Coral extension rate analysis using computed axial tomography

Abstract

Biology, geology, and paleoclimatology studies of coral reefs often rely on annual rates of corallite extension determined from cores of skeletal material recovered from scleractinian corals. To examine how corallite orientation within such cores may affect measured extension rates, we used high-resolution X-ray computed axial tomography (CT) to image a suite of short Orbicella faveolata cores from corals living in the northwest Gulf of Mexico. This nondestructive method enabled skeletal extension rates to be determined several ways: (1) along transects that cross individual slice/slab images, (2) along transects that cross both a 0.6-mm-thick CT slice and the corresponding conventional X-ray image from a 8.4-mm-thick slab of skeletal material, and (3) along transects that cross slices/slab images oriented at varied angles relative to a coral core’s longitudinal axis. Additionally, the true three-dimensional (3D) extension rates of individual corallites were determined and compared to the extension rates apparent on a two-dimensional (2D) slice/slab. Results suggest that differences between all methods are not statistically significant, confirming that the conventional extension rate methodology is suitable for studies if the core’s longitudinal axis is appropriately aligned with the growth axis of the corallites. Effects of core misalignment on extension rates measurement and the appearance of skeletal structures in 2D images are described, and the applicability of 3D imagery for investigations of coral polyp growth patterns and coral colony development is demonstrated.