Morphological and genetic evaluation of the hydrocoral Millepora species complex in the Caribbean

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, Curacao, 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 (H-d = 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.