Quills of Confusion! Genomic insight into the evolution and morphology of sea pens (Octocorallia: Scleralcyonacea: Pennatuloidea)

Background: Sea pens, Superfamily Pennatuloidea (Ehrenberg 1832), are a group of colonial octocorals so named because of the resemblance of some species to quill pens. These octocorals were grouped together due to their ability to live anchored in soft-sedimented sea floors using the peduncle. A modification of the peduncle in the species of one genus allow them to attach onto hard rocky substrates. Diverse colony morphology is seen among the sea pens, and some taxa have polyp-bearing secondary leaf-like extensions arising from the main axis (rachis). A few phylogenetic studies have investigated the evolutionary history of this group; however, this is the first genomic-scale investigation, in the form of Ultra Conserved Elements (UCEs), focused on Pennatuloidea. A mtMutS phylogeny with wider taxon sampling has also been constructed and compared to the UCE tree to address questions regarding the diversification of the different groups of sea pens. We also investigated the evolution of diverse colony morphology in sea pens by conducting ancestral state reconstruction analyses. Results: We found evidence that the sea pen lineage spilts into two major clades, which is not reflected in their current taxonomy. Our results show that most families of sea pens are not monophyletic and require reclassification. We also found that colony morphology has a complex evolutionary history, with at least seven independent origins of colonies bearing leaf-like extensions ("branches") arising from the main axis. Conclusion: Our results demonstrate incongruence among morphology, taxonomy, and phylogeny in most families of sea pens. We were able to confirm the monophyly of only 4 out of the 16 extant families. This incongruence is also evident in the presence of leaf-like extensions from which secondary polyps arise.