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Tardigrades (Tardigrada), or "water bears", are a phylum of small, segmented animals, similar and related to the Arthropods. Tardigrades were first described by J.A.E. Goeze in 1773 ('kleiner Wasserbär'=little waterbear). The name Tardigrada means "slow walker" and had been given by Spallanzani in 1776. Tardigrades are small animals. The biggest adults may reach a body length of 1.5 mm, the smallest below 0.1 mm. Fresh hatched larvae may be smaller than 0.05 mm.
There are about 750 species described. Tardigrades occur over the whole world, from the high Himalaya (above 6,000 m) to the deep sea (below 4,000 m) and from the polar regions to the equator. Some species are able to dry out (cryptobiosis) and resist extreme conditions (dryness, temperatures above 350 K and below 30 K, radiation). All species need to be in liquid water for active life. Some species are marine, other live in freshwater environments.
The most convenient place to find tardigrades is roof mosses, where they live in the water films around lichens and mosses. Other environments are dunes, beaches, soil and marine or freshwater sediments, where they may occur quite frequent (up to 25,000 animals/l).
Scientists have reported their existence in hot springs, on top of the Himalayas, under layers of solid ice and in ocean sediments. Many species can be found in a milder environment like lakes, ponds and meadows, while others can be found in stone walls and roofs.
It is possible to spend some time in mushroom-smelling forests, scrutinizing mosses on rotten trunks in a search for water bears until coming to the conclusion that they can be found virtually everywhere.
Most tardigrades are phytophage or bacteriophage, some are predators (e.g. Milnesium tardigradum).
Fossil records are sparse. The species Beorn leggi has been recorded from Canadian Cretaceous amber. Aysheaia from the middle Cambrian Burgess shale might be related to tardigrades.
Recent DNA and RNA sequencing data indicate that Tardigrades are the closest sister group to the Arthropods, even closer than Onychophora. These groups have been traditionally thought of as close relatives of the Annelids, but newer schemes consider them Ecdysozoa, together with the roundworms and several smaller phyla. The ecdysozoa-concept resolves the problem of the nematode-like pharynx as well as some data from 18S-rRNA and HOX (homoeobox) gene data, which indicate a relation to roundworms.
Tardigrade characteristics
References
http://www.itis.usda.gov
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