An ultrastructural study of silica deposition in the freshwater sponge Spongilla lacustris

Abstract

The secretion of siliceous spicules in Spongilla lacustris involves an axial filament and a unit type membrane, the silicalemma. There are no obvious connections between the plasma membrane and silicalemma. Ultrastructural studies of spicules grown in pond water and in germanium indicate that silica is deposited by the silicalemma; the axial filament may function primarily in establishing the overall geometry of the spicule. Cytoplasmic, membranous pockets and vesicles possibly form connections between the plasma membrane and the silicalemma. Germanium inhibits the formation of axial filaments and the silicalemma and prevents silica deposition even when filaments and the silicalemma have formed. In the presence of germanium, vesicles, membranous pockets, and vacuole-like expansions of the silicalemma are prevalent and possibly represent functionless stages in silica deposition. The silica of young spicules contains subunits which are interpreted as sectioning artifacts.