To elucidate these patterns, we followed the temporal succession of gene expression during metamorphosis and settlement of coral planulae.Ĭorals have a biphasic life cycle with planktonic larval stages and benthic adults. However, the temporal and spatial patterns of the molecules responsible for skeletal formation and organization are very poorly understood. The secreted biomolecules, collectively termed the skeletal organic matrix (SOM), have long been hypothesized to aid in the stabilization, nucleation, growth, spatial orientation and structural integrity of the skeleton (e.g. These cells appear to control the extracellular precipitation of calcium carbonate as microscopic fibrils that develop into a skeletal framework with structural features that are genetically derived. The biomineralization process in corals is associated with desmocytes, the differentiated calicoblastic cells that secrete proteins, phospholipids and polysaccharides. The interface between the aboral ectoderm and the skeleton, referred to as the calicoblastic epithelium, is the site where extracellular aragonite crystals (orthorhombic CaCO 3) are precipitated and organized to form a macroscopic biomineral structure. In stony symbiotic corals, the endoderm contains intracellular photosynthetic dinoflagellates of the genus Symbiodinium. Their radial body plan consists of only two cell layers: an ectoderm and an endoderm, separated by the mesoglea, a non-cellular gelatinous matrix. Corals (Cnidaria) belong to one of the most anciently diverged invertebrate phyla and are among the first known metazoans to precipitate calcium carbonate.
Although carbonates from corals abound in the fossil record, how these organisms precipitate their mineral skeletons remains enigmatic.
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