Morphological and genetic variation in the Caribbean species of the hydrocoral genus Millepora

Millepora, an important structural component of tropical coral reefs, is a ubiquitous hydrocoral genus in the shallow water reefs of the Caribbean and is traditionally thought to consist of four species: M. alcicornis Linnaeus 1758, M. complanata Lamarck 1816, M. striata Duchassaing & Michelotti 1864 and M. squarrosa Lamark 1816. Intermediate forms and high phenotypic variability oftentimes hinder the correct identification of colonies. A multicharacter approach based on morphology and DNA sequences was used to evaluate taxonomic differences among Caribbean Millepora species. Samples of M. alcicornis, M. complanata and M. squarrosa were collected from the La Parguera reef system in southwest Puerto Rico; samples from M. striata were collected at Bocas del Toro, Panama. Morphological traits of gastropore and dactylopore diameter, distances among gastropores, among dactylopores and from gastropore to the nearest dactylopore were compared between M. complanata and M. alcicornis . High intraspecific variability and overlap among the morphotypes was observed; thus, the traits were more powerful delimiting species when used in conjunction than when used alone. A portion of the Cytochrome Oxidase Subunit I (COI) gene was used to examine the genetic differences among the four putative species. High levels of haplotypic diversity (Hd=0.94) were observed and the most common haplotypes were shared by M. alcicornis, M. complanata and M. striata. Sequence divergence ranged from 0--3% among colonies identified as M. alcicornis, M. complanata and M. striata, contrasting with the 25% divergence observed among these species and M. squarrosa. Bayesian analysis of the genealogy of Millepora resulted in paraphyletic clades suggesting that only two species of Millepora are present in the Caribbean: M. squarrosa and the species currently described as M. complanata, M. alcicornis and M. striata. The lack of congruence between the molecular and morphometric results indicates an uncoupling of morphological and molecular evolution in the genus Millepora.