Ability of mobile cells of the freshwater sponge Ephydatia fluviatilis (Porifera, Demospongiae) to digest diatoms

Abstract

In Lake Piediluco (Central Italy) the reed Phragmites australis is colonized by the freshwater sponge Ephydatia fluviatilis, which assumes a different shape according to the season: highly expanded in June and July; gradually reducing in August; fairly round and minute (from less than 1 to 10 mm) from September to March, following the die‐back of the reeds. Ultrastructural investigations carried out on the minute sponges gave evidence that they were functional and contained diatoms both free in the sponge mesohyl and enclosed in cytoplasmic vacuoles of the wandering cells. Vacuoles contained a single diatom of different size (about 9×5 urn the largest; about 5.7 × 3.8 μm the smallest) and in various phases of degradation. Although sponges are known to host auto‐ and heterotrophic symbionts, this is the first report of their progressive digestion of endocellular diatoms up to the final fragmentation of their frustule.