CHARACTERISTICS OF CARBONATES OF GORGONIAN AXES (COELENTERATA, OCTOCORALLIA)

Axial skeletons of 13 species of gorgonians were examined by SEM, X-ray diffraction, and polarizing microscopy. Calcite, though occasionally amorphous is the major biogenic carbonate of axes. Non-biogenic mineralization may be calcitic, amorphous, or aragonitic. Axes of Plexaurella contain numerous, lenticular, calcitic loculi of spherulitic prismatic crystals. Mineralization in Ellisella barbadensis is in the form of concentric layers of perpendicularly oriented, lath-shaped crystals that extend through the annulations. Numerous longitudinally oriented collagen fibers perforate the crystals. Mineralization in Lophogorgia cardinalis is in the form of crescentic, shield-shaped, flat, laminated plates composed of alternating layers of calcified (sheathed) and uncalcified collagen fibers. The fibrous component in all species is oriented parallel to the longitudinal axis of axes. Fine striae on transversely fractured crystals of species of Plexaurella and E. barbadensis probably represent daily growth banding. The functional associations of mineral forms with stiffness, resistance to twist, and water movement zones are discussed.