LATE PLEISTOCENE EVOLUTION OF A DOMINANT CARIBBEAN REEF CORAL: SPECIATION AND EXTINCTION WITHIN THE MONTASTRAEA ANNULARIS COMPLEX

Evolutionary patterns are examined within the Montastraea annularis species complex during the late Pleistocene in order to assess the roles of selection and metapopulation factors in the long-term evolution of the clade. Specimens were collected along community transects across reef terraces (~125Ka) on three distant Bahamian islands (Andros, Great Inagua, San Salvador). Species are recognized using new landmark methods developed using genetically characterized modern colonies. The data emphasize non-traditional features in transverse thin section, involving the corallite wall and associated costosepta. Cluster and discriminant analyses show that at least four species were present in the Bahamian region during late Pleistocene time. These species differ not only in corallite morphology but also in colony form. Despite differences in community composition among islands, three species were abundant on all three islands.

Multivariate statistical comparisons between the Pleistocene Bahamian species and the three modern species of the complex show that the morphology of only one of the four Bahamian species closely matches that of a modern species. Although not in the Bahamian collections, a second modern species occurs in the Plio-Pleistocene of Costa Rica. These results suggest that diversity may have been higher within the complex during Pleistocene time, and that the modern species may be survivors of latest or post-Pleistocene extinction. Comparisons with species from the upper Pleistocene of Barbados and the Dominican Republic show that at least three of the Pleistocene Bahamian species also occurred at these other two locations, and were thus widely distributed across the Caribbean region. Contrary to expectations, extinction victims, therefore, included abundant species with wide distributions. Despite extinction, a broad array of colony forms (plates, mounds, columns, pipes) was maintained on Caribbean reefs from Pleistocene to Recent time, and is interpreted as being the result of co-evolution of species within the complex.