Gastrotricha are microscopic (0.06-3.0 mm in body length) free-living, acoelomate, aquatic worms, characterised by a meiobenthic life style. In marine habitats they are mainly interstitial, whereas in fresh waters they are an ubiquitous component of periphyton and benthos and to a more limited extend also of the plankton. In marine sediments, gastrotrich density may reach 364 individuals/10 cm2; typically they rank third in abundance following the Nematoda and the harpacticoid Copepoda, although in several instances they have been found to be first or the second most abundant meiofaunal taxon. In freshwater ecosystems population density may reach 158 ind/10 cm2 making the taxon rank among the top 5 most abundant groups. In aquatic environments the ecological role of the gastrotrichs is realised within the microphagous, detritivorous, benthic community. Like free-living nematodes, gastrotrichs swallow their food, which is made up of microalgae, bacteria and small protozoans, by means of the powerful sucking action of the muscular pharynx, and in turn they are preyed upon by turbellarians and small macrofauna.
The phylum is cosmopolitan with about 765 species grouped into two orders: Macrodasyida, with 310 strap-shaped species, all but two of which are marine or estuarine, and Chaetonotida with 455 tenpin shaped species, three-fourth of which are freshwater. Macrodasyida include 8 families and 32 genera, whereas Chaetonotida counts 8 families and 31 genera. However due to the numerous species, and at least three new genera that wait to be described, these statistics should be considered as very conservative, particularly for the Chaetonotida. Despite their diversity and abundance, the phylogenetic relationships of the Gastrotricha are still unclear. Based on morphology, most researchers, though considering the evolutionary connections of these worms to be quite obscure, regard them as close allies of the Gnatostomulida, the Rotifera, or the Nematoda. On the other hand, a re-examination of the "Aschelminthes" phylogeny based on the SSU rRNA gene sequence analysis showed the Gastrotricha as the sister taxon of the Platyhelminthes, while later studies placed them close to the Ecdysozoa, the Lophotrochozoa, or neither one. Most recent studies based on molecules (18S rRNA and EST) indicate Gastrotricha as members of the Platyzoa. Such discrepancies between the traditional and the modern views on the gastrotrich phylogeny suggest that further research in this direction is necessary.