THE PHYSIOLOGY OF SKELETON FORMATION IN CORALS. II. CALCIUM DEPOSITION BY HERMATYPIC CORALS UNDER VARIOUS CONDITIONS IN THE REEF

Abstract

Ca uptakes of 13 hermatypic corals and 2 hydro-corallines were measured by a modified Ca-45 method under conditions approximating those of natural environment of the reef in experiments lasting 4-8 hours. When measured on the basis of N content, the growth rates of the branching corals were higher than those of the massive corals. On the basis of area, however, the latter appear to grow as fast or faster than the former. Light intensity had a profound influence on the growth rate under the conditions of the experiments. All corals tested deposited Ca fastest in sunlight, less during cloudy weather and least in darkness. Bleached zooxanthella-less colonies deposited Ca at lower rates in the light than normal colonies with zooxanthellae did in darkness. Systematic calcification gradients were observed in branching corals but not in massive species. Although there was a considerable variance in the growth rate from place to place within individual colonies large and significant differences in the growth rate were observed between individual colonies of the same species, size and shape under similar conditions. It is suggested that the organization of the skeleton, which is really composed of many separate lamellae with spaces in between, indicates that the calcification process itself may be discontinuous, and that this may in turn be responsible for the growth fluctuations that were observed within the coral polyp populations of individual colonies. No hard and fast correlation was observed between the calcification rates/unit of N and the relative abundance of the species on the reef. Although some of the commonest shallow water corals are very fast calcifiers as well, the most important West Indian reef builders are the slower growing massive corals. An explanation is put forward to the effect that growth of massive corals in the reef is enhanced by the formation of buttresses which serve to increase the available surface area for calcification.