Untangling deep-sea corals systematics: Description of a new family, Stephanocyathidae (Anthozoa, Scleractinia), through a genomic approach

Once considered the most speciose mostly azooxanthellate scleractinian group, the family Caryophylliidae is found to be highly polyphyletic based on molecular data and is undergoing a process of systematic revision. High-throughput sequencing methods coupled with morphological analyses have facilitated revision of several scleractinian lineages, including the aforementioned family. In previous studies that relied on few mitochondrial and nuclear markers, the caryophylliid genera Stephanocyathus and Vaughanella were phylogenetically recovered in separate clades from the lineage that includes the type genus of the family, Caryophyllia, which is considered the 'true' Caryophylliidae. To help untangle the relationship among taxa of this family, here a new deep-sea scleractinian family (Stephanocyathidae Vaga, Cairns & Kitahara fam. nov.) is proposed based on phylogenomic reconstructions coupled with molecular features, specifically gene order, of the complete mitochondrial genome. Evolutionary reconstructions were based on both mitochondrial and nuclear ultraconserved elements (UCEs) and exon loci data sets and confirmed the divergent position of the genera Stephanocyathus and Vaughanella. The new family shows a specific gene transposition in the mitochondrial genome, not present in the 'true' caryophylliid lineage, but instead already observed for the species Paraconotrochus antarcticus, recovered as sister taxon of the here proposed new family. Although its phylogenetic position is unknown, the genus Ericiocyathus is also added to the new family, based on macromorphological similarities. This study represents a step forward in our understanding of deep-sea corals relationships and provide further information (e.g., mitochondrial gene order) that will aid in future efforts of assessing the systematic of caryophylliid lineages.