The phylum Gastrotricha includes around 450 to 700 known species of small marine, brackish, and freshwater animals. Most are less than 1 mm long, although a few reach 3 mm. Gastrotrichs live in the watery spaces between grains of sand in loose sediments, in surface detritus, and on aquatic plants; a few are planktonic. (Zrzavy 2002; Brusca and Brusca 2003; Todaro et al. 2006) In marine interstitial habitats, gastrotrichs typically account for around 1 to 8% of the benthic meiofauna numerically, ranking third in abundance in the meiofauna (behind the Nematoda and the harpacticoid copepods), and in some studies they have ranked first or second. The population density of gastrotrichs in freshwater habitats may reach over 150 individuals per 10 cm2, ranking gastrotrichs among the five most abundant groups. (Todaro et al. 2006 and references therein)
The gastrotrich body includes a head and a trunk (and sometimes a distinct tail) and the body sports various combinations of spines, bristles, and scales or plates. Nearly all gastrotrichs bear two or more "adhesive tubes" with glands that secrete attachment and releaser substances used for temporarily attaching to objects. Adults lack a coelom (body cavity). Gastrotrichs feed on nearly any organic material, alive or dead, that is of appropriate size (e.g., protists, unicellular algae, bacteria, detritus). As is the case for many groups of tiny animals, circulation and gas exchange are accomplished by simple diffusion rather than by specialized structures. Sensory structures are mainly tactile and consist of spines and bristles concentrated especially on the head. Most gastrotrich species are hermaphroditic (or are known only from parthenogenetic females) and mating individuals mutually cross-fertilize. Zygotes (fertilized eggs) are released singly or a few at a time by rupture of the body wall. Parthenogenesis predominates among freshwater forms, with sperm-producing hermaphrodites appearing only infrequently. Development is direct (i.e., there is no distinct larval stage). Time to hatching is variable, but once juveniles hatch out, maturation is rapid and sexual maturity is usually reached within a few days. (Zrzavy 2002; Brusca and Brusca 2003; Todaro et al. 2006)
As of 2010, the phylogenetic placement of gastrotrichs was still not clearly resolved, but a recent analysis of a large data set including multiple nuclear genes and 90 taxa representing more than 2 dozen bilaterian phyla found moderately strong support for Gnathostomulida as the sister taxon to Gastrotricha, a result that is at least consistent with a number of previous analyses based on both morphological and molecular data (Paps et al. 2009 and references therein).
Todaro and Hummon (2008) provide a dichotomous key to gastrotrich genera; Smith (2001) includes a dichotomous key to genera known from freshwater in the United States. Balsamo et al. (2008) review the limited information available on the biogeography of freshwater gastrotrichs. Balsamo et al. (2009) include a world list of known freshwater gastrotrichs.