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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