Studies on reef corals. I. Skeleton formation by newly settled planula larva of Pocillopora damicornis

Abstract

Initiation of coral-skeleton formation was studied in the reef-coral Pocillopora damicornis Lamarck. Observations were made on sequential skeletal growth stages of newly settled planula larvae during the first 22 days following settling onto glass microscope slides. Techniques used include phase light microscopy, scanning and transmission electron microscopy, and powder X-ray and selected area electron micro-diffraction. Formation of the skeleton is initiated immediately on settling of the larva. The primary calcareous elements are of two types — flattened spherulitic platelets, and smaller rod-like granules. Rudimentary primary septa are clearly defined within 6 h after settling. Fusion of the primary calcareous elements results in the formation of the larval basal disc within 48 to 72 h. With transmission electron microscopy, this basal disc is found to differ from subsequent adult calcification in (1) considerably lesser degree of mineralization, (2) smaller crystal size, (3) more random orientation of the crystals, and (4) the presence of trace amounts of calcite in addition to aragonite. The basal disc with its septal rudiments constitutes a true larval skeleton, differing in morphology, micro-architecture, and crystal type from the fibrous growth characterizing the adult skeleton.