Phylogenetic relationships within the Octocorallia (Cnidaria : Anthozoa) based on nuclear 18S rRNA sequences

We determined the nuclear 18S rRNA sequences for 41 species of octocorals a nd used these to address the validity of the historical ordinal divisions a nd the current subordinal divisions within the subclass Octocorallia. We al so explored the phylogenetic affinities of the species Dendrobrachia paucis pina, which was originally classified in the order Antipatharia (subclass C eriantipatharia) although polyp structure indicates it belongs in the subcl ass Octocorallia. Trees constructed using maximum likelihood techniques are incongruent with the current and historical taxonomy of the Octocorallia. There appeared to be three major clades of octocorals. The first clade incl uded most, but not all, pennatulaceans as a monophyletic group. The second clade contained 21 species, representing all major octocoral groups other t han pennatulaceans. The third clade contained members from three suborders of the Alcyonacea and one member of the Pennatulacea. These data could not be used to distinguish the branching order of the three major clades. The s pecies D. paucispina had a close affinity with the genera Corallium and Par agorgia (Alcyonacea: Scleraxonia), although its morphology suggests it is m ore similar to the genus Chrysogorgia (Alcyonacea: Calcaxonia). The morphol ogical character of dimorphism (the presence of both autozooids and siphono zooids within a single colony) corresponded loosely with the topology of th e most likely trees, and a single origin of dimorphism could not be rejecte d. Despite sampling from the majority of families within the Octocorallia, many of the relationships within this group remain ambiguous.