The growth and formation of branch tips of Pocillopora damicornis (Linnaeus)

Abstract

The rate of skeletal extension and morphology of branch tips of Pocillopora damicornis (Linnaeus) was investigated using alizarin red S staining, light microscopy, and scanning electron microscopy. Branch tips were found to extend along an apical growth axis with little or no growth in other directions. There was no difference between the lateral extension of colonies from either fore- or back-reef sites. The skeletal extension of both individual branch tips (0.2–2.9 mm · 35 days −1) and individual colonies (0.35–1.89 mm · 35 days −1) was very variable. Examination of the apices of individual branches revealed seven morphological categories of branch tip that characterized their growth and bifurcation. Five of these branch-tip categories had a mean skeletal extension of between 1.54–1.79 mm · 35 days −1, and the two others between 0.54–0.75 mm · 35 days −1. Variation between the skeletal growth of individual colonies was caused by a predominance of either high (> 2.0 mm) or low (< 1.0 mm) extension of branch tips. The formation of the tabulate floor of corallites was observed to take place by the deposition of a primary layer followed by the deposition of a secondary layer. The orientation of individual crystals in the primary and secondary layers was at right angles to each other. New corallites were formed as outgrowths of the older corallite walls at the apex of branches and on the coenosteum between corallites on the sides of branch tips. A scheme describing the growth and branching of branch tips of P. damicornis is developed and the relationship between the measurement of coral growth, endogenous cycles of growth, and environmental parameters is discussed.