Systema Naturae includes representatives of every major lineage of the animal phylum Cnidaria.  However, Linnaeus did not classify the members of the phylum as is now done, and the diversity of the group is not well represented.  We contrast the Linnaean perspective on cnidarian diversity with the modern, phylogenetic perspective.  For each order, we detail diversity at the family level, providing phylogenetic context where possible.

Cnidaria, the sister group to Bilateria, is a highly diverse group of animals in terms of morphology, lifecycles, ecology, and development. How this diversity originated and evolved is not well understood because phylogenetic relationships among major cnidarian lineages are unclear, and recent studies present contrasting phylogenetic hypotheses. Here, we use transcriptome data from 15 newly-sequenced species in combination with 26 publicly available genomes and transcriptomes to assess phylogenetic relationships among major cnidarian lineages. Phylogenetic analyses using different partition schemes and models of molecular evolution, as well as topology tests for alternative phylogenetic relationships, support the monophyly of Medusozoa, Anthozoa, Octocorallia, Hydrozoa, and a clade consisting of Staurozoa, Cubozoa, and Scyphozoa. Support for the monophyly of Hexacorallia is weak due to the equivocal position of Ceriantharia. Taken together, these results further resolve deep cnidarian relationships, largely support traditional phylogenetic views on relationships, and provide a historical framework for studying the evolutionary processes involved in one of the most ancient animal radiations.

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.

Gastrulation is a central event in metazoan development, involving many cellular behaviors including invagination, delamination, and ingression. Understanding the cell biology underlying gastrulation in many different taxa will help clarify the evolution of gastrulation mechanisms. Gastrulation in the anthozoan cnidarian Nematostella vectensis has been described as a combination of invagination and unipolar ingression through epithelial to mesenchymal transitions (EMT), possibly controlled by snail genes, important regulators of EMT in other organisms. Our examination, however, fails to reveal evidence of ingressing cells. Rather, we observe that endodermal cells constrict their apices, adopting bottle-like morphologies especially pronounced adjacent to the blastopore lip. They retain apical projections extending to the archenteron throughout gastrulation. Basally, they form actin-rich protrusions, including interdigitating filopodia that may be important in pulling the ectodermal and endodermal cells together. Endodermal cells retain cell-cell junctions while invaginating, and are organized throughout development. Never is the blastocoel filled by a mass of mesenchyme. Additionally, injection of splice-blocking morpholinos to Nematostella snail genes does not result in a phenotype despite dramatically reducing wild-type transcript, and overexpression of Snail-GFP in different clonal domains has no effect on cell behavior. These data indicate that EMT is not a major factor during gastrulation in Nematostella.

The composition of and relationships among higher-level groups within the anthozoan subclass Hexacorallia ( = Zoantharia) has been controversial because independent analyses of anatomy, life history, ultrastructure, and molecular sequences have failed to provide a consistent framework for drawing taxonomic boundaries or understanding phylogenetic relationships. The relationship among stony corals (order Scleractinia), sea anemones (order Actiniaria), and corallimorpharians (order Corallimorpharia) has been particularly problematic. We synthesize existing studies and provide new anatomical and molecular evidence that bear on the question of ordinal circumscription and relationships. We find that orders Actiniaria, Antipatharia, Ceriantharia, Corallimorpharia, Scleractinia, and Zoanthidea are monophyletic; Corallimorpharia is most closely related to Scleractinia. We infer that many traditional diagnostic characters are shared primitive features and thus poor indicators of phylogenetic relationships. Although the major nodes of the hexacorallian tree are well supported by multiple types of data, questions about skeletal evolution and subordinal taxonomy remain unanswered pending denser taxonomic and character sampling. CC, Willis BL. 1986. Synchronous spawnings of 105 scleractinian coral species on the Great Barrier Reef. Marine Biology 90: 379-394. Bayer FM, Grasshoff M, Verseveldt J. 1983. Illustrated trilingual glossary of morphological and anatomical terms applied to Octocorallia. Leiden: EJ Brill. Berntson EA, France SC, Mullineaux LS. 1999. Phylogenetic relationships within the class Anthozoa (Phylum Cnidaria) based on nuclear 18S rDNA sequences. Molecular Phylogenetics and Evolution 13: 417-433. Brown WM, Pranger EM, Wang A, Wilson AC. 1982. Mitochondrial DNA sequences of primates: tempo and mode of evolution.