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Geometric morphometrics are a useful tool for studying morphological variation in scleractinian corals, extant and extinct. In the case of modern specimens, three-dimensional features of the calical surface can be measured. These features are rarely preserved in fossil corals, however, necessitating measurement of 2-D landmarks in transverse thin-sections of corallites. Unfortunately, 2-D and 3-D methods often yield differing answers to questions about interspecific, intraspecific and intracolonial variation. This issue is addressed in the present study by directly comparing results of 2-D and 3-D geometric morphometric analyses of identical colonies of extant members of the Montastraea "annularis" species complex.
Ten colonies of each extant species in the complex (M. annularis s.s., M. faveolata and M. franksi), identified in the field during collection and verified by molecular data, were selected for analysis. Slabs of colony surfaces and transverse thin-sections from ~1 cm below the surface were cut from tops and edges of each colony. Six corallites from each slab were measured in 3-D using a Reflex microscope, and six measured in 2-D on digital images of each transverse thin-section. Both datasets were explored using geometric morphometric methods and analyzed statistically to address questions related to measurement error, intracolonial variation in corallite morphology between tops and edges of colonies, and interspecific morphological differences. The shape data were superimposed using Procrustes generalized least squares, and examined using principal components and canonical variates analyses. Shape differences implied by axes obtained from PCA and CVA were depicted as deformations using the thin-plate spline, to identify which morphological features are correlated with axes of greatest total variance (PCA) and greatest between-group variance (CVA). Goodall's F-test was used to detect significant morphological differences among species and colony positions. All of the data used in these analyses are available in the supplementary file that accompanies this thesis (see Appendix C for a description of the contents of this file).
Measurement error analyses show significant differences among variances associated with replicate measurements of 2-D and 3-D landmarks. In many cases the variance is asymmetrical, and for 2D data especially, this asymmetry coincides with orientation of anatomical features. Significant shape differences between corallites from tops and edges of colonies of M. annularis and M. faveolata are found when 3-D data are used. These intracolonial differences are due in large part to height and shape of the septal margin. As a result, 2-D data are unable to find significant differences within colonies.
Both datasets find significant interspecific differences, but different anatomical features are found to be responsible. Important interspecific differences for 2-D data are relative thickness of the corallite wall and lengths of septa and costae. When 3-D data are used, results are most influenced by height of primary and secondary septa above the calical surface, as well as length of septa from the corallite wall toward the columella. Patterns of relative morphological similarity among species also differ between datasets. 2-D data show closest similarity between M. annularis and M. faveolata, while M. faveolata and M. franksi are most similar when 3-D data are used. The former result is consistent with previous 2-D analyses, while the latter conclusion is without precedent. Neither is consistent with relationships inferred using molecular data.