Ultrastructure of cells associated with excavation of calcium carbonate substrates by boring sponges

Boring sponges excavate calcium carbonate substrates by chemical and mechanical methods. The chemical phase of excavation is accomplished at the cellular level by a localized secretion (possibly an acid or an enzyme) at the cell-substrate interface (Cotte, 1902; Nassonow, 1883, 1924; Warburton, 1958; Rützler & Rieger, 1973; Cobb, 1969, 1971, 1975). Chemical etching detaches a chip of calcium carbonate (about 40–60 µm diameter) which is then mechanically removed from the substrate through the sponge tissue and out through the excurrent canal system. Only 2–3% of the substrate is dissolved by the etching cells (Rützler & Rieger, 1973), while the rest is removed as characteristically shaped chips. Recent studies of the mechanism of sponge boring have denned (using light microscopy) the spatial relationships sponge cells establish with the substrate (Cobb, 1971), identified (using transmission electron microscopy) the cell type (etching cell) responsible for chemical etching (Rützler & Rieger, 1973), and described (using scanning electron microscopy) the architectural nature of substrate destruction by sponges (Cobb, 1969, 1971, 1975; Rützler & Rieger, 1973; Pomponi, 1976, 1977, Ph.D. Thesis, University of Miami).