Toward a natural classification: phylogeny of acontiate sea anemones (Cnidaria, Anthozoa, Actiniaria)

Acontia—nematocyst‐dense, thread‐like extensions of the mesenterial filaments―are the characteristic feature of the actiniarian group Acontiaria. Phylogenetic analyses have shown that acontiate taxa form a clade that also includes some taxa without acontia. We analyse five molecular markers from 85 actiniarians to explore the phylogenetic relationships among families in Acontiaria, including acontiate species assigned to other higher taxa and species without acontia that have been allied to Acontiaria. Based on our results, we redefine the group to accommodate those lineages that have lost acontia, and formalize it as superfamily Metridioidea. Based on stable and well supported clades, we resurrect Phelliidae and Amphianthidae, redefine Kadosactinidae and Actinoscyphiidae, and move two species to new genera: that previously termed Sagartiogeton erythraios belongs in Jasonactis gen. nov.; and that previously termed Anthosactis pearseae belongs in Ostiactis gen. nov., type genus of Ostiactinidae fam. nov. We also synonymized Halcampoididae and Halcampidae (as Halcampidae) and Andvakiidae and Isophelliidae (as Andvakiidae). The results of our phylogenetic analyses indicate that the diagnostic morphological characters used in the family‐level taxonomy of acontiate actiniarians such as the nematocysts of the acontia, the marginal sphincter muscle, and mesenteries divisible into macro‐ and micro‐cnemes, have to be revisited, as these features are highly homoplasious.