Silicification Processes in Sponges: Geodia Asters and the Problem of Morphogenesis of Spicule Shape

Abstract

The deposition of silica by sponges involves a special membrane and an internal organic matrix, the axial filament. A hexagonal cross sectional shape of the filament may act as a basis, through differential growth, of the formation of a trigonal shape. Dynamic processes involved in silica deposition include membrane and organic filament syntheses and assembly; transport, accumulation and polymerization of silicic acid; and termination of these processes. The theoretical details of these processes are discussed. The pattern of development of complex siliceous structures (asters) in Geodia cydonium involve the formation of short, terminal processes which then become fused together and finally form a star-shaped pattern. It is difficult to invoke the presence of a centrally located silicification center as a basis for this sequence of morphogenesis. Such centers appear to be present during the development of other morphological types of siliceous sponge structures and such a center may be present in the early stages of development of Geodia asters before the maturation of the tips of the structures. Both sodium and calcium ions appear to interact with the sponge silicification system as determined by their effects on spicules formed in the presence of germanic acid. The absence of central bulbs in marine spicules grown in germanic acid may be partly due to the high concentration of these (and other) ions in sea water.