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Tardigrada
Scientific name: phylum Tardigrada — “slow ones”
Common name: water bears
Description
Tardigrades are microscopic animals found in a variety of marine, freshwater and terrestrial habitats. They are often present in soil and litter, but seldom noticed by us due to their minute size - the majority of tardigrades are 0.3-0.5 mm in length. Tardigrada were first discovered in 1773, when German naturalist and parish priest J.A.E. Goeze observed a freshwater tardigrade and called it a “Kleiner Wasser Bär”, a “little water bear”. In 1776, the Italian abbot and professor of natural history Lazzaro Spallanzani gave these animals their present name — “Il Tardigrado” (“slow ones”) (Nelson and Higgins, 1990).
As these names imply, tardigrades have
a micro-ursine appearance — with four pairs of short
stumpy legs equipped with large claws — and walk slowly.
Tardigrades are often white, or colourless and more or less
transparent; some are brown, yellow, orange, pink, red or
green. The body colour can reflect the pigmentation of the
body cuticle itself, or can be due to the gut contents showing
through the colourless body wall. Tardigrada have no specialized
larval stage, and juveniles look like miniature adults. The
tardigrade taxonomy is based primarily on the morphology of
claws, cuticle, and mouth apparatus.
Notes on biology
Most tardigrades feed on plants, but a few species are carnivorous, feeding on soil nematodes and other minute soil animals, including other tardigrades. The tardigrade mouth contains a pair of stylets, and they feed by piercing plant or animal cells with the stylets, and sucking out the cell contents.
Similar to nematods and rotifers, tardigrads exhibit cryptobiosis (“hidden life”), a bizarre ability to dehydrate and slow down metabolic rate when environmental conditions are unfavourable. In the state of cryptobiosis tardigrades are able to withstand severe dessication, high pressure, and extremely low temperatures (such as being plunged into liquid nitrogen!). A tardigrade can survive in this state for more than 10 years, and sometimes more than a 120 years. Life span is less than a year otherwise.
Where to find them?
Essentially aquatic animals, all Tardigrada
live in the thin water film on the surface of wet terrestrial
substrates.
Tardigrades are commonly found on the surface of mosses, lichens
and liverworts, which cover stones and trees. Some tardigrades
can be also found in the top 1-2 cm of many soils, and in
leaf litter.
To find tardigrades, collect samples of
moss or plant litter. First, spread moss samples in a thin
layer, and leave at room temperature until dry. Then, place
dried sample into a container with water for several hours
to rehydrate. Agitate the sample, then squeeze out and discard
the moss (litter). After the excess water is decanted, the
remaining sediment will contain live tardigrades. To observe
them, examine the sediment in a small amount of water under
a dissecting microscope, at magnification x30 or higher. Boiling
water or 75% alcohol can be used to fix tardigrades. They
should be stored in alcohol for preservation.
Distribution and conservation
The ability of tardigrades to withstand
long periods of dehydration probably contributes to their
dispersal throughout the world, as eggs, cysts and anhydrobiotic
adults are disseminated by wind, water, and migrating animals.
Many genera and species are cosmopolitan. Tardigrades are
found throughout New Zealand. The diversity and ecology of
Tardigrada are not well studied, and their role in ecosystems
and conservation value are unknown.
Further information on New Zealand
Tradigrada:
Horning D.S. Jr, Shuster R.O., Grigarick
A.A. 1978. Tardigrada of New Zealand. NZ Journal of Zoology
5, p. 185-280.
McInnes S.J., Pugh P.J.A. 1998. Biogeography
of limno-terrestrial Tardigrada, with particular reference
to the Antarctic fauna. Journal of Biogeography 25, p. 31-36.
Nelson D.R., Higgins R.P. 1990. Tardigrada.
In: Dindal D.L. (ed.). Soil Biology Guide. J. Wiley &
Sons, New York, p. 393-419.
Pilato G., Binda M.G. 1997. Remarks on some
tardigrades of New Zealand with the description of Hypsibius
novaezeelandiae n. sp. (Eutardigrada). NZ Journal of
Zoology 24, p. 65-67.
Suren A.M. 1992. Meiofaunal communities
associated with bryophites and gravels in shaded and unshaded
alpine streams in New Zealand. NZ Journal of Marine and Freshwater
Research 26, p. 115-125.
Tardigrada Resources on the Web:
Tardigrades, Museum of New Zealand Te Papa Tongarewa, Wellington, NZ. Information on Tardigrada of New Zealand.
Tardigrades (Tardigrada): Images, animations, and full text, by Martin and Sabine Mach.
Wonderfully illustrated overview of Tardigrades.
Tardigrades: Bears of the Moss, by William R. Miller. A very informative description of appearance, biology, distribution, and study of tardigrades.
The Water Bears (Phylum Tardigrada ).
An introduction to the biology, classification and ecology of Tardigrada.
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