Tardigrades: Animal Survivors
We’ve covered the nigh invulnerable bacteria Deinococcus radiodurans and near-indestructible prions, but what about animals? Generally, animals do not come to mind when we think of surviving in extreme environments; we’re a little too squishy to survive in vacuums, sub-Antarctic temperatures, or sans water. However, there are a few astonishing animals that have mastered survival skills that put the rest of us to shame, and even offer up a challenge to D. radiodurans. The animal phylum Tardigrada contains species of Tardigrades that can survive 10 years without water, subsist in temperatures from -196°C (-320°F) to 151°C (304°F), live in a vacuum or under six times the pressure of the deepest point in the ocean, shrug off 5,000 Gy of gamma radiation and 8,000 Gy of heavy ion radiation, and dodge the toxic effects of most environmental toxins. These astounding animals have been given very dignified and fearsome nicknames to live up to their reputation; they are known as Water Bears and Moss Piglets.
Meet the Critter
Water bears are microscopic animals that range from 100-1000 micro-meters in length (that’s 0.1 cm at the high end of the scale) . These animals were discovered in 1773, when the German zoologist Goeze spotted them under the microscope. Apparently, he thought they looked like miniature bears (he must have had a vivid imagination), and thus dubbed them kleiner wasserbar, or water bear. Three years later, an Italian scientist by the name of Spallanzani renamed them Tardigrada . Only about 35 groups of animals are unique enough to qualify them for their own phylum classification, and water bears are one of them. The phylum Tardigrada contains about 1000 described species of water bears so far [2,7].
These little animals are aquatic invertebrates that can be found worldwide in oceans, freshwater, and terrestrial environments. In terrestrial environments they most frequently make their homes in mosses and lichens where they live in tiny pockets of water, but they’re also found in hot springs, rainforests, mountains, and Antarctica [2,4]. They contain a wide spectrum of pigments across the species, resulting in water bears that are translucent, white, red-orange, brown, green, yellow, or multicolored . Each water bear is made up of only about 1060 cells (in comparison, humans contain just under 100 trillion cells). They have a brain, muscular system, nervous system, reproductive system, and digestive system. Their body cavities contain free-floating storage cells that are used for nutrient and gas exchange, removing the necessity for a circulatory and respiratory system. Externally, they have a head followed by four trunk segments, each with a pair of legs (that’s 8 legs in total), and the entire body is covered in a chitinous cuticle, similar to many insects. Their diets include plants, bacteria, and other microscopic animals, which they catch with their claws [2,7].
When a mommy water bear loves a daddy water bear very much there are several possible methods of reproduction. Tardigrades reproduce through gametes (sperm and eggs), but their method of reproduction varies between species and based on the environment. In the ocean, water bears tend to be gonochoristic, like mammals and birds: each individual is one unique sex. Females will produce a single egg at a time, repeated throughout her life, while males are semelparous (they get one shot to reproduce and then kick the bucket). In other species, hermaphroditism is the main method of reproduction, with each individual being able to carry out the role of either sex. In water bears, this method comes with a tendency to self-fertilize. In non-marine species, thelytoky is more common; females will lay clutches of eggs, and unfertilized eggs will develop into females while those that are fertilized by a visiting male develop into males. Some species contain only females, with unfertilized eggs developing into embryos in a form of asexual reproduction (parthenogenesis) .
At all stages of their lives, water bears exhibit an extraordinary resistance to conditions that would kill most other organisms. As scientists have only recently begun to attempt to unravel the secrets of their survival, most of the mechanisms behind water bear resistances remain a mystery. However, along with general resistances, water bears have been found to enter various states of dormancy or suspended animation in order to persist in the most extreme situations. There are two main types of dormancy that water bears utilize: diapauses and quiescence [4,7].
Diapause is not directly caused by a hostile environment, but instead by an internal, physiological mechanism. Consequently, the initiation and termination of this state of dormancy may not exactly correspond to environmental conditions, although there is some interplay with stimulation from the environment affecting the physiological trigger. Diapause tends to have season changes and can vary between times, conditions, and individuals .
