OVERLAPPING SPECIES BOUNDARIES AND RETICULATE EVOLUTION IN REEF CORALS: EVIDENCE FROM THE FOSSIL RECORD

Recent molecular analyses suggest that many reef coral species belong to hybridizing species complexes. Species in one such complex, Montastraea “annularis” s.l., exhibit significant genetic differences in Panama, which do not exist in the Bahamas. Although there is no evidence for hybridization in the Bahamas today, it has been hypothesized that an ancestral polymorphism, caused by hybridization in the geologic past, may be responsible for the observed lack of genetic distinctiveness. Here we test this hypothesis by comparing patterns of ecologic and morphologic overlap in genetically-characterized modern species with their fossil counterparts.

Our study focuses on colonies measured and collected along linear transects across Pleistocene reef terraces of last interglacial age (~125 Ka) on three islands in the Bahamas. Both ecologic and morphologic analyses reveal moderate to high overlap among Pleistocene species(massive, column, organ-pipe). Ecological analyses performed using multidimensional scaling shows that, although organ-pipes had higher abundances in patch reef environments, the three species co-occurred. Morphologic analyses were performed using canonical discriminant analysis on landmark data and linear measurements. The results show that columnar species overlap moderately with organ-pipe and massive species. Comparisons with genetically-characterized modern colonies show that the observed moderate overlap differs from the morphologic separation exhibited among the three modern species in Panama. It also differs from morphologic distinctions observed in massive and columnar species from the Pleistocene of the Dominican Republic.

These results support the hypothesis of hybridization among species within the complex in the Bahamas during the Pleistocene. Hybridization most likely occurred in association with sea level and temperature fluctuations, and appears to have been limited geographically primarily to the Bahamas and the northern Caribbean. These geographic differences have persisted since the Pleistocene, and have played an important role in shaping the genetic structure of modern species.