Schizas, N.V.

It has been proposed that the elkhorn coral Acropora palmata is genetically separated into two distinct provinces in the Caribbean, an eastern and a western population admixing in Western Puerto Rico and around the Mona Passage. In this study, the genetic structure ofA. palmata sampled at 11 Puerto Rican localities and localities from Curaçao, the Bahamas and Guadeloupe were examined. Analyses using five microsatellite markers showed that 75% of sampled colonies had unique genotypes, the rest being clone mates. Genetic diversity among genets was high (HE = 0.761) and consistent across localities (0.685–0.844). FST ranged from −0.011 to 0.047, supporting low but significant genetic differentiation between localities within the previously reported eastern and western genetic provinces. Plots of genetic per geographic distances and significant Mantel tests supported isolation-by-distance (IBD) within Puerto Rico. Analysis with the software STRUCTURE favored a scenario with weak differentiation between two populations, assigning Eastern Puerto Rican locations (Fajardo and Culebra), Guadeloupe and Curaçao to the Caribbean eastern population and Western Puerto Rican locations (west of Vega Baja and Ponce), Mona and the Bahamas to the Caribbean western population. Vieques and San Juan area harbored admixed profiles. Standardized FST per 1000 km unit further supported higher differentiation between localities belonging to different STRUCTURE populations, with IBD being stronger within Puerto Rico than on larger regional scales. This stronger genetic transition seems to separate localities between putative eastern and western provinces in the Eastern Puerto Rican region, but not around the Mona Passage.

Abstract:

Background: The hydrocoral Millepora is an important framework builder that dominates shallow turbulent environments in the Indo-Pacific and the Atlantic-Caribbean. The Caribbean representatives of the genus are classified in four species - Millepora alcicornis, Millepora complanata, Millepora striata, and Millepora squarrosa - but their taxonomic boundaries are not clearly defined. We used mitochondrial gene sequences to delineate the four Millepora species and evaluated whether morphological traits and mitochondrial sequence divergence were correlated for two most common species M. alcicornis and M. complanata.

Results: Samples were collected from Puerto Rico, Guadeloupe, Curaçao, Grand Cayman, and Panama during 2006 to 2007. Diameter of dactylopores distinguished the branching and encrusting morphotypes of M. alcicornis and M. complanata, and gastropore diameter discriminated between M. alcicornis and M. complanata. High levels of haplotypic diversity (Hd = 0.94) were observed, with the most common haplotypes shared by M. alcicornis and M. complanata. Sequence divergence ranged from 0% to 3% among M. alcicornis, M. complanata, and M. striata to 25% between these three species and M. squarrosa. Bayesian analysis of cytochrome oxidase subunit I (COI) gene indicated the presence of three Caribbean taxa: M. squarrosa, M. striata, and the ‘species complex’ encompassing the morphologies displayed by M. complanata and M. alcicornis.

Conclusions: The branched M. alcicornis and encrusted M. alcicornis and M. complanata can be differentiated morphologically but not genetically. Phylogenetic analysis suggests that the Caribbean milleporids include three species - M. squarrosa, M. striata, and the species complex of M. alcicornis-M. complanata. Millepora striata is closely related to the M. alcicornis-M. complanata species complex.