Sea anemones (order Actiniaria) are among the most diverse and successful members of the anthozoan subclass Hexacorallia, occupying benthic marine habitats across all depths and latitudes. Actiniaria comprises approximately 1,200 species of solitary and skeleton-less polyps and lacks any anatomical synapomorphy. Although monophyly is anticipated based on higher-level molecular phylogenies of Cnidaria, to date, monophyly has not been explicitly tested and at least some hypotheses on the diversification of Hexacorallia have suggested that actiniarians are para- or poly-phyletic. Published phylogenies have demonstrated the inadequacy of existing morphological-based classifications within Actiniaria. Superfamilial groups and most families and genera that have been rigorously studied are not monophyletic, indicating conflict with the current hierarchical classification. We test the monophyly of Actiniaria using two nuclear and three mitochondrial genes with multiple analytical methods. These analyses are the first to include representatives of all three currently-recognized suborders within Actiniaria. We do not recover Actiniaria as a monophyletic clade: the deep-sea anemone Boloceroides daphneae, previously included within the infraorder Boloceroidaria, is resolved outside of Actiniaria in several of the analyses. We erect a new genus and family for B. daphneae, and rank this taxon incerti ordinis. Based on our comprehensive phylogeny, we propose a new formal higher-level classification for Actiniaria composed of only two suborders, Anenthemonae and Enthemonae. Suborder Anenthemonae includes actiniarians with a unique arrangement of mesenteries (members of Edwardsiidae and former suborder Endocoelantheae). Suborder Enthemonae includes actiniarians with the typical arrangement of mesenteries for actiniarians (members of former suborders Protantheae, Ptychodacteae, and Nynantheae and subgroups therein). We also erect subgroups within these two newly-erected suborders. Although some relationships among these newly-defined groups are still ambiguous, morphological and molecular results are consistent enough to proceed with a new higher-level classification and to discuss the putative functional and evolutionary significance of several morphological attributes within Actiniaria.

Sea anemones of the genus Aiptasia Gosse, 1858 are conspicuous members of shallow-water environments worldwide and serve as a model system for studies of cnidarian-dinoflagellate symbiosis. However, to date there have been no comprehensive analyses investigating the systematics of the group. In addition, previously published phylogenetic studies of sea anemones have shown that the genus is not monophyletic. Herein we revise the genus Aiptasia and the family Aiptasiidae Carlgren, 1924 using newly-collected material. We find that the formerly-named A. pallida (Agassiz in Verrill, 1864) (now Exaiptasia pallida comb. nov.) encompasses a single, widespread species from the tropics and subtropics; we erect a new genus, Exaiptasia gen. nov., for this species primarily based on cnidae, mode of asexual reproduction and symbionts. We also find morphological evidence that supports splitting A. mutabilis into two species: A. couchii (Cocks, 1851) and A. mutabilis. In addition, we find Bellactis Dube, 1983 (formerly placed within Sagartiidae Gosse, 1858) and Laviactis gen. nov. (formerly known Ragactis Andres, 1883, whose familial placement was previously uncertain) belonging within Aiptasiidae. Aiptasiidae is a morphologically homogeneous family whose members (those species in genera Aiptasia, Aiptasiogeton Schmidt, 1972, Bartholomea Duchassaing de Fombressin & Michelotti, 1864, Bellactis, Exaiptasia gen. nov., and Laviactis gen. nov.) are characterized by ectodermal longitudinal muscles in the distal column, rows of cinclides in mid-column, microbasic b-mastigophores in the column, and acontia with basitrichs and microbasic p-amastigophores.