We’ve talked about some very small animals here before, but none have been as small as the tardigrades. Tardigrades are microscopic animals who live in water, and might just be the toughest animals on Earth. The zoologist who discovered them in 1773, Johannes Goeze, named them “water bears” (because their claws resembled those of a bear), and they’ve also been called “moss piglets”. They were officially named tardigrades in 1777 by Lazzaro Spallanzani, an Italian biologist.
Of the five mass extinctions that we know about in the history of Earth, tardigrades have survived all of them. They can survive great pressure and extreme temperatures (both hot and cold), as well as have immunity to gamma radiation. Even eggs, if subjected to radiation in late developmental stages, can withstand many times a lethal dose for humans. They can survive without food or water for up to 30 years, and then go on living normally after re-hydrating. Tardigrades have been sent directly into the vacuum of outer space and were revived back to normal not even an hour after returning to an atmosphere. Tardigrades are not categorized as extremophiles, because they do not flourish based on extreme conditions. Rather, they are just very good at tolerating them.
Tardigrades live in moist environments, and their favorite place to live is in moss or lichen. They also live in lakes, beaches, the floor of oceans and rivers, and damp undergrowth or fallen leaves. Some species of tardigrades live in barnacles. They feed on plant particles and algae, as well as microscopic organisms that are even smaller than they are.
To give some perspective on exactly how tiny these animals are, the largest species is less than 0.05 inches long, and most species are significantly smaller. Adults may have as many as 40,000 cells, at most. Tardigrades have four pairs of legs that are without joints, and which end in four to eight claws. Of the four pairs of legs, three are used for movement. The last pair of legs is used for grasping. The young tardigrades hatch from their eggs after only 14 days, and are already grown to full size at that time.
Scientists in Tokyo have been studying the DNA sequences of one of the most resilient species of tardigrades since 2015. They have discovered the absence of the genes which cause the organism to be damaged under extreme stress. They also have discovered the pattern which protects the tardigrades against radiation. Experiments to add this sequence to the human genome have been very successful. The human DNA with the added radiation-resistant sequence is up to 40% more resistant that regular human DNA.
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