Aspects of skeletal growth in the Indo-Pacific staghorn coral Acropora formosa

This thesis examines variations in the branch extension of the staghorn coral Acropora formosa at different spatial and temporal scales. The relationship between branch extension and tissue or area specific calcification is first examined. This is followed by investigations of variation in extension within a colony and between colonies at different depths. Seasonal variations in extension are also examined for several colonies at two different sites. Finally the reproductive biology of A. formosa is described and the possibility of competition in energetic investment between gametogenesis and growth is investigated. If a simplified model of branch shape and structure is adopted, then linear measurements of branch extension can provide a good index of total skeletal deposition and the rate of calcification. The model assumes that all branches consist of a solid cylinder of uniform width and density, and a tapered cap of varying density. Empirical data which support the predictions of this model are presented. In addition it is shown that the weight of newly extended skeleton is less satisfactory as a measure of total skeletal deposition, than linear extension. Intra-colony growth variations between different tips in a colony are shown to be correlated with tip colour, shape and skeletal structure. White-tipped branches lack zooxanthellae, are lightly calcified, have a well developed axial corallite, and exhibit a range of extension rates. On the other hand, brown- tipped branches possess zooxanthellae, are heavily calcified, have smaller axial corallites, and exhibit no active branch extension. It is suggested that brown tips arise in zones of unfavourable micro-environment, such as are found in the interior of large arborescent colonies. This may provide a mechanism for avoiding anastomosis, and maintaining the overall form of branching coral colonies. Significant variations in extension rate were found between colonies growing at three different depths at Davies Reef, a midshelf platform reef. There was a trend in which extension increased with increasing depth. This trend, although obvious in the first year of sampling, became obscured during the second year, possibly due to over-harvesting. A reciprocal transplant experiment between the shallow and deep colonies indicated that the observed growth differences were environmentally induced, rather than being due to genetically based differences between the colonies. Although extension rates were lower at the shallow site, branching was much higher, and resulted in a higher overall rate of calcification at the shallow site. It is proposed that the higher extension rates at the deep site, are made possible by an increase in translocated metabolites from a greater area of tissue than at the shallow site. Significant seasonal variations in branch extension were found in 7 colonies for 2 different sites (Davies Reef and Nelly Bay, an inshore fringing reef). Extension rates exhibited 2 maxima and 2 minima per year. Minima occurred during mid-winter and mid to late summer. A multiple linear regression analysis between extension and a variety of environmental parameters was able to explain 28% of the variance in the growth rates. An inhibitory effect for high temperature was incorporated into a temperature index, and this index was found to be the most important parameter in the regression model. It is hypothesized that high summer, and low winter temperatures cause the observed growth minima. Acropora formosa has an annual gametogenic cycle and spawns once a year during mass multispecific spawning events. The period of peak gametogenesis does not appear to be correlated with any period of reduced branch extension. Isolated colony fragments, as small as single branches 35mm in length, still develop gonads of nearly normal size and fecundity. On the other hand juvenile corals up to 143mm in mean diameter were sterile. Branch fragments greater than 40mm in length, isolated from the parent colony half way through the gametogenic cycle, appear to continue normal gonad development.