SKELETON AND SCLERITE FORMATION IN THE PRECIOUS RED CORAL CORALLIUM-RUBRUM

The carbonate skeleton of the gorgonian coral Corallium rubrum (L.) is composed of both a skeletal axis and numerous sclerites scattered in the mesoglea. Studies carried out on these skeletal elements and their associated tissues using microscopy a-nd X-ray microanalysis, suggest a close relationship between the process of sclerite formation and sk eletogenesis. The skeleton is surrounded by an axial epithelium compos ed of a single cell type. These cells associate intimately with mesogl eal sclerites and scleroblasts, incorporating them into a nascent skel eton at the branch tip. Subsequent (sub-apical) growth appears to occu r solely through the agency of the axis epithelial cells that serve to physically separate mesogleal sclerites and scleroblasts from contact with the axis. The epithelium is associated with the production of la yered calcite crystals and irregular protuberances that constitute the mature, calcareous skeleton. Free sclerites in the mesoglea appear to be the product of multiple cells that are cytologically indistinguish able from those in the axis epithelium. Like the axis, sclerites are p roduced as layers of calcite crystals with irregular protuberances. Th e protuberances differ only slightly from those of the axis, and the s keleton is mineralogically indistinguishable from the sclerites. Thus, the skeleton of red coral is not primarily the product of fused scler ites. Instead, we suggest that the axis epithelium treats the incipien t skeleton as if it were the core of a single sclerite, and conversely , that the mesogleal scleroblasts of C. rubrum constitute a fragmented axis epithelium.