Penetration of Calcium Carbonate Substrates by the Boring Sponge,Cliona

Abstract

Clionid sponges are noted for their capacity to bore into calcareous substrates. During penetration the substrate is gradually destroyed as the sponge hollows out an extensive system of cavities and tunnels. Preliminary studies revealed that these excavations are produced as small fragments of calcareous material are removed by a special type of amoebocyte which exhibits an etching activity. Cellular penetration occurs along the interface where these cells contact the substrate and is characterized by a unique pattern of cell-substrate relationships.Each active cell releases a substance which dissolves the substrate around its edge, forming a linear etching which corresponds in size and shape to the contours of the cell. Deeper etching occurs as the cell edge, moving gradually downward through the initial etching, sinks into the substrate in a noose-like fashion. During this movement the cell border is drawn down through the slit-like crevice cut by the cell edge, while the nucleus remains in position on the surfaceof the substrate within the original etched outline. Eventually the undercutting action is completed and a small chip is freed from the substrate. Penetration is achieved by the precise cellular release of a chemical agent which dissolves the calcareous substrate along restricted zones of contact between cell and substrate.