Tardigrada (water bears, tardigrades)

Life > Eukaryotes > Opisthokonta > Bilateria > Ecdysozoa > Panarthropoda > Tritocerebra

Echiniscus sp.

Minibiotus sp.

Milnesium sp.

What Are They?

Water Bears are members of a largely unknown phylum of invertebrate animals, the Tardigrada. The first tardigrades were discovered in 1773 after microscopes were invented. Over 800 species have been described since then. The largest tardigrades grow to a size of just over 1 mm, but they can easily be seen with microscopes. Tardigrade bodies are divided into segments, roughly cylindrical and posses four pairs of lobopodial limbs (poorly articulated limbs without joints, which are found in soft bodied animals like Onycophorans). The limbs terminate in four to eight claws or discs. They crawl about with a bear-like pawing motion of the legs (that originated the name water bears) over sand grains in the seas, soil, lichen or pieces of plant material etc.

Macrobiotus cf hufelandi on underside of glass coverslip of microscope slide, showing the claws at the end of its legs.

Limno-terrestrial tardigrades are regarded as amongst the most indestructible animals that exist when they enter their resistant state—called a tun, and can survive in extreme conditions during cryptobiosis (the most extreme form of suspended animation).

Where are they found?

Tardigrades live in marine or fresh water ecosystems, and also semi-aquatic terrestrial (limno-terrestrial) environments. If you sample the mosses and lichens around your yard, you are likely to find them. Water bears can be found in almost every type of ecosystem around the world, from moss in tropical rain forests to the Antarctic,from the deepest ocean to the highest mountain. They can all, however, be considered aquatic since they must have a film of water surrounding their body to permit gas exchange and prevent uncontrolled desiccation.

Some tardigrades exist in environments that experience changes in temperature and water levels, so tardigrades must be able to survive these changes or they would die. Tardigrades have the ability to go into cryptobiosis, an extreme form of hibernation, in order to survive the fluctuating conditions in their environment.

General Biology

Water bears feed on the fluids of plant and animal cells, fungi etc. They are unique in having a pair of piercing stylets which they extend out of their mouth to pierce plant cells or animal body walls. A sucking pharyngeal bulb enables them to suck up and then ingest the internal contents of their food. Some tardigrades eat entire live organisms, such as rotifers, nematodes or other tardigrades.

Typically tardigrades are dioecious, sexually reproducing with both male and females. Each has a single gonad which lies dorsally to the gut. However, some species are hermaphrodite and the absence of males has been reported in many populations, the females then reproduce asexually by parthenogenesis.This gives biologists a major problem in terms of defining a species and how they fit in evolutionary schemes where these clones are usually regarded as a ‘dead end’ which should be quickly out competed by sexual species.

Tardigrades express eutely, which means that the number of cells in some organs of the body is fixed from birth, growth occurring by increase in size only and not cell division. They do not have circulatory and respiratory systems and the excretory system may also be minimal.

The widespread distribution of tardigrades may be attributed to the fact that their eggs, and tuns are light enough to be distributed by wind or animals for great distances possibly in the upper atmosphere. There is however, little evidence as yet for this. They are also now thought to be a most ancient lineage having evolved in the Cambrian period 530 million years ago (or earlier) as newly discovered fossils date from this period, pre-dating the famous Burgess Shale fossils of the Cambrian explosion of animal phyla.

Macrobiotus cf. hufelandi egg.

Macrobiotus cf. richtersi egg

Cryptobiosis

The way that tardigrades have adapted to environmental stress is to reversibly suspend their metabolism and to effectively isolate themselves from the changes. This state is known as cryptobiosis and is a truly death-like state. Cessation of metabolism in other organisms is called death. Metabolism in tardigrades can lower to less than 0.01% of normal, or be entirely undetectable and the water content of the body may decrease to less than 1%. Tardigrades have been revived from this state after more than 100 years and shown signs of life! However, experiments have only shown cryptobiosis under natural conditions of 12 years but frozen they could theoretically survive for ever! The change in environmental conditions determine which cryptobiotic method—anhydrobiosis, cryobiosis, osmobiosis, and anoxybiosis—will occur.

Most studies of cryptobiosis have been done on anhydrobiosis, the form of cryptobiosis initiated by desiccation. Tardigrades living in a limno-terrestrial habitat, such as moss or lichens, require that they can survive long periods of dryness. Anhydrobiosis involves an almost complete loss of body water and the animal can stay in this state for an long period of time. Tun formation is an essential part of the process, which can be done repeatedly throughout the life cycle, even by adults, resulting in a body that is constricted and folded up. The limbs invaginate, the body contracts and becomes folded.The tun formation is an active process requiring metabolism and synthesis of a protective sugar Trehelose. After the tun is formed further desiccation can take place in 0 % relative humidity and the tardigrade can still survive. Revival typically takes a few hours from natural habitats but depends upon how long the tardigrade has been in the anhydrobiotic condition.

Cryobiosis is cryptobiosis which is initiated by a reduction in temperature to below 0 °C and involves the ordered freezing of water within the body cells . Osmobiosis is a form of cryptobiosis initiated by a decreased water potential due to increased solute concentration in the surrounding solution.A reduction of the % of oxygen initiates a suspended state in tardigrades, but is mostly not considered a form of cryptobiosis—tardigrades in this state remain extended, turgid, and immobile but it can only last for days before they die.

