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BNA Guide – An Introduction to “Water
Bears” or Tardigrades
by Roger Tabor
See also BNA
Archive : Tardigrades
With the English name of ‘Water
Bears’ how can we not be drawn to these minute but unusual
animals, the Tardigrades. Despite that, and their amazing
abundance, they are rarely looked for so we have only a sketchy
idea of their true distribution in the British Isles. The
first publication about Tardigrades was by Pastor J.A.E. Goeze
in Germany in 1773, and he said that as “it closely resembles
a bear en miniature” so he called it “Little Water
Bear”. Tardigrade itself just means “slow-walker”.
They gain their aquatic common name from living in wet films.
Some species can reach 1mm long, but most are less than half
that, so to see them in any detail a microscope is required,
and view at x50 (try both dark ground and bright field illuminations).
They are most likely to be found
in damp habitats, and are usually to be found in moss. The
standard way of being able to examine them is to soak the
moss in water for 24 hours. Tardigrades have a remarkable
ability to overcome a lack of water by becoming dormant (then
called a “tun”) until water appears again. Tardigrades will
leave this cryptobiotic state when soaked in water for 24
hours.
Morgan & King recommend using 4%
acetic acid or 20% ethyl alcohol to narcotise them sufficiently
so they relax and fall off the moss. They recommend wringing
the moss dry and re-soaking, and repeating several times into
a dish to ensure the Tardigrades are dislodged into the dish.
(Detailed species determination may depend on the use of an
oil immersion lens with fixed specimens; however the locomotion
movements of non-asphyxiated animals can be sufficient to
indicate identification.
However Mach’s procedure is simpler,
to just stand a cushion moss upside down in water alone in
a dish. After leaving overnight take the moss out then carefully
examine with a x10 hand lens (where they will be visible but
very small) or use a binocular dissection-style microscope
at x25 with top to side light which can even be provided with
a torch. They are easier to see if the dish is on a black
base. Once having found one transfer it by glass-pipette to
a microscope slide. To see them does not require costly equipment,
for they can be seen reasonably well even under a child’s
“toy” or beginner’s microscope. It is fine to start off with
a low magnification of just x40-50. Martin Mach recommends
that to avoid damaging the water bears and to be able to watch
their movements that a spacing device between slide and coverslip
is used, which can be as simple as placing 2 cover slips on
a slide either side of the viewing area, such that they then
support a third coverslip placed over them. An appropriate
sized thickness of water is kept.
Due to a Tardigrade’s survival
trick of turning into a tun on drying, from which it can be
readily revived, those studying Tardigrades commonly make
a collection of dry mosses holding particular tardigrade populations.
Not all species of moss are equally
laden with Tardigrades, and Sphagnum species have relatively
few. Although found virtually anywhere, fewer live in woodland
moss as it tends to stay damp whilst water bears ironically
prefer wet moss that regularly dries out! You can find Tardigrades
very close to home including in such damp habitats as household
gutters.
Although they are found on mosses
they are also common in the surface water film of lichens
and liverworts, also in aquatic mud. They are also found in
the soil, and marine Tardigrades are also encountered in the
water in between sand grains.
In Britain over 70 species of Tardigrade
have been identified (over 10x that worldwide). Some have
a certain resemblance to tiny tail-less armadillos in those
that have plates on their dorsal cuticle. Others lack the
cuticle plates. Macrobiotidae family species are devoid of
plates, while in the species of the Echiniscidae family plates
are present. If there is an additional pseudosegmental plate
then the animal is in the genus pseudechiniscus. (For more
detailed species level identification consult the key of Morgan
& King).
They all have 4 pairs of stumpy
legs (although depending on how viewed they will not all be
visible) and each of the legs have claws on the end. (A number
of marine species have suckers rather than claws). The form
of the claws can be diagnostic in some species and some genus
identification.
Tardigrades have a mouth at their
front end, with a pair of stylets that are normally sheathed,
but emerge from the mouth to pierce the cells of plants or
animals. They have a brain and eye spots.
The Phylum Tardigrade is formed
from the order Heterotardigrada (which consists of the sub-orders
Arthrotardigrada and Echiniscoides), and the Order Eutarcligrada.
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