In the past decade, molecular phylogenetic analyses of octocorals have shown that the current morphological taxonomic classification of these organisms needs to be revised. The latest phylogenetic analyses show that most octocorals can be divided into three main clades. One of these clades contains the families Coralliidae and Paragorgiidae. These families share several taxonomically important characters and it has been suggested that they may not be monophyletic; with the possibility of the Coralliidae being a derived branch of the Paragorgiidae. Uncertainty exists not only in the relationship of these two families, but also in the classification of the two genera that make up the Coralliidae, Corallium and Paracorallium. Molecular analyses suggest that the genus Corallium is paraphyletic, and it can be divided into two main clades, with the Paracorallium as members of one of these clades. In this study we sequenced the whole mitochondrial genome of five species of Paragorgia and of five species of Corallium to use in a phylogenetic analysis to achieve two main objectives; the first to elucidate the phylogenetic relationship between the Paragorgiidae and Coralliidae and the second to determine whether the genera Corallium and Paracorallium are monophyletic. Our results show that other members of the Coralliidae share the two novel mitochondrial gene arrangements found in a previous study in Corallium konojoi and Paracorallium japonicum; and that the Corallium konojoi arrangement is also found in the Paragorgiidae. Our phylogenetic reconstruction based on all the protein coding genes and ribosomal RNAs of the mitochondrial genome suggest that the Coralliidae are not a derived branch of the Paragorgiidae, but rather a monophyletic sister branch to the Paragorgiidae.
While our manuscript was in review a study was published using morphological data and several fragments from mitochondrial genes to redefine the taxonomy of the Coralliidae. Paracorallium was subsumed into Corallium and the genus Hemicorallium was resurrected. This left two disjunct clades as Corallium, making that genus paraphyletic. One of the clades includes the type specimens of Corallium, indicating that clade should remain Corallium. For the other clade, we support the resurrection of the genus Pleurocorallium to fix the paraphyly of Corallium. Based on congruent phylogenies in both studies, the genus Pleurocorallium includes the species C. secundum, C. kishinouyei, C. konojoi, C. elatius, and C. niveum.
