Monorhaphis chuni sponges have evolved the genes to synthesize giant basal spicule that are nearly 3 meters long. The sponge’s cells absorb minute traces of silicic acid from sea water and use it to produce a monolithic amorphous silica rod by concentric deposition. Sclerocytes (a type of cell) form rings around a nascent spicule and produce a protein called silicatein to convert silicic acid to amorphous silica. Each ring is only 10 microns wide but repeated cycles result in the macroscopic structure that supports the sponge above the sea floor.
“The giant basal spicule of the hexactinellid sponge Monorhaphis chuni represents the longest natural siliceous structure on Earth [3 m long]. This spicule is composed of concentrically arranged lamellae that are approximately 10 ?μm thick…It is shown that the formation of an outermost lamella begins with the association of cell clusters with the surface of the thickening and/or growing spicule. The cells release silica for controlled formation of a lamella. The pericellular (silica) material fuses to a delimited and textured layer of silica with depressions approximately 20–30? μm in diameter. The newly formed layer initially displays 40? μm wide, well-structured banded ribbons and only attains its plain surface in a final step…the depressions are the nests for the silica-forming cells and that silica formation starts with a direct association of silica-forming cells with the outer surface of the spicule, where they remain and initiate the development of the next lamellae.” (Wang et al. 2011:2047)
“Among the metazoans, the siliceous sponges (Porifera:Demospongiae and Hexactinellida) are the only taxa that build their skeleton (spiculae) of amorphous silica (Morse, 1999; Uriz, 2006; Müller et al., 2007d; Ehrlich et al., 2010b). Their silicification deposition–polycondensation pathway is distinguished from others by their ability to take up and accumulate silicic acid from a very silicon-poor aqueous environment (5 ?μmol?l–1)”. (Wang et al. 2011:2047)