Ability to Resist Environmental Extremes

While in a state of cryptobiosis tardigrades are able to resist environmental extremes that would be instantly letha to animals if in the active state. In 1842, the French naturalist Doyere first discovered tardigrades were able to withstand being heated for a few minutes to 125 °C, later Rham in 1929 increased this figure to 150°C. Adults have been able to survive being cooled to temperatures of almost absolute zero (-272.8°C) where there is no free molecular vibration and so no metabolism can exist. While in this state the organisms are also greatly resistant to X-Rays of 570,000 Roentgens (only 500 Roentgens would be fatal to a human). Water bears are also resistant to a vacuum (like outer space), some noxious chemicals, boiling alcohol, and pressures six time greater than the bottom of the deepest ocean etc.

Implications and Future Research

As science obtains a better understanding of biological processes we may have problems with previous beliefs or understandings. This is perhaps exemplified by cryptobiosis. The issue pertains to the definition of death and the question of whether or not tardigrades can be dead (ametaboilic) and come back to life. The answer is no since although metabolism may cease they maintain structural continuity and the potential for metabolism. However, for nearly all other animals, the cessation of metabolic activity is associated with death, and death is considered an irreversible state. It could be suggested that life can be described as the continuity of structural integrity with the potential of maetabolic activity and death as the destruction of structural integrity. But even this is a problem as how can a potential to become active be assesed?

Cryptobiosis is an amazing adaptation that may have arisen very early in the evolution of life. Scientists have discovered how to apply this phenomenon to larger organisms. Preservation of sperm, seeds, blood, and food is an emerging new discipline that involves cryobiology. Cryosurgery and suspended animation also present some exciting possibilities. The long-range implications may even include the ability to travel long distances in space. This could occur through suspending metabolism i.e. cryptobiosis, in humans.

Recently scientists from Japan have used the tardigrade sugar trehelose to enable them to keep a rat's heart in a fridge at 4 degrees Celsius for 10 days and then revive it, this is exciting as at present doctors can normally only keep a human heart alive for 4 hours before a transplant.

Where do Tardigrades fit in with other Animals?

Tardigrades have often been called a minor phylum on account of their small numbers of species, compared with others such as the millions of Arthropods. However, it would be better to call them a lesser known phylum as they appear to have been completely separate in body plan (bauplan) since at least the Cambrian time of 530 million years ago.They also have a world-wide distribution though some distributions indicate links to ancient continental break-ups. Most biologists the regard tardigrades as being close to the Arthropods probably branching off earlier from the ancestral line.

Even the most recent molecular investigations have been unable to decide their position. Analysis of 18S rRNA by some scientists place them next to the Onycophora and Uniramia —one of the polyphyletic Arthropod groups—while other scientists analysing the same molecules suggest they split off before the protosomes (Annelid/Arthropod line) even evolved! The search for the position of the phylum Tardigrada in a natural phylogeny is still ongoing.

What about Southern Africa?

Tardigrades were first recorded from South Africa in 1907. However, this was done like all the other records, by experts based elsewhere.and since then there has not been as much research undertaken compared with other areas of the world. Records from sites (recorded now for example in 1°squares of latitude/longitude) are limited to ten in South Africa. Other countries have less, and some countries such as Zambia and Swaziland do not yet have records at all. Botswana and Lesotho now have records produced by the author, in Lesotho only two sites, while Botswana has records from 17 out of 62 of the 1° squares making it the most covered country on the continent. Overall in Africa less than 2% of the area has been recorded in any way. Some species such as Echiniscus africanus (first found in South Africa) seem not been found in Southern Africa for 90 years! Some records are dubious and other visiting experts have found unique endemic species only found once.

Can you Collect and See Tardigrades?

Yes, taking samples for limno-terrestrial tardigrades is extremely easy, it is mainly the processing and identification that are difficult and time consuming. To see tardigrades you will normally need a microscope (a hand lens could be used). A moss or lichen sample should be soaked in water (preferably distilled or rainwater) for a few hours, then it can be squeezed or shaken, to release tardigrades, and the water examined first under low power such as X30 (best with a stereo microscope). If possible a micro-pipette can be used to transfer tardigrades to a slide, which can be looked at with a higher power compound microscope.

If you wish to help in this area of research, you only have to find lichens or moss growing on trees or rocks etc. scrape them off into a paper envelope or bag and send them for identification with site data or examine them yourself.

Specialists who are undertaking surveys involving soil/freshwater/marine invertebrates should seek advice from the author.

References:

Copley, J. 1998. Putting life on hold. New Scientist 7 November 1998: 7.
Copley, J. 1999. Indestructable. New Scientist 23 October 1999: 44-46.
Kinchin, I. M. 1994. Biology of Tardigrades. Portland Press, London.
Kristenssen, R. M. (Ed) 2001. Tardigrada. Zoologischer Anzeiger 240: 3-4.
Mc Innes, S. J. and Morman, D. B. (Eds) 1996. Tardigrade Biology. Zoological Journal of the Linnaean Society 116.