 Cyanobacteria contain toxins and indigestible mucopolysaccharides, and green algae have cellulose in their cell walls ("Theodoxus" species have no cellulase enzymes to digest cellulose).They also graze on zygotes and germlings of brown alga "Fucus vesiculosus", when the alga is small up to 1 mm.Peters and Traunspurger (2012) studied the effect of the grazing of "Theodoxus fluviatilis" on epilithic meiofauna and algae.