Cellular mechanisms of coral calcification

Calcification is a basic process by which the controlled deposition of calcium salts provides the basis of supportive structures in both vertebrate and invertebrate organisms. While vertebrates use calcium phosphate in their skeletons, many invertebrates use calcium carbonate in the form of calcite and/or aragonite. It is assumed that vertebrates and invertebrates calcifying structures evolved separately and represent discontinuous evolutionary systems. However, the fact that some components of the calcification repertoire such as carbonic anhydrases and calcium homeostasis mechanisms have an ancestral origin suggests that while extracellular components of calcifying matrices have been recruited independently in different taxa, basic cellular ion homeostasis and transport mechanisms are likely to be conserved.\ud \ud It has been suggested that calcification in corals evolved from an ancestral calcium sink mechanism. As well as being a successful evolutionary strategy, the evolution of scleractinian exoskeletons has had important ecological consequences, as corals constitute the framework of tropical reef ecosystems. Corals are the only anthozoan group in which settlement and tissue reorganization during metamorphosis correlates with the deposition of a juvenile skeleton. While the oral ectoderm retains the columnar organization seen in planulae, the aboral ectoderm is transformed from a columnar epithelium into a squamous calcifying cell type known as the “calicoblast”. This cell type secrets and matures an organic matrix that regulates the nucleation of calcium carbonate crystals.\ud \ud This study focuses on metamorphosis and the early stages of calcification in the scleractinian coral Acropora millepora, and focuses on proteins of the galaxin family and EF-hand calcium sensors as potential regulators of larval morphogenesis and skeleton deposition. In parallel, primary cell cultures were established from five key developmental stages and gene expression microarray analysis was used to investigate changes in gene expression during metamorphosis and post-settlement life. Although\ud the results reported here reveal a high level of complexity, they will facilitate the elucidation of the gene networks involved in processes such as lineage specific cell differentiation and juvenile calcification