In actuality, tardigrades are translucent and display a variety of colors—white, green, orange, red. In the microenvironments made by water that coheres in the fissures of mosses and lichens due to surface tension, tardigrades thrive by feeding on smaller organisms and by sucking contents out of plant cells.
Their moist realm is transient, and in response tardigrades have evolved an array of strategies based on induced cryptobiosis—the suspension of metabolism by drying or freezing. In their cryptobiotic state, desiccated or frozen, they are astonishingly durable. These organisms survive extreme conditions—of temperature, pressure and radiation—to a degree unparalleled in nature.
Terrestrial species live in the interior dampness of moss, lichen, leaf litter and soil; other species are found in fresh or salt water. They are commonly known as water bears, a name derived from their resemblance to eight-legged pandas. Some call them moss piglets and they have also been compared to pygmy rhinoceroses and armadillos. On seeing them, most people say tardigrades are the cutest invertebrate. At one time water bears were candidates to be the main model organism for studies of development.
That role is now held most prominently by the roundworm Caenorhabditis elegans , the object of study for the many distinguished researchers following in the trail opened by Nobel Prize laureate Sydney Brenner, who began working on C. Water bears offer the same virtues that have made C.
Some species may, like C. Tardigrades have somewhere over 1, cells. I and others use water bears as a model educational organism to teach a wide range of principles in life science.
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Tardigrades are nearly translucent and they average about half a millimeter 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 to power magnification—as charismatic miniature animals. Most tiny invertebrates dart about frantically. Tardigrades move slowly as they clamber around on bits of debris.
These stout legs propel them unhurriedly and deliberately about their habitat. Tardigrades have five body sections, a well-defined head and four body segments, each of which has a pair of legs fitted with claws. The claws vary in different species from familiarly bearlike to strangely medieval fistfuls of hooked weaponry. The hindmost legs are attached backwards, in a configuration unlike that of any other animal.
These legs are used for grasping and slow-motion acrobatics rather than for walking. Figure 2. A light-microscope image of the anterior end of a tardigrade left shows the mouth; stylets, strutures that help them feed; the buccal apparatus, part of the digestive tract; and the pharynx at the top of the alimentary system. A cross section of a generic water bear right shows the relative positions of the organ systems.
Lacking are circulatory and respiratory systems. At this tiny scale, an open hemocoel cavity is sufficient to distribute oxygen and nutrients through the organism. David J. Illustration at bottom by Tom Dunne, adapted from a figure by the author. Inside these tiny beasts we find anatomy and physiology similar to that of larger animals, including a full alimentary canal and digestive system.
Mouth parts and a sucking pharynx lead to an esophagus, stomach, intestine and anus. There are well-developed muscles but only a single gonad. Tardigrades have a dorsal brain atop a paired ventral nervous system. Humans have a dorsal brain and a single dorsal nervous system. The body cavity of tardigrades is an open hemocoel that touches every cell, allowing efficient nutrition and gas exchange with no need for circulatory or respiratory systems.
Taxonomists divide life on Earth into three domains: Bacteria, Archaea an ancient line of bacterialike cells without nuclei that are likely closer in evolutionary terms to organisms with nucleated cells than to bacteria , and Eukarya. Eukarya is divided into four kingdoms: Protista, Plantae, Fungi and Animalia. Phylum Tardigrada is one of the 36 phyla roughly, depending on whom one asks within Animalia—making water bears a significantly distinctive branch on the tree of life.
Tardigrades are encased in a rugged but flexible cuticle that must be shed as the organism grows. Thus they have been placed among the phyla on the ecdysozoa line of evolution between animals such as nematodes and arthropods that also shed their cuticles to grow. Animals grow in either of two ways, by adding more cells or by making each cell larger. Tardigrades generally do the latter. If an animal has a hard cuticle or exoskeleton, it must break out of that shell in order to grow.
For example, in summer in many parts of the world, one encounters the shed exoskeletons of locusts on trees everywhere.
