Molecular and morphological variation supports monotypy of the octocoral Briareum Blainville, 1830 (Octocorallia: Alcyonacea) in the Western Atlantic

The current diagnosis and description of the octocoral genus Briareum ascribes to it a single Caribbean species, Briareum asbestinum. Nonetheless, the combination Briareum polyanthes is commonly used in the scientific literature for the encrusting form of this species, and it has been proposed that growth form, allozyme differences, and secondary chemistry support recognition of the digitate form and the encrusting form as separate species. Through comparison of sclerite sizes and nuclear and mitochondrial DNA sequences, as well as a review of studies of secondary metabolites, we find that variation in these characters is not correlated as much to growth form as to location or habitat. Analyses of traditional morphological characters and molecular data support the monotypy of Briareum in the western Atlantic. The subjective junior synonymy of Ammothea (=Briareum) polyanthes with B. asbestinum is reiterated and a modified description of the species is provided, including an assessment of intraspecific sclerite variation. Partial mitochondrial gene sequences showed no variation among all forms and locations (Bermuda and The Bahamas), whereas the 18S-ITS1-5.8S-ITS2-28S nuclear ribosomal locus contained multiple unique genotypes within and among individuals, growth forms and geographic locations. Extensive intragenomic heterogeneity in the nuclear ribosomal locus, existing in excess of normal heterozygosity, indicates that the multiple copies of the 18S-ITS1-5.8S-ITS2-28S array are not homogenized through concerted evolution. Since this intragenomic variation requires extensive screening to determine the homology of haplotypes and can confound comparison between individuals, we recommend the future use of single-copy nuclear markers in studies of octocoral systematics and evolution.