The consortium of the sponge Ephydatia fluviatilis (L.) living on the common reed Phragmites australis in Lake Piediluco (central Italy)

Lake Piediluco ( Central Italy) displays a discontinuous reed-belt along its shore, the dominant macrophyte being Phragmites australis (Cav.) Trin ex Steud., the reeds of which support the growth of the sponge Ephydatia fluviatilis ( L.). Samplings were carried out at ten stations located along the reed-belt with the aim of investigating the 'consortium', which is the result of the association between the edificatory E. fluviatilis and the related organisms. These last were considered at three different levels: ( a) spatial localisation (epibionts, endobionts); (b) kind of association ( occasional, strictly linked); ( c) model of organisation ( unicellular, multicellular). The associated fauna was investigated by considering both sponge biomass and life-cycle. The faunal composition of associated organisms and their quantitative rates were determined, and statistical differences among stations were tested by using 1-way ANOVA (p < 0.05). It emerged that there were significant differences among stations and that the dipteran Chironomus sp. was the most common taxon, its presence being independent of the spatial gradient. By contrast, the mean density ( total number of hosted macroinvertebrates/spongewet-weight) did not vary significantly from station to station. A positive relationship between sponge biomass (wet-weight) and the total number of taxa was found, but a sponge weight of 2000 g seemed to provide a better habitat for macroinvertebrates than higher values. This finding emerged from the non-linear relationship between sponge biomass and the number of hosted organisms. Histological analysis of the gut content of some associated organisms enhanced knowledge of their ability to utilize sponge tissues as food.