ON THE NATURE AND CAUSES OF DENSITY BANDING IN MASSIVE CORAL SKELETONS

A mechanistic account of skeletal density band formation in massive colonies of Porites is developed by linking observations from vital staining of growing skeletons with measurements of density band characteristics, dissepiment spacing and the depth to which skeleton was occupied by tissue. Three growth processes contribute to density band formation. First, addition of new skeleton at the outer surface of the colony. Second, thickening of existing skeleton through the depth of the tissue layer (tissue usually occupies skeleton formed over the last 4-13 months). Third, the density pattern is then modified by periodic and abrupt uplift of the lower margin of the tissue layer which occurs about every 30 days. Uplift is associated with formation of new dissepiments and produces a fine density band at the lower margin of the tissue layer. Less regular, fine bands are formed within the tissue layer probably as a result of variations in calcification at the colonial surface. X-radiography of skeletal slices compresses and combines the results of these mechanisms making it difficult to distinguish the contribution of individual mechanisms to the overall X-ray image. The relative importance of the three mechanisms - extension, thickening and tissue uplift - to overall skeletal growth may differ between individual colonies. Variability in the way the mechanisms operate and combine may account for widely differing reports of coral density banding.