Skeletal formation in the modern but ultraconservative chaetetid sponge Spirastrella (Acanthochaetetes) wellsi (Demospongiae, Porifera)

The modern hadromerid coralline sponge Spirastrella (Acanthochaetetes) wellsi exhibits a unique secondary high-Mg calcite (> 19 mol % MgCO3) basal skeleton. The basal skeleton is constructed of bundles of elongated crystals more or less tangentially orientated. The initial formation of these crystals is controlled by soluble highly acidic aspartic and glutamic-rich (40%) macromolecules. The skeletal mineralization occurs in four different loci: in the top of the calicles, at the tabulae, on collagenous anchor fibres, and within closed spaces between the tabulae. The calicle walls are formed on the uppermost top of the basal skeleton as a continous process. Based on long term stainings with Ca2+-chelating fluorochroms (calcein, chlorotetracyclines) the growth rate of this sponge is extremely low with ca. 50-100 mu m/a. The skeletal formation takes places outside the sponge, within a narrow zone (300-500 nm) between the basopinacoderm and the mature basal skeleton. The sponge produces thread-like folded templates ('spaghetti fibres') of 0,5-2 mu m size, the shape controlling insoluble organic matrix. These templates become mineralized in a first step as MgCO3, then are stretched. A soluble organic matrix is also secreted, and remains are included inside the mineralized skeleton. This organic matrix consists of in a complex mixture containing small very acidic proteins (5, 13, 31 KD; 40% Asp and Glu and therefore most probably Ca2+-binding) and high molecular weight glycoproteins among several other organic compounds. The mature crystals are high-Mg calcites. During calcification large cells with large reserve granules (LCG) are always present in a tight connection with the basopinacoderm. These cells form also the collagenous anchor fibres. Primary tabulae are formed by a non-collagenous organic sheet. Calcification happens only when LCG cells are enriched on the organic sheet. Randomly oriented high-Mg calcite crystals are growing on the collagenous anchor fibres. The same type of the mineralization is observed within the spaces of the tabulae. This particular case of mineralization is controlled by decaying sponge tissue (ammonification). The gamma(13)C values are in equilibrium with the ambient sea water and vary between +3.2 and +2.8 parts per thousand. The mode of mineralization of the basal skeleton can be described as biologically induced resp. matrix mediated.