Latin: tardus = slow; gradu = step
Water bear overview
Tardigrades are commonly
known as water bears. There are just over 1000 living species; around 50 are known in the UK, mainly
terrestrial, and they range in length from 0.05 - 1.2 mm (no bigger than the
dot over this i).
similarities to the Rotifers, e.g., cryptobiosis;
but they are usually placed as an offshoot of the Annelid-Arthropod line, and some say they are related
to the Onychophora.
Tardigrada natural history, life cycle and behaviour
They are mainly found in the water films surrounding algae,
mosses, lichens and sand grains. These are all non-permanent habitats, and to
survive the Tardigrades have evolved resistant stages. They can allow their
body water content to fall from 85% to just 3%, and can withstand temperatures
well below freezing and above boiling. Macrobiotus huflandi can survive after being immersed in liquid helium- a temperature of -271oC!
During the resistant stage they pull in
their legs and curl into a ball. There are records of water bears on a dried
museum specimen returning to life after being dried out for 120 years.
They are unable to swim, and move around by clinging to the surface of mosses etc. with their claws.
The cuticle is moulted
about four times during the life cycle. Some species lay their eggs inside the
moulted cuticle, into which males shed sperm. It appears that egg laying and
defecation can only happen during moulting.
are also highly resistant to desiccation and temperature extremes. The eggs hatch into miniature adults.
They feed on plant or animal juices
by piercing the cell or body wall with their stylets and sucking out the
contents, though one species is known to be a parasite of sea cucumbers.
Water bear body
The legs end in 4 - 8 claws. The structure of the claws can be used in identification, so a microscope of high power is needed.
They have a muscular pharynx (see above left) which allows them to suck up plant juices.