Patterns of settlement, survival and growth of oysters across the Great Barrier Reef

Abstract

The effects of substratum irregularity on abundance, dispersion patterns, and generic di- versity of juvenile corals (newly settled coral spat) were investigated on Britomart Reef, central region of the Great Barrier Reef, Australia, Pieces of freshly killed plating coral Pachyseris speciosa Dana (Coelenterata: Scleractinia), which varied naturally in their struc- tural complexity, were utilized as settling substrata. These pieces were placed within territories of the damselfish Hemiglyphidodon plagiometopon Bleeker, where grazing by herbivorous fishes is reduced, to maximize possible success of coral settlement. After 4 months, plates were retrieved and examined for coral settlement and substratum characteristics. Coral spat were identified and analyzed for exposure, location on the substratum, and specific angle of settlement. Substratum was analyzed for surface area, surface irregularity, average surface angle, and plate angle. This was done by means of a three-dimensional profile gauge developed specifically for this purpose. Five measures derived from geomorphological studies were tested and compared for their suitability for quantifying surface irregularity. These included vector dispersion, vector strength, deviations vertical from a best-fit plane, deviations normal to a best-fit plane, and deviations vertical to the mean horizontal plane. Vector dispersion was determined to be most appropriate for measuring surface irregularity. A scale of I em was found to be appropriate for examining the relationship between substratum irregularity and settlement in corals or other sessile invertebrates with similar larval search behaviors. Coral spat abundance correlated positively with surface irregularity and average surface angle. Generic diversity and degree of aggregation were also correlated with surface irregularity and average surface angle.A disproportionately large number of spat successfully settled on steeper surfaces, indicating that recruitment was not random with respect to placement angle. Highest success of coral settlement (90%, N = 288) occurred cryptically (on the underside of plates).