Question of the Week, 18.5.2011
Sponge spicules are between some of the most studied biomaterials nowadays, and depending on the species they can be made of silica (Demosponges and Hexactinellid) or calcium carbonate (Calcarea).
This different classes have, of course, different habitats and the presence of either a calcium carbonate or silica spicule endoskeleton can be partially justified by the local environment (deeper vs. shallower waters, for example).
Nevertheless, sponges with silica spicules are far more abundant than calcarea sponges (about 90% higher). When you look at it, silicon (or silicic acid as it appears usually in the sea) concentrations in sea water are quite lower than that of calcium. Besides in an evolution point of view, most skeletons present nowadays are composed of biomaterials with calcium (corals, shells, hydroxyapatite in human bone, for example).
So the question regarding the predominance of sponges with silica spicules still leaves room for debate, with several different opinions like the simple natural selection regarding the fact that in deeper waters where the silica containing sponges are more dominant, there are less predators in opposite to the shallower waters where sponges containing calcium carbonate spicules are generally more proliferous. Other opinions focus on the evolution of highly effective mechanisms of transport and accumulation of silicon intracellularly which allowed the taxa to establish successfully in a small niche.