A time-calibrated molecular phylogeny of the precious corals: reconciling discrepancies in the taxonomic classification and insights into their evolutionary history
Authors: Ardila Néstor E , Giribet Gonzalo , Sánchez Juan A 
Published: December 2012

Background

Seamount-associated faunas are often considered highly endemic but isolation and diversification processes leading to such endemism have been poorly documented at those depths. Likewise, species delimitation and phylogenetic studies in deep-sea organisms remain scarce, due to the difficulty in obtaining samples, and sometimes controversial. The phylogenetic relationships within the precious coral family Coralliidae remain largely unexplored and the monophyly of its two constituent genera,CoralliumCuvier andParacoralliumBayer & Cairns, has not been resolved. As traditionally recognized, the diversity of colonial forms among the various species correlates with the diversity in shape of their supporting axis, but the phylogenetic significance of these characters remains to be tested. We thus used mitochondrial sequence data to evaluate the monophyly ofCoralliumandParacoralliumand the species boundaries for nearly all named taxa in the family. Species from across the coralliid range, including material from Antarctica, Hawaii, Japan, New Zealand, Taiwan, Tasmania, the eastern Pacific and the western Atlantic were examined.

Results

The concatenated analysis of five mitochondrial regions (COI, 16S rRNA, ND2, and ND3-ND6) recovered two major coralliid clades. One clade is composed of two subgroups, the first includingCorallium rubrum, the type species of the genus, together with a small group ofParacoralliumspecies (P. japonicumandP. tortuosum) andC. medea(clade I-A); the other subgroup includes a poorly-resolved assemblage of sixCoralliumspecies (C. abyssale, C. ducale, C. imperiale, C. laauense, C. niobe,andC. sulcatum; clade I-B). The second major clade is well resolved and includes species ofCoralliumandParacorallium(C. elatius, C. kishinouyei, C. konojoi, C. niveum, C. secundum, Coralliumsp.,Paracorallium nix, Paracorallium thrinaxandParacoralliumspp.). A traditional taxonomic study of this clade delineated 11 morphospecies that were congruent with the general mixed Yule-coalescent (GMYC) model. A multilocus species-tree approach also identified the same two well-supported clades, being Clade I-B more recent in the species tree (18.0-15.9 mya) than in the gene tree (35.2-15.9 mya). In contrast, the diversification times for Clade II were more ancient in the species tree (136.4-41.7 mya) than in the gene tree (66.3-16.9 mya).

Conclusions

Our results provide no support for the taxonomic status of the two currently recognized genera in the family Coralliidae. Given thatParacoralliumspecies were all nested withinCorallium, we recognize the coralliid genusCorallium, which includes the type species of the family, and thus considerParacoralliuma junior synonym ofCorallium. We propose the use of the genusHemicoralliumGray for clade I-B (species with long rod sclerites, cylindrical autozooids and smooth axis). Species delimitation in clade I-B remains unclear and the molecular resolution for Coralliidae species is inconsistent in the two main clades. Some species have wide distributions, recent diversification times and low mtDNA divergence whereas other species exhibit narrower allopatric distributions, older diversification times and greater levels of mtDNA resolution.