The limno-terrestrial species are classified into two groups; Heterotardigrada and Eutardigrada.
In the heterotardigrades, which include the genus Echiniscus, the body is covered with sclerotizised plates and the head is equipped with two anterior cirri. The eutardigrades, to which Macrobiotus and Hybsibius belong, have an elastic cuticle and lack cirri on the head.
The terrestrial species may sometimes occur in rather high population densities. In a pine-forest soil in central Sweden 20 000-200 000 tardigrades were found per square-metre (Sohlenius 1982). In an ombrotrophic mire in northern Sweden (Abisko) 400000 tardigrades were found per square-meter (Sohlenius et al. 1997). In arable land tardigrades only occur occasionally.
rying, dispersal, food
The terrestrial tardigrades have a very high ability to survive extreme conditions in a dehydrated state, which explains their occurrence in tufts of lichens and mosses on stones and on mountain peaks and in polar areas. In Antarctic nunataks tardigrades were found in 40 % of the examined samples with population densities up to 580 specimens per gram soil material (dry weight) (Sohlenius et al. 1996). Because of their small size and ability to dehydrate they have a very great ability of long distance dispersal and many species thus have a cosmopolitic distribution.
Some species are predacious and feed on other microscopic animals such as nematodes. It has been demonstrated that tardigrades by their consumption can reduce the number of nematodes. However, the tardigrades themselves are vulnerable to predation by micro-arthropods (mites and springtails).
Since tardigrades are rather neglected in soil ecological studies, comparatively little is known about their ecology. They appear to be sensitive to air pollution and moss living species may be a useful tool to indicate the occurrence of harmful substances in the air.
Text och photo: Björn Sohlenius, Department of Inevertebrate Zoology, Naturhistoriska riksmuseet