By M.A. Todaro (2003 - updated November 2019)
Gastrotrichs are microscopic (0.06-3.0 mm in body length) free-living, acoelomate, aquatic worms, characterized by a meiobenthic lifestyle. In marine habitats, they are mainly interstitial, whereas in fresh waters they are a ubiquitous component of periphyton and benthos and to a more limited extent also of 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 are 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 realized within the microphagous, detritivorous, benthic community. Like free-living nematodes, gastrotrichs swallow their food, which is made up of microalgae, bacteria, and small protozoans, using 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, as of November 2019, about 860 species grouped into two orders: Macrodasyida, with about 377 strap-shaped species, all but four of which are marine or estuarine, and Chaetonotida with 483 mostly tenpin-shaped species, three-fourth of which are freshwater. Macrodasyida includes 10 families and 36 genera, whereas Chaetonotida counts 8 families and 32 genera. However, due to the numerous species, and at least one new genus that wait to be described, these statistics should be considered 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 a close ally 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. Later studies based on molecules (18S rRNA and EST) indicated Gastrotricha as members of the Platyzoa. Subsequently, the monophyly of the Platyzoa has been questioned by phylogenomic analyses that found the Gastrotricha to be the sister taxon of the Platyhelminthes, forming a clade named Rouphozoa nested within Spiralia. Such discrepancies between the traditional and the modern views on the gastrotrich phylogeny suggest that further research in this direction is necessary.