COMPARISON OF GEOMETRIC MORPHOMETRIC ANALYSES OF THE MONTASTRAEA "ANNULARIS" REEF-CORAL SPECIES COMPLEX: DIFFERING RESULTS FROM 2D AND 3D DATA

Evolutionary studies of the Montastraea “annularis” species complex (M. annularis s.s., M. faveolata and M. franksi) using traditional morphological and molecular data yield conflicting answers. Geometric morphometrics have shed light on evolutionary relationships within the group, but 3D landmarks (used for modern specimens) cannot be measured on fossils due to destruction of delicate surface features. Instead, 2D landmarks are measured on transverse thin sections. In order to assess the comparability of 2D and 3D methods, twenty-one 3D landmarks were measured on the calical surfaces of corallites and eighteen 2D landmarks on transverse thin sections from the same genetically characterized colonies (12 corallites/colony, 10 colonies/species). The data were then superimposed by Procrustes GLS.

CVA of the 2D data grouped by species produced two CVs, both of which are effective discriminators (Bartlett’s test: p<0.001), explaining 59.2% and 40.8% of the variance. The data overlap, but group means are distinct. CV1 separates M. franksi from the other species, while CV2 distinguishes M. faveolata from the others. Assignment of specimens to their nearest group means was 79.4% successful overall. Likewise, CVA of the 3D data resulted in two axes that explain 63.6% and 36.4% of the variance and are both effective discriminators (Bartlett’s test: p<0.001). CV1 distinguishes M. faveolata from M. annularis and M. franksi, CV2 separates M. franksi from the other species. A posteriori classifications were 73.1% successful overall.

Pairwise Goodall’s F-tests showed highly significant differences between each species pairing (p<0.001) for both 2D and 3D datasets. However, the 2D data clearly show closest similarity (least partial Procrustes distance) between M. annularis and M. faveolata, while 3D data suggest somewhat closer similarity between M. faveolata and M. franksi. These results differ from those of molecular data, which indicate a more recent divergence between M. annularis and M. franksi. In thin section, the characters that differentiate M. franksi from the other species are its thicker corallite wall and relatively shorter septa and costae. In 3D, however, thickness of the wall cannot be observed. Instead, vertical relief of calical surfaces (least in M. annularis) is the most distinguishing feature.