On the origin of metazoan calcification

Abstract

Mucous materials excreted by external epithelia of the metazoans form highly diverse macromolecular mixtures providing binding sites for potentially harmful invaders such as viruses and bacteria. In the human respiratory system, for example, the removal of these invaders with the mucus to which they are bound provides an important protective mechanism. We have evidence showing that in the mucus of marine invertebrates special moieties are capable of specifically trapping and binding calcium carbonate crystals and their nuclei. Removal of these crystalline materials, e.g. by ciliary activity, would protect the soft tissues from spontaneous mineral overgrowth when the supersaturation of the ambient water is sufficiently high. We propose the term anti-calcification to denote this protective function. The available geological evidence suggests that in the Late Precambrian the spontaneous crystallisation of calcium carbonate in the ocean water was more widespread than it is now. The sudden appearance of calcified skeletons among many different invertebrate taxa at the Precambrian Cambrian transition may have required minor reorganizations of pre-existing secretory functions. In particular, factors of the skeletal organic matrix responsible for regulating crystal growth by inhibition may be derived from the anti-calcifying mucous excretions. We tested this hypothesis by comparing the serological properties of skeletal water-soluble matrices (SM) and mucous excretions of three invertebrates -the scleractinian coral Galaxea fascicularis, and the bivalve mollusks Mytilus edulis and Mercenaria mercenaria. Cross-reactivities recorded between muci and SM suggested that these different secretory products have a high degree of homology. Furthermore, freshly extracted muci of Mytilus were found to inhibit calcium carbonate precipitation in solution.