Encystment is the main type of diapause, in which the water bear encloses itself in a cyst. When initiated, the water bear sheds its outer layer of sclerified cuticle before growing several new layers that are far more simplified. These new layers lack claws and legs, which the water bear keeps tucked inside, resulting in an oval, opaque cyst .
Far more well-researched is the water bear’s second type of dormancy, that of quiescence. This form is directly induced and maintained by environmental factors; stresses such as lack of water or cold temperatures will cause the water bear to enter quiescence, and the state will be maintained until favorable conditions are restored. This type of dormancy in water bears is called cryptobiosis, a state in which the metabolic activities of the animal are reduced to an undetectable level and physiological changes allow persistence through nearly any natural catastrophe [2,4]. Water bears can persist in this state for years, although there is a positive correlation between the time spent in this state and the time required to recover active life (the longer the water bear remains in dormancy, the longer it takes to recover from it) .
The ability of water bears to enter cryptobiosis allows them to colonize and form new niches in different environments. Additionally, while they’re in cryptobiosis, water bears can be widely dispersed and cross physical barriers (carried over mountains on winds, etc). Being able to survive in extreme environments reduces the number of competitors, predators, and parasites water bears have to deal with as fewer species can tolerate such conditions . All in all, it’s a very cool adaptation. Cryptobiosis is further broken down by what type of environmental factor triggers it, as the physiological details of cryptobiosis vary depending on what condition the water bears need to survive. Without further ado, let’s look at some of the jaw-dropping resistances water bears have!
Desiccation Resistance: Anhydrobiosis
Desiccation is a state of dryness. Organisms require water for the majority of their biological and chemical processes; humans would only survive three days without water. As water bears are aquatic animals, it is even more important for them. Despite this, water bears can survive for up to ten years in a complete absence of water by entering a cryptobiosis state called anhydrobiosis. An anhydrobiotic water bear lacks all features of a living organism, with an undetectable metabolism. This allows water bears to persist in glaciers and deserts, awakening when there’s enough water available to support living and reproduction .
Anhydrobiosis is characterized by the formation of a tun: the water bear withdraws its legs and contracts its body, reducing the surface from which water can be lost due to evaporation and reducing the permeability of its cuticle shell. These measures allow the water bear to retain more water at the beginning of desiccation. A complex web of factors comes into play, including various bioprotectant molecules and physiological changes, which are not well understood. Proteins known as molecular chaperones are created by the water bear that help prevent damage done to other cellular protein due to the lack of water. Another molecule is thought to be made that replaces water in the cell, so that as nearly all water within the animal’s cells is lost, their structure is maintained and the damaged minimized [4,7].
Water bears can remain in anhydrobiosis for years with no effect on their aging or longevity. While damage to proteins and DNA accumulate in proportion to the length of time spent dry, these damages are apparently flawlessly repaired upon termination of the dormant state .
Temperature Resistance: Cryobiosis
Many species of water bear can survive being completely frozen and then resume their normal life cycles after thawing out. These cryobiotic water bears can survive in temperatures as low as -80°C (-112°F) for years as well as persist through exposures to temperatures as low as -196°C (-320°F) . This resistance to cold exists in all life stages, from egg through embryo and adulthood. While the exact methods are still being determined, scientists have determined both freeze tolerance and an accumulation of cryoprotectants to be involved. By controlling the ice formation and ice crystal size, water bears can cause smaller and less damaging crystals to occur in their cells. Additionally, cryoprotectants such as sugars and alcohols help prevent specific cellular content to remain unfrozen and thus reduce damage .
Water bears are also very heat tolerant, surviving temperatures up to 151°C (304°F), although even less appears to be known about this resistance. Special proteins called heat-shock proteins have been proposed to act as molecular chaperones. These proteins may protect existing proteins and structures from heat damage, preventing proteins from denaturing or changing shape which often occurs under higher temperatures. They may additionally be involved in refolding proteins correctly when the cells are recovering .
