Interstitial gastrotrichs from lotic Italian fresh waters

Twenty-three species of gastrotrichs were found in samples collected from 10 of the 19 water-courses examined. The richness of species and the faunistic composition are comparable with those in lentic sediments, even if specimens are more numerous in the latter. The Zambra riverlet in the Pisa Mountains is the richest (17 species) of the sampling biotopes. As in freshwater lentic environments, the presence of organic detritus causes an increase in both the number of specimens and species, whereas the fine fraction of the sediment (silt) leads to their sharp decrease. Lepidodermella squamata, Chaetonotus maximus and C. persetosus have proved to be ubiquitous species, while C. fluviatilis and Ichthydium squamigerum were only found in sand. Most species show a cuticular covering with a particular morphology which has probably favoured their moving from the surface of sediment towards interstitial spaces.