BY Mohammad Namjoo
Meet The Toughest Creature On Earth
Meet The Toughest Creature On Earth
Tardigrades (/ˈtɑːrdᵻˌɡreɪd/; also known as water bears, space bears, pudgy wudgies, or moss piglets) are water-dwelling, eight-legged, segmented micro-animals.They were first discovered by the German zoologist Johann August Ephraim Goeze in 1773. The name Tardigrada (meaning "slow stepper") was given three years later by the Italian biologist Lazzaro Spallanzani. They have been found everywhere from mountaintops to the deep sea, from tropical rain forests to the Antarctic.
Tardigrades are the most resilient animal known: they can survive extreme conditions that would be rapidly fatal to nearly all other known life forms. They can withstand temperature ranges from 1 K (−458 °F; −272 °C) (close to absolute zero) to about 420 K (300 °F; 150 °C), pressures about six times greater than those found in the deepest ocean trenches, ionizing radiation at doses hundreds of times higher than the lethal dose for a human, and the vacuum of outer space.They can go without food or water for more than 30 years, drying out to the point where they are 3% or less water, only to rehydrate, forage, and reproduce.
They are not considered extremophilic because they are not adapted to exploit these conditions. This means that their chances of dying increase the longer they are exposed to the extreme environments, whereas true extremophiles thrive in a physically or geochemically extreme environment that would harm most other organisms.
Usually, tardigrades are about 0.5 mm (0.02 in) long when they are fully grown. They are short and plump with four pairs of legs, each with four to eight claws also known as "disks".The first three pairs of legs are directed ventrolaterally and are the primary means of locomotion, while the fourth pair is directed posteriorly on the terminal segment of the trunk and is used primarily for grasping the substrate. Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a very low-power microscope, making them accessible to students and amateur scientists.
Tardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. It is an ancient group, with fossils dating from 530 million years ago. About 1,150 species of tardigrades have been described. Tardigrades can be found throughout the world, from the Himalayas (above 6,000 m (20,000 ft)), to the deep sea (below 4,000 m (13,000 ft)) and from the polar regions to the equator.
Tardigrades are able to survive in extreme environments that would kill almost any other animal. Extremes at which tardigrades can survive include those of:
- Temperature – tardigrades can survive:
- A few minutes at 151 °C (304 °F)
- 30 years at −20 °C (−4 °F)
- A few days at −200 °C (−328 °F)
- A few minutes at −272 °C (−458 °F; 1 K)
- Pressure – they can withstand the extremely low pressure of a vacuum and also very high pressures, more than 1,200 times atmospheric pressure. Tardigrades can survive the vacuum of open space and solar radiation combined for at least 10 days. Some species can also withstand pressure of 6,000 atmospheres, which is nearly six times the pressure of water in the deepest ocean trench, the Mariana trench.
- Dehydration – the longest that living tardigrades have been shown to survive in a dry state is nearly 10 years, although there is one report of leg movement, not generally considered "survival", in a 120-year-old specimen from dried moss. When exposed to extremely low temperatures, their body composition goes from 85% water to only 3%. As water expands upon freezing, dehydration ensures the tardigrades do not get ripped apart by the freezing ice.
- Radiation – tardigrades can withstand 1,000 times more radiation than other animals, median lethal doses of 5,000 Gy (of gamma rays) and 6,200 Gy (of heavy ions) in hydrated animals (5 to 10 Gy could be fatal to a human). The only explanation found in earlier experiments for this ability was that their lowered water state provides fewer reactants for ionizing radiation. However, subsequent research found that tardigrades, when hydrated, still remain highly resistant to shortwave UV radiation in comparison to other animals, and that one factor for this is their ability to efficiently repair damage to their DNA resulting from that exposure.
- Irradiation of tardigrade eggs collected directly from a natural substrate (moss) showed a clear dose-related response, with a steep decline in hatchability at doses up to 4 kGy, above which no eggs hatched. The eggs were more tolerant to radiation late in development. No eggs irradiated at the early developmental stage hatched, and only one egg at middle stage hatched, while eggs irradiated in the late stage hatched at a rate indistinguishable from controls.
- Environmental toxins – tardigrades are reported to undergo chemobiosis, a cryptobiotic response to high levels of environmental toxins. However, As of 2001, these laboratory results have yet to be verified.
- Outer space – tardigrades are the first known animal to survive in space. In September 2007, dehydrated tardigrades were taken into low Earth orbit on the FOTON-M3 mission carrying the BIOPAN astrobiology payload. For 10 days, groups of tardigrades were exposed to the hard vacuum of outer space, or vacuum and solar UV radiation.