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What Are They?

Water Bears belong to a lesser known phylum of invertebrate animals, the Tardigrada. The first tardigrades were discovered by Goetz in 1773. Over 400 species have been described since that time.

Tardigrades grow only to a size of about 1mm, but they can easily be seen with a microscope. Tardigrade bodies are short, plump, and contain four pairs of lobopodial limbs (poorly articulated limbs which are typical of soft bodied animals). Each limb terminates in four to eight claws or discs. They lumber about in a slow bear-like gait over sand grains or pieces of plant material.

All tardigrades possess a bucco-pharyngeal apparatus, a complex structure. The claws and the bucco-pharyngeal apparatus are morphological characteristics used to identify the different species. The body is covered with a cuticle which contains chitin, proteins, and lipids (Kinchin, 1994).

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Location

Tardigrades live in marine, fresh water, and semiaquatic terrestrial environments. If you sample the mosses and lichens in your backyard, your will likely find these tiny creatures. You can find water bears in almost every type of habitat around the world, from moss in a tropical forest to the freezing waters of the Arctic Ocean. They are all, however, considered aquatic to some extent because they must have a film of water surrounding their body to permit gas exchange and prohibit uncontrolled desiccation. About ten percent of the known species are marine and the other ninety percent are fresh water. Many are limnoterrestrial, living in wet terrestrial habitats such as moss or leaf litter.

Many of these environments experience changes in temperature and humidity throughout the year. Tardigrades must be able to adapt to these changes or they will die. Recent studies have indicated that some tardigrades in Antarctica can survive in the hydrated state in temperatures as low as -80 C. Tardigrades have the ability to go into cryptobiosis, a hibernation-like state in order to survive these fluctuating conditions in their environment (Kinchin, 1994, Somme, 1995b, & Somme, 1996).

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Biology

Water bears feed on the fluids of plant and animal cells. They have stylets which allow them to pierce plant cells or animal body walls. A sucking pharyngeal bulb enables them to then ingest the internal contents of their food items. Some species of water bears are known to eat entire live organisms, such as rotifers and other tardigrades.

Typically, tardigrades are dioecious, that is they have male and female individuals in the population. Each has a single gonad which lies dorsal (above) to the gut. The presence of dwarf males or no males has been reported in some populations. Fertilization can occur through a gonopore or the male can deposit his sperm on the eggs after they have been laid either on the substratum or in the molted cuticle. Females lay from 1 to 30 eggs at a time. Development is direct (no larval stages) with juveniles hatching from eggs. Tardigrades express eutely, which means that the number of cells in the body is fixed from birth.

The wide spread distribution of tardigrades can be attributed to the fact that their eggs, cysts, and tuns are light enough to be distributed by wind or animals for great distances.

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Cryptobiosis

One way in which tardigrades have adapted to various types of environments, has been to reversibly suspend their metabolism. This state is known as cryptobiosis and is a truly deathlike state. Metabolism lowers to 0.01% of normal or is entirely undetectable and the water content of the body decreases to less than 1%. The environmental extreme determines which of four crypto biotic pathways -- anhydrobiosis, cryobiosis, osmobiosis, and anoxybiosis---will occur.

The most intensely studied type of cryptobiosis is anhydrobiosis, a form of cryptobiosis initiated by desiccation. Living in a limnoterrestrial habitat, such as moss, requires that these organisms can survive periods of dryness. Anhydrobiosis is an almost complete loss of body water and the animal can stay in this state for an extended period of time. Tun formation, a vital part of the process, results in a body that is constricted and folded. The first step is invagination of the limbs, longitudinal contraction of the body, and infolding of the intersegmental cuticle. Wax extrusion covers the surface and may help to reduce transpiration (water loss by evaporation). The tun formation process requires active metabolism. The relative humidity required for tun formation to be successful varies between 70-95%, depending on the species. Once the tun is formed further desiccation can take place in 0% relative humidity and the tardigrade can still survive (Wright, 1989b). Revival from this state typically takes a few hours but is dependent on how long the tardigrade has been in the anhydrobiotic state (Somme, 1996).

