The phylogeny of Pacifigorgia (Coelenterata, Octocorallia, Gorgoniidae): a case study of the use of continuous characters in the systematics of the Octocorallia

The Octocorallia constitute a group of organisms that inhabit a wide spectrum of habitats ranging from abyssal to shallow marine environments and from tropical to polar waters. As a subclass, the group is clearly defined and the clade is considered monophyletic based on a series of morphological and molecular synapomorphies. In contrast, the systematic treatment of octocorals below the subclass level is complex given the continuous nature of many of the characters used in their taxonomy. The use of continuous characters for the deduction of phylogenies generally has been considered a philosophical and methodological challenge; within the Octocorallia it has led researchers to rearrange genera, to consider character analysis problematic, or to prefer molecular data sets, which avoid morphological characters, for the study of octocoral phylogenetic relationships. In this study, we assessed the role of continuous morphological variation on the deduction of octocoral phylogenies. Specifically, we analyzed the morphologically diverse genus Pacifigorgia Bayer, 1951 and show that continuous variation, after adequate coding, represents a valuable source of information suitable for phylogenetic inference.