Absence of Oxygen (Anyoxybiosis) and Pressure
Not only can water bears survive in environments without any oxygen, but they can also survive in an absence of pressure–in other words, in a vacuum. These conditions might sound familiar, as they’re what you encounter in outer space. There was actually a project called the Tardigrade Resistance to Space Effects (TARSE) project, which was part of a mission called LIFE, that sent water bears into space for twelve days in 2007. Not only did they survive the high levels of space radiation (which we will get to next), but they persisted in the vacuum of space. In fact, during the mission they not only molted but actually laid clutches of eggs, which later hatched back on Earth !
Radiation is extremely dangerous to living organisms as it results in not only damaged proteins, but even fragmented and mutated DNA. More details concerning radiation damage was covered in this post, including the susceptibility of humans: being exposed to 7.5 Gray of gamma radiation will kill us. While water bears aren’t quite as radiation resistant as Deinococcus radiodurans, they still put humans to shame by surviving up to 5,000 Gy of gamma radiation. They can additionally survive 8,000 Gy of heavy ion radiation and have a high resistance to UV radiation, which allows them to persist in space .
Water bears have a variety of other resistances which remain less explored and understood. They are tolerant to high-salt concentrations (osmobiosis) and can survive exposure to many deadly toxins and chemicals (chembiosis). They can also survive at extreme pressures of up to 6,000 atmospheres, far beyond the pressure found at the deepest points in the ocean .
Resistances in EggsWater bear eggs not only look awesome, but they are well equipped to survive all the challenges that they might encounter in extreme climates. Eggs can enter quiescence and diapause, allowing them to tolerate all of the aforementioned conditions. Not only can eggs match adults for survival, but they are possibly even more resistant (the duration of survival of an egg in a desiccated state rivals the adult’s). Once an egg enters a cryptobiosis state, its resistance is increased to radiation, temperature, and vacuum conditions. Additionally, a female water bear lays her eggs in various places to maximize survival chances. The eggs will hatch at varying times within 62 days, a mechanism that could increase the chance of survival success by varying the conditions the hatching water bears will face [4,6].
Resistances in a Non-Dormant State
While a key to water bears’ extreme resistances is their ability to enter dormant states, they are also quite resistant when in a fully active, living state. While “virtually nothing” is known about their physiology, researchers suspect a high-metabolic state enhancing DNA repair is partially behind their survival success . Water bears have multiple DNA protective pathways and repair functions for both DNA and proteins. Some of these protective and repair pathways are identical to those found in humans and other animals, though scientists believe they have been enhanced in these species. Additionally, water bears have 46 unique protein adaptations which likely encode the molecular chaperones mentioned previously along with damage repair and resistant mechanisms . Part of the ability of water bears to survive extreme circumstances is likely very similar to that of Deinococcus radiodurans: while temperature, radiation, pressure, and lack of water may damage the DNA in cells, they have extremely efficient mechanisms to repair that damage and prevent cell and organism death . Water bear cell membranes also contain several special molecules and phospholipids which stabilize cellular membranes against adverse conditions [3,7]. Finally, pigmentation might be another protective trait. The wide spectrum of colors found in water bear cells and bodies are not just there to dazzle; the pigments themselves have antioxidant functions. Most of the pigments found coloring water bears are acquired through their diet, and when investigated were found to be carotenoids. These types of pigments are known for their ability to collect free-radicals, which would otherwise damage proteins and DNA . Consequently, the collection and utilization of pigments for resistance to oxidative is one of many physiological adaptations to enhance their survival.
Can we adapt these strategies?
Researchers are using water bears to investigate increasing oxidative stress resistance in other organisms, such as humans. Not only have scientists identified various molecules (ranging from carbohydrates to proteins) that water bears build up in their cells to help protect from them damage, but they are also using various proteins formed by water bears in cell cultures to test their effects. Once more is known about these protective molecules and pathways, scientists can find out which, if any, are at all conserved in humans and possibly use drugs to enhance our protective and repairing pathways [3,7]. As oxidative stress and DNA damage are main factors behind both aging and disease, this is an exciting avenue of research.
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