Cryobiosis is a form of cryptobiosis which is initiated by a reduction in temperature and involves the ordered freezing of water within the cells. Recent studies done by Somme in 1995 and 1996 have helped to develop a greater understanding of the mechanism tardigrades use to survive extreme temperatures. John Wright (1992) claimed that organisms which live in polar regions must be able to withstand periods of freezing without becoming frozen themselves. However, certain animals that live in such environments are able to remain viable in the frozen state. These include some arthropod insects which may spend ten months in a completely frozen solid state (Storey, 1990). Cryobiosis allows tardigrades to tolerate rapid freezing and thawing cycles and allows for tardigrades in Arctic and Antarctic habits to withstand the temperature changes which occur (Wright, 1992). Recent work on two species, Adorybiotus coroniferandAmphibolus nebulosus found in the Arctic demonstrate the ability of tardigrades to survive super-cooling to &endash;6 Celcius.

Osmobiosis is a form of cryptobiosis initiated by a decreased water potential due to increased solute concentration in the surrounding solution. Osmobiosis has been poorly studied with only two studies (Collin and May, 1950 and Wright, 1987) concerning Tardigrada to date. Upon immersion in non-ambient saline solutions tardigrades commonly contract rapidly into a tun. However, this is not necessary since active animals can survive high salinity. Viability decreases with prolonged exposure. Some tardigrades are found in the marine intertidal zone and can tolerate changes in the salinity of the water. Echiniscoides sigismundi, species found on rocky shores, can tolerate tidal cycles of seawater and severe desiccation, combined with fluctuations in osmolality during evaporation and rainfall (Wright, 1992). The process by which osmobiosis occurs is not understood but does appear to involve the cessation of metabolism.

A reduction of oxygen tension initiates a suspended state in tardigrades, but is not really considered a form of cryptobiosis. Animals in this state remain extended, turgid, and immobile. Tardigrades are very sensitive to changes in oxygen tension and prolonged reduction of oxygen leads to osmoregulatory failure. Unlike other types of cryptobiosis, anoxybiosis involves the uptake of water and the animals become turgid. Revival time is directly proportional to duration of the dormant state. John Wright (1992) explained that the survival rate of a tardigrade in anoxybiosis is questioned because studies done by John Crowe (1975), show that specimens were only viable for up to 3-4 days, while Kristensen and Hallas (1980) reported survival for up to six months in closed vials.

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Ability to Resist Environmental Extremes

While in a state of cryptobiosis organisms are able to resist environmental extremes that would be instantly lethal to the animal if in the active state. In a review of cryptobiosis, Crowe (1971) discusses some of the findings regarding the abilities of tardigrades to withstand these environmental extremes. In the 1920's P. G. Rahm of the University of Freidburg discovered tardigrades were able to withstand being heated for a few minutes in 151 degrees Celsius and survive being chilled for days in temperatures up to minus 200 degrees Celsius. While in this state the organisms are also greatly resistant to ionizing radiation as shown by Raul M. May from the University of Paris who found that 570,00 roentgens were required to kill 50% of exposed tardigrades (only 500 roentgens would be fatal to a human). Water bears are also resistant to vacuums. Specimens exposed to high vacuum and electron bombardment in a SEM for 0.5 hours were then revived and survived for a few minutes before dying. Why are organisms in the cryptobiotic state able to withstand extreme conditions? Crowe (1971) hypothesized that the importance of water, heat, and oxygen in destructive reactions may explain why the lack of at least one of these characters in animals in cryptobiosis provides resistance to such cellular breakdown.

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Implications and Future Research

As science obtains a better understanding of biological processes we must at times re-examine previous beliefs or understandings. This is perhaps exemplified by cryptobiosis. The issue pertains to the question of whether or not tardigrades can die and come back to life. The answer is no. However, normally, the cessation of metabolic activity is associated with death and death is considered an irreversible state.

Crowe (1971a) has suggested that life can be described as the continuity of structural integrity and death as the destruction of structural integrity. 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 (Crowe, 1971a). Preservation of sperm, seeds, blood, and food is an emerging new disciplines 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 suspended metabolism--cryptobiosis--in humans.

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Tardigrade Facts
Website Authors: Karen Lindahl and Professor Susie Balser in affiliation
with Illinois Wesleyan University. Last revised 2 Oct. 1999.