Use of X-radiographs to distinguish members of the Montastraea annularis reef-coral species complex

Recent work suggests the Montastraea annularis species complex consists of at least three species, which can be distinguished qualitatively in the field using features related to colony growth (e.g. overall growth form, bumpiness, growth along the colony edge). However, when whole colonies are not available and surfaces are eroded, identification becomes problematic when relying on such characteristics. Characters based on internal skeletal structures are less prone to loss due to taphonomic processes. Previous work has shown that internal corallite architectural features measured in transverse thin sections can be used to distinguish species. To determine whether internal colony-level features measured on X-radiographs can be used, eight characters related to corallite budding and accretionary growth were measured on specimens representing three modern members of the M. annularis species complex (M. annularis, M. faveolata and M. franksi), as well as two fossil forms (columnar and organ-pipe). All eight characters showed significant differences among species. Discriminant function analysis using seven of these characters resulted in distinct species groupings in canonical scores plots and a 100% classification success for specimens from Panamá. These results suggest that measurements made on X-radiographs provide a useful tool for quantitatively distinguishing members of the M. annularis complex as well as between other massive reef corals.

Key words: Montastraea, coral reef, Pleistocene, coral banding, species complex