Tales of the Tardigrades
Several hundred million years ago life was much different than it is now. Organisms were really just beginning to become complex. After billions of years of evolution multicellular life was finally starting to emerge. Then, at a certain point in deep time, tardigrades miniaturized from their larger lobopodian ancestors, which were worm-like creatures with stubby legs. As such, tardigrades are most closely related to velvet worms and arthropods. They are also quite unique, in many ways though. Tardigrades are known colloquially as “water bears” or “moss piglets”. Regardless, there are about 1,150 known species of those weird water-dwelling, eight-legged, segmented micro-animals in that particular phylum. The thing is that water bears not only survive, but actually thrive almost everywhere, including in the ocean depths, on mountaintops, within rainforests, among hot springs, across frozen tundras, and even in the harsh vacuum of outer space. They are nearly omnipresent on the planet, being found on every continent. There is probably at least one tardigrade somewhere close to you right now. Typically speaking though, these bizarre little animals are most prevalent in lichens and mosses, where they tend to feed on plant cells, algae, and small invertebrates. There are herbivorous, carnivorous, and even cannibalistic tardigrades, though. They are somehow both sort of cute and creepy at the same time.
Part of this stems from the fact that tardigrades are tiny little critters with rather weird looking bodies. For one thing, a specific class in the phylum only has one orifice for both digestion and reproduction. Either way, tardigrades all range from somewhere between 0.1 to 1.5 millimeters in length. So, not surprisingly, they are rather rudimentary animals, with adults only being composed of 40,000 cells or less. As such, the slow-moving microscopic organisms have very small brains, so they have simple mating and hunting habits. The brain includes multiple lobes and is attached to a large ganglion below the esophagus, from which a double ventral nerve cord possesses one ganglion per segment, each of which produces lateral nerve fibers that extend into the appendages. Many species also possess a pair of pigment-cup eyes, as well as many sensory bristles on the surface of their skin. As part of this primitive body plan, tardigrades lack respiratory organs because gases can be exchanged across the entirety of their bodies. They also predated the development of several Hox genes, which typically determine the form and function of the various different segments in the bodies of other animals. Thus, the main portion of a tardigrade, except for the last pair of legs, is made up of the segments that are homologous to the head region in arthropods. So, they don’t have a thorax and abdomen, like insects. The body of a tardigrade consists of a head, three body segments each with a pair of legs, and a caudal segment with a fourth pair of legs. The legs are jointless, and the feet have four to eight claws each. The first three pairs are directed downward along the sides, and are the primary means of locomotion, whereas the fourth pair is directed backward on the last segment and is used primarily for grasping the substrate.
I often find it funny when people talk about how cockroaches could live through an apocalypse, or whatever. Little do they know, tardigrades are actually a far more resilient kind of bug than roaches could ever be. Simply put, millions, billions, and even trillions of years from now there could still be tardigrades somewhere in the universe. As the local spacetime continuum grows bigger, colder, and darker throughout the eons the tardigrades will continue to exist. This is because ultimately those incredibly robust little creatures can handle almost anything. Their hardy kind has already survived five great mass extinction events, and they will easily live through the sixth, which is currently underway. Although humans might not last, tardigrades certainly will. They will even make it through the seventh mass extinction event, and the eighth and ninth and so on and so forth. If they can somehow manage to live through the impending collision of the Milky Way and Andromeda galaxies, as well as the death of the Sun, then they might even make their way to other worlds, by hitching a ride on cosmic debris, like comets and asteroids. One day they could even seed life on an otherwise uninhabited planet or moon numerous lightyears away, giving rise to a whole new genesis in some other solar system.
This is all made possible by the fact that individual species are able to survive extreme conditions that would be rapidly fatal to nearly all other life forms, such as exposure to extreme pressures and temperatures, dehydration, starvation, air deprivation, and even radiation. There are tardigrades that can be frozen to almost absolute zero, at which point all molecular motion ceases. Of course, due to the specific pressures they face, terrestrial tardigrades are far more resilient than their aquatic cousins. As part of this, a human can only withstand the pressure of 1 atmosphere, but some tardigrades could resist a 1,000, if not more. Along with this, if the entire Earth were somehow ever exposed to just 10 grays of radiation then humans would go extinct, but tardigrades can withstand at least 5,000. In other words, the toughest animals in existence are highly resistant to things that would easily kill basically any other organism in the universe. As if that wasn’t impressive enough, certain tardigrades can enter a state of suspended animation called cryptobiosis. This is a reversible near-death condition in which the animals curl up, dry out, and shrivel down into dried husks known as “tuns”. This results in anhydrobiosis, transforming a tardigrade into a powdered clump of intrinsically disordered proteins. Then, just the tiniest drop of water can fully revive the primitive life form, regardless of how much time has passed. It is as though tardigrades are somehow immortal. Such is the weird world of water bears.