Tardigrada

About 800 species of tardigrades have been described from marine, freshwater and terrestrial habitats1,2, although more species are expected to be as yet undescribed. Mature animals are generally less than 500 μm long. The body is short, stout and cylindrical with four pairs of latero-ventral legs. Their movements resemble that of a bear and this is the reason why the group is often called "water bears".

• Tardigrades are mostly plant feeders, because they have mouth stylets that help them to pierce moss leaflet cells to suck the fluid content by pharyngeal pumping action. Some species use the body fluids of other metazoans (rotifers, nematodes) as food.
• Sexes are separate, but the great majority of individuals are females. Two distinct eggs have been observed in tardigrades, equivalent to the summer and winter eggs produced by rotifers. In most true aquatic species the rule is an "external" fertilisation while in semi/aquatic forms fertilisation is internal.
• Few species, such as those of the genus Macrobiotus and Hypsibius, are truly aquatic and there are many semi-aquatic species living in terrestrial habitats. They usually live associated with mosse and lichens, but the thick cellulose walls of mosses do not sustain Tardigrada. The species tolerate a wide range of temperatures, however, their oxygen requirements are not well known yet.
• Tardigrades are renowned for their ability to withstand extreme environmental conditions through cryptobiosis. If the body of tardigrads stored enough food, they are, similar to bdelloid rotifers, capable of surviving periods of dessication, extreme temperatures and pressure as well as other hazards through anabiosis. They also form cysts when damaged or under adverse ecological conditions.
• The taxonomy of this group includes the examination of the structure of the buccal apparatus, claws, filaments and eggs1,2.