselves back to normal.
Many species of tardigrades, especially the land-dwelling and fresh-water species, can survive extreme conditions that would be fatal to almost all other species on the planet. They can also survive mass extinction events or astrophysical events that can wipe out most other life on Earth. They can withstand over six times the pressure found in the deepest ocean trenches. They can also survive ionizing radiation a hundred times the lethal dose for humans. They can survive in the vacuum of outer space and dry up to having just three percent of water, then rehydrate themselves, feed and reproduce. However, the tardigrades are not considered true extremophiles as they did not develop any adaptive mechanisms to benefit from the extreme conditions in which they can survive.
Another astounding discovery about tardigrades is that 17.5% of their DNA is foreign. They acquire DNA belonging to bacteria, fungi, plants, archaeans, and viruses via a process called horizontal gene transfer.
After sequencing the genome of one of the tardigrade species, the scientists found that almost one-sixth of the genes are foreign. Though many animals have a small degree of horizontal gene transfer, including humans, this is by far the highest amount. The transfer occurs when the tardigrades dry up, fragmenting the cell membrane and the DNA. This lets big molecules like proteins and fragments of foreign DNA to move in and out of the cell. The fragmented tardigrade DNA then repairs itself with the foreign DNA. Scientists believe that this DNA transfer could also be the reason why they are capable of surviving in extreme conditions as they imbibe the strengths of the animals to which the foreign DNA belonged.