WHY DO NO NON-PORIFERAN METAZOANS BUILD THEIR SKELETONS OUT OF SILICA?

With the exception of Porifera, no known metazoans construct their skeletons or shells primarily out of silica, and most marine invertebrates that biomineralize principally use calcium carbonate. This stands in contrast to single-celled marine eukaryotes, in which silica is a favored material for biomineralization and siliceous tests are estimated to have independently originated at least 10 times (Wallace et al., 2015). A number of metazoans construct minor components such as teeth out of biogenic silica, indicating the capacity for silicification is present within Metazoa but not used for the main skeleton or shell. This suggests a selective advantage of calcium carbonate over silica that applies to metazoans and not to single-celled eukaryotes. Several hypotheses have been proposed to address this question but none have been quantitatively tested. Here, we use a simple biomineralization model to test the hypothesis that silica skeletal biomineralization in marine invertebrates is unfavorable due to their size and the low concentration of silica in seawater.­­­­