Unusual presence and intranuclear storage of silica crystals in the freshwater sponges Ephydatia muelleri and Spongilla lacustris (Porifera: Spongillidae).

Crystal structures have been identified in nuclei of cells from the freshwater sponges Ephydatia muelleri and Spongilla lacustris. Their length is approximately 5 microns and their diameter varies from 0.8 to 1 micron. They occur in several different cell types, among them pinacocytes, cystencytes (E. muelleri) and granulocytes as well as differentiating archaeocytes (S. lacustris). In contrast to earlier reports it is now shown that the crystals are geometrically periodic and anisotropic structures composed of silica. We assume that the crystals play a functional role during differentiation of the cells.