Tardigrades are divided into two classes, Eutardigrada and Heterotardigrada. As a general rule, the members of Eutardigrada have a naked or smooth cuticle without plates, whereas the Heterotardigrada boast a cuticle armored with plates. A few years ago, the Discovery network show Animal Planet aired a countdown story about the most rugged creatures on Earth.
Tardigrades left were for a time considered competitors with the round worm Caenorhabditis elegans right and the fruit fly Drosophila melangaster as major invertebrate model organisms. Tardigrades have played that role less over the years, but research attention is increasing as new genetic research tools allow deeper inspection of their extreme durability and adaptivity in response to changing environmental conditions. Tardigrades are predators of nematode worms such as C.
Under the microscope, tardigrade researchers occasionally encounter a water bear grabbing a nematode around the middle. The nematode wriggles furiously all over the dish, with the tardigrade hanging on like a bronco rider, until the drained nematode surrenders. But extreme survivorship applies only to some species of terrestrial tardigrades. Marine and aquatic tardigrades did not evolve these characteristics because their environments are stable. It appears that the extravagant survival adaptations have been selected in direct response to rapidly changing terrestrial microenvironments of damp flora subject to rapid drying and extreme weather.
Terrestrial tardigrades have three basic states of being: active, anoxybiosis and cryptobiosis. In the active state, they eat, grow, fight, reproduce, move and enact the normal routines of life. Anoxybiosis occurs in response to low oxygen.
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Tardigrades are quite sensitive to oxygen tension. Prolonged asphyxia results in failure of the osmoregulatory controls that regulate body water, causing the tardigrade to puff up like the Michelin Man and float around for a few days until its habitat dries out and it can resume active life. Cryptobiosis is a reversible ametabolic state—the suspension of metabolism—that has inevitably been compared to death and resurrection.
In cryptobiosis, brought on by extreme desiccation, metabolic activity is paralyzed due to the absence of liquid water.
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Terrestrial water bears are only limnoterrestrial—aquatic animals living within a film of water found in their terrestrial habitats. Moss and lichens provide spongelike habitats featuring a myriad of small pockets of water and, like sponges, these habitats dry out slowly. As its surroundings lose water, the tardigrade desiccates with them.
It has no choice. The creature loses up to 97 percent of its body moisture and shrivels into a structure about one-third its original size, called a tun. In this state, a form of cryptobiosis called anhydrobiosis—meaning life without water—the animal can survive just about anything. Figure 4. Tardigrades have evolved a suite of survival tactics to escape the vagaries of their localized and vulnerable environments.
Anoxybiosis and encystment, described in the upper part of this figure, are responses one might see in a variety of organisms. The bottom half of the chart shows three states of cryptobiosis, in which metabolism is suspended—an act usually diagnostic of death. Cryobiosis occurs in response to freezing, and anhydrobiosis in response to drying. During the latter, an organism surrenders its internal water to become a desiccated pellet. Both result in the formation of a durable shrunken state called a tun. More rarely, a tun is created to resist osmotic assault, which requires water.
In the tun state, tardigrades can survive for many years, impervious to extremes far beyond those encountered in their natural environments. Tardigrades have been experimentally subjected to temperatures of 0. They have been stored at — degrees Celsius for 20 months and have survived. They have been exposed to Celsius, far above the boiling point of water, and have been revived. They have been subjected to more than 40, kilopascals of pressure and excess concentrations of suffocating gasses carbon monoxide, carbon dioxide, nitrogen, sulfur dioxide , and still they returned to active life.
In the cryptobiotic state, the animals even survived the burning ultraviolet radiation of space. Challenging student scientists to ponder the astonishing durability of tardigrades brings their understanding of physics, chemistry and biology into play. They recall that water expands as it approaches the freezing point, which is why ice floats. At 4 degrees celsius the expansion of water exerts sufficient force to split boulders, rupture metal containers and explode living cells. A cell is more than 95 percent water. The rupturing forces and icy microshards that form in frozen cells are the same that cause frost bite.
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