The ultrastructure of the skeleton and skeletogenic tissues of the temperate coral Caryophyllia smithii

The skeleton and calicoblastic ectoderm of the scleractinian non-zooxanthellate coral Caryophyllia smithii were investigated by light microscopy, scanning and transmission electron microscopy. Except for some costal spines, the skeleton was fasciculate. Fasciculi were made up of bundles of crystalline needles, each crystalline needle consisting of a number of linear series of small (<1 μm) rounded crystals. Fractured skeletons showed the fasciculi to be arranged into layers and that within some septa, theca and costal spines there were spaces that contained neither mineral nor organic matter. These spaces could also be found at the growing edges of septa and theca. Demineralization of the skeleton revealed an organic matrix whose configuration mirrored the architecture of the skeleton. In areas of the skeleton where deposition was occurring the overlying calicoblastic ectoderm was relatively thin with prominent intercellular spaces and secretory vesicles. In contrast, over non-depositing areas the calicoblastic ectoderm was thick and contained residual bodies, nematocysts and membrane-bound granules. The results are compared and contrasted with those from scleractinian corals that have endosymbiotic zooxanthellae.