Complete mitochondrial genomes of the Japanese pink coral (Corallium elatius) and the Mediterranean red coral (Corallium rubrum): a reevaluation of the phylogeny of the family Coralliidae based on molecular data

Precious corals are soft corals belonging to the family Coralliidae (Anthozoa: Octocorallia: Alcyonacea) and class Anthozoa, whose skeletal axes are used for jewelry. The family Coralliidae includes ca. 40 species and was originally thought to comprise of the single genus Corallium. In 2003, Corallium was split into two genera, Corallium and Paracorallium, and seven species were moved to this newly identified genus on the bases of morphological features. Previously, we determined the complete mitochondrial genome sequence of two precious corals Paracorallium japonicum and Corallium konojoi, in order to clarify their systematic positions. The two genomes showed high nucleotide sequence identity, but their gene order arrangements were not identical. Here, we determined three complete mitochondrial genome sequences from the one specimen of Mediterranean Corallium rubrum and two specimens of Corallium elatius coming from Kagoshima (South Japan). The circular mitochondrial genomes of C. rubrum and C. elatius are 18,915 bp and 18,969–18,970 bp in length, respectively, and encode 14 typical octocorallian protein-coding genes (nad1–6, nad4L, cox1–3, cob, atp6, atp8, and mtMutS, which is an octocoral-specific mismatch repair gene homologue), two ribosomal RNA genes (rns and rnl), and one transfer RNA (trnM). The overall nucleotide differences between C. konojoi and each C. elatius haplotype (T2007 and I2011) are only 10 and 11 nucleotides, respectively; this degree of similarity indicates that C. elatius and C. konojoi are very closely related species. Notably, the C. rubrum mitochondrial genome shows more nucleotide sequence identity to P. japonicum (99.5%) than to its congeneric species C. konojoi (95.3%) and C. elatius (95.3%). Moreover, the gene order arrangement of C. rubrum was the same as that of P. japonicum, while that of C. elatius was the same as C. konojoi. Phylogenetic analysis based on three mitochondrial genes from 24 scleraxonian species shows that the family Coralliidae is separated into two distinct groups, recovering Corallium as a paraphyletic genus. Our results indicate that the currently accepted generic classification of Coralliidae should be reconsidered.