Fresh-water sponges (Spongillidae) are one of the most common and abundant groups of hard substratum inhabitants in water ecosystems. Their filtration activity and excretion of biologically active metabolites make sponges powerful agents of environment formation. They provide specific conditions for life of many plants and animals. The current study dealt with the main characters of ecosystems where sponges are central, and with their role in increasing the biodiversity of hard substratum inhabitants. The organisms trophically linked with sponges include both heterotrophs and autotrophs. Autotroph inhabitants of sponges themselves can be divided by two groups: obligate intracellular symbionts and facultative symbionts of sponge body cavities (these are plankton algae taken in by a sponge and living for some time in its subdermal and intercellular cavities). It was found that different species of algae could be intracellular symbionts of sponges. These included Choricystis and Chlorella (I.Yu. Kostikov is thanked for helping with identifications). The presence of numerous living plankton algae in internal cavities of sponges can be explained by the filtration activity of sponges and by favorable habitat conditions (a stable medium saturated with carbon dioxide and sponge metabolites). Invertebrates associated with sponges can also be divided into several ecological groups, first of all by their position in trophic chains. These groups are: species obligately feeding on sponges; species facultatively feeding on sponges; species feeding on detritus; species feeding on plants; predators. The localities where they live link them all with sponges, where they find shelter and high concentrations of food. In different habitats the species composition of organisms associated with sponges can vary, but their structure is sufficiently stable and in almost all cases the groups listed are involved. The species composition of the organisms feeding obligately on sponges is the most stable: the same species occur in different ecological conditions. Species commonly associated with sponges are larvae of Sisyra sp. (Burmister) (Neuroptera), Athripsodes alboguttatus (Hagen) (Trichoptera), Xenochironomus xenolabis (Kief.), Glyptotendipes glaucus (Meigen) and Demeijera rufipes (L.) (Chironomidae). Sponge colonies are thus complex aggregates of organisms with lives tightly connected to sponges. The species diversity of these aggregates seems to be lower than on Dreisena colonies for example, but colonization of substrata by sponges considerably increases biomass and biodiversity of periphyton.