Studies on reef corals. III. Fine structural changes of calicoblast cells in Pocillopora damicornis during settling and calcification

The site of reef-coral calcification has been studied in the branching coral Pocillopora damicornis Lamarck. Electron microscopy and X-ray microprobe analysis were performed on the calicoblast epidermis of newly settled larval stages and of adult coral. During settling, the heterogeneous columnar cell composition of the planktonic larva epidermis is replaced by a simple epithelium consisting of a single cell type, the calicoblast cell. Metamorphosis appears tightly linked to settling, with cell changes occurring within hours after attachment, and is marked by the appearance of a new secretory cell. The calicoblast cell of the adult coral is extremely flattened, and interdigitates extensively with adjacent calicoblast cells. This cell possesses a featureless plasma membrane lacking microvilli or flagella. It characteristically contains large membrane-bound vesicles with homogeneously fine granular contents. Preliminary microprobe analysis indicated a higher calcium content in these vesicles than in surrounding tissue; however, not in concentrations suggesting calcium-carbonate precipitation. They may represent sites of organic matrix synthesis. The calicoblast epidermis is separated from the underlying coral skeleton by a narrow gap. This gap appeared devoid of substructure, either organic or inorganic. The coral soft tissues are attached to the skeleton by mesogleal attachment processes, the desmoidal processes. These consist of a complex fibrous network originating in the mesoglea, and inserting onto the skeleton via specialized attachment regions consisting of electron-opaque membranous plaques. Skeletogenesis in reef-corals probably occurs extra-cellularly, external to the calicoblast epidermis, by simple overgrowth of the skeleton.