Calcite in the skeletons of annelids

Calcite is a common biomineral in animals. In annelids, as in many other invertebrates, calcite is mostly used to reinforce a skeleton. Calcite skeletons first appeared in polychaetes in the Ordovician. Most advanced skeletons containing calcite occur in serpulid polychaetes. Mg-Calcite crystals in their tubes are deposited between thin organic sheets and can have biologically controlled orientation. There are at least six different calcitic animal skeletal ultrastructures in serpulids, one being unique to them. The biomineralization of calcite in serpulids is probably organic matrix mediated, but can be influenced by seawater chemistry and temperature. Serpulids can locally be important calcifiers and producers of calcitic sediments, especially in temperate seas. Calcitic serpulid species can also build small reefs.