Some aspects of silica deposition in lithistid demosponge desmas.

Transmitted light microscope and SEM observations of various growth stages, including very young forms, of lithistid demosponge spicules called desmas provided a wealth of new observations on silica deposition in desmas of most lithistid demosponge groups. In typical (pachastrellid) demosponges a basic feature of silica deposition in young spicules is the formation of silica granules (100-160 nm in diameter) deposited in more or less regular concentric layers. Further growth stages in typical demosponges are similar, only silica granules are smaller and more densely packed. The shape of the spicule is controlled by an organic axial filament, while features of the outer spicule surface are also determined by silicalemma. In lithistid desmas the early stage of silica deposition is controlled by an organic axial filament or, in some cases, dispersed organic molecules only. The next step, after early arrest of axial filament growth and its total encasing by silica, is the deposition of various silica granules (40-300 nm in diameter), spheres, and/or cylinders (1,300-3,330 nm), which are either the result of precipitation or the effect of templating by organic molecules (proteins and polysaccharides), without direct control by the silicalemma. The later stages of desma growth are under direct control of the silicalemma, which molds secondary branches and/or elements of sculpture of the desmas. The tips of desmas, which articulate with older desmas, are also controlled by local spatial relationships. Differentiation of morphological forms of silica in desmas, which is at least genus-specific, clearly supports the polyphyletic nature of lithistid sponges.