Molecular systematics of the bubblegum coral genera (Paragorgiidae, Octocorallia) and description of a new deep-sea species

Bubblegum octocorals (Paragorgia and Sibogagorgia) play an important ecological role in many deep-sea ecosystems. However, these organisms are currently threatened by destructive fishing methods such as bottom trawling. Taxonomic knowledge of conservation targets is necessary for the creation and implementation of efficient conservation strategies. However, for most deep-sea coral groups this knowledge remains incomplete. For instance, despite its similarities with Paragorgia, Sibogagorgia is particular in lacking polyp sclerites, which are present in groups like Paragorgia and the Coralliidae. Although two kinds of sclerites are very similar between Paragorgia and Sibogagorgia, other characters challenge the monophyly of these genera. Here we help to clarify the taxonomy and evolutionary relationships of the bubblegum octocorals and related taxa by examining molecular data. We employed nucleotide sequences of mitochondrial (ND6, ND6-ND3 intergenic spacer, ND3, ND2, COI, msh1 and 16S) and nuclear (28S and ITS2) genomic regions from several taxa to infer molecular phylogenetics and to examine the correspondence of morphological features with the underlying genetic information. Our data strongly supported the monophyly of the genus Paragorgia, the family Coralliidae (precious corals), and a group of undescribed specimens resembling Sibogagorgia. Further morphological observations were congruent regarding the uniqueness of the undescribed specimens, here defined as a new species, Sibogagorgia cauliflora sp. nov., which occurs in both sides of the North American landmass at depths below 1700 m. This new species resembles S. dennisgordoni with branching in one plane but has fairly different radiate sclerites and significantly divergent DNA sequences. The existence of several diagnostic characters of Sibogagorgia in S. cauliflora indicates that they indeed belong to this genus. It is however remarkable that a small number of medullar canals are also found in this species; medullar canals have been considered as the main diagnostic character of Paragorgia. Thus, the evidence generated here indicates that the presence or absence of these canals per se is not a conclusively diagnostic character for either genus. The lack of internal-node resolution in the inferred phylogenetic hypotheses of these genera does not allow us to propose a clear scenario regarding the evolution of these traits.