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HISTORY
Discovery
HABITAT
Where they are found
SCIENTIFIC STUDY
Ability to Adapt
Characteristics
Reproduction
Cryptobiosis Summary
CITATIONS
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Water bears feed on the fluids of plant and
animal cells, fungi and stuff. 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 lets them suck up and then ingest the
internal contents of their food. Some tardigrades eat entire live organisms,
such as rotifers, nematodes or other water bears.
Typically water bears are “dioecious”, which means reproducing with both
male and females. Each has a single organ which lies around the gut.
However, some species have both parts. 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.
Water bears grow differently, “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 digestive system may also be minimal.
The widespread distribution of water bears may be attributed to the fact
that their eggs, and “Tuns” (the hibernating forms of them) are light
enough to be distributed by wind or animals for great distances possibly in
the upper atmosphere. There is however, little evidence for this. They are
also now thought to be a 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. |