Gastrotricha: adaptations and evolution in fresh waters

Gastrotricha are a small phylum widely present in aquatic systems as a common and significant component of meiofauna. Two orders are recognized by current taxonomy, Macrodasyida and Chetonotida, very different in morphology, biology and ecology. All but two freshwater species globally known so far (ca. 330 species) belong to Chaetonotida. Most freshwater gastrotrichs are known from eutrophic-mesotrophic habitats in standing waters, where they live as epibenthic and periphytic, and reach considerable densities (up to 170 ind/cm2). But also few interstitial species, even in running waters, have been detected in freshwater sandy sediments. Morphological as well as biological adaptations of freshwater species clearly related to different habitats have been reported: an overview of these adaptations is advanced. Fresh waters appear to host a large majority of species of Chaetonotida, an order also present in marine and brackish waters, but only two species of Macrodasyida, an order entirely marine/estuarine: the question of the relationships and evolution of Gastrotricha between the sea and the inland waters is discussed