Sea anemones (Cnidaria: Class Anthozoa: Order Actiniaria) belong to the relatively simple and diverse phylum Cnidaria, with about 10,000 species of mostly marine animals including corals, hydroids, jellyfishes, sea fans, sea anemones, etc. The Order Actiniaria (sea anemones sensu stricto) comprises about 1,200 solitary and skeleton-less species present in all marine ecosystems and depths. Sea anemones are diverse morphologically but also in their biology, displaying a high variety of reproductive and feeding strategies and varied symbiotic interactions.

Although their long and sessile life and ubiquity in marine systems makes them good targets for a great variety of studies, sea anemones are often omitted from general or more applied studies because identifying them is difficult. There are few active specialists for the group, their taxonomy is based on histological internal characters, and there can be considerable disparity between preserved and living specimens.

Actiniarians are characterized by an absence of attributes that define other hexacorallian orders and yet are more diverse in terms of polyp anatomy, leading to conjecture that the order is paraphyletic (reviewed in Rodríguez et al. 2014). Nonetheless, genetic data consistently find Actiniaria monophyletic with respect to the other hexacorallian orders (Rodríguez et al. 2014; Quattrini et al. 2020). Ultrastructure of cnidae identified apical flaps (three plaques that fold backwards when the capsule discharges) (reviewed in Reft & Daly 2012) as a synapomorphy for the order.

Perhaps because their taxonomy is difficult to master, the subordinal classification of actiniarians was relatively stable for a long time. Carlgren (1949) formalized a taxonomic system for the order that remained in place for nearly 60 years, when Daly et al. (2008) demonstrated the inadequacy of that morphology-based classification in the first comprehensive molecular phylogenetic analysis of the order. Subsequent studies have affirmed the major finding: none of the superfamilial groupings nor most families and genera in Carlgren (1949) classification are monophyletic.

In addition to the need to re-evaluate the traditional morphological characters used in these classifications, sea anemones (like all anthozoans) show an extremely slow rate of mitochondrial evolution (Shearer et al. 2002) and have insufficient variability in the standard molecular markers (12S, 16S, CO3, 18S, 28S) used for most broad-scale phylogenetic reconstruction (reviewed in Rodríguez et al. 2014). These factors together limit our ability to elucidate relationships. This led to a broad collaboration to establish a robust and comprehensive phylogenetic framework for sea anemones at the higher level, while dedicating attention to more focused studies of particular lineages and problems using both molecular and morphological data (e.g. Gusmão & Daly 2010; Rodríguez & Daly 2010; Rodríguez et al. 2012, 2014; Grajales & Rodríguez, 2016; Daly et al. 2017; Sanamyan et al. 2019; Yap et al. 2020; Hamilton et al. In press).

According to the current formal higher-level classification for the order, Actiniaria is composed of three main suborders (Rodríguez et al. 2014, Xiao et al. 2019). Suborder Anenthemonae includes former suborder Endocoelantheae and family Edwarsiidae and is characterized by an unusual arrangement of mesenteries and no muscles in the base. Suborder Enthemonae includes former suborders Nynantheae and Protantheae and former infraorder Boloceroidaria and is characterized by a generally hexamerous arrangement of mesenteries. Enthemonae is the most species-rich group, with three superfamilies:

1. Actinostoloidea (pp. Former Mesomyaria, polar taxa with mesogleal sphincter and no acontia);
2. Actinioidea (former Endomyaria, mainly shallow water taxa with endodermal sphincter but including derived members that have lost sphincter and basilar muscles)
3. Metridioidea (former Acontiatia, mainly acontiate taxa, but including derived lineages that have lost acontia).

Finally, a third suborder of actiniarians, Helenmonae Daly & Rodríguez in Xiao et al. (2019), was recently described to accommodate the enigmatic divergent deep-sea taxon Relicanthus daphnae (Daly, 2006). Relationships in the current formal classification of the order are supported by molecular, morphological and by attributes of the cnidae.

This section of the World Register of Marine Species (WoRMS) provides for sea anemones (order Actiniaria) a continuously updated overview of scientific names, classifications, geographical distributions, and bibliographic references to the work in which each taxon was originally described.

This list started from relational databases built by Dr. Daphne Fautin from 1994–2013 as “Hexacorallians of the World”. Taxa have been entered in the WoRMS database since 2004, for which the online Aphia platform of the Flanders Marine Institute (VLIZ) is used.

A separate portal for the World List of Actiniaria has been introduced in 2022 to continue and capitalize on former efforts within “Hexacorallians of the World” to facilitate the study of actiniarian sea anemones. It is aims to present names that result from a science-based taxonomy, in which morphological and molecular criteria are used (Rodríguez et al. 2014). In the last decade, various phylogenetic reconstructions of actiniarian higher taxa, families and genera have been published (e.g. Daly et al. 2008; Lauretta et al. 2013; Rodríguez et al 2014; Grajales & Rodríguez 2016; Izumi et al. 2016; Daly et al. 2017; Sanamyan et al. 2019; Yap et al. 2020; Hamilton et al. In press). Application of these phylogenetic models to taxonomic revisions resulted in new classifications, in which taxa moved from one genus or family to another one, usually forming new combination names. We keep an updated record of these systematic advances. In addition to species names, we provide:

1. Bibliographic reference to the work in which each taxon was originally described;
2. Synonymy of each taxon;
3. Taxonomic status of each taxon;
4. Images of many species - from the published literature as well as previously unpublished ones;
5. Type specimens - by species and by museum;
6. Type locality of each species;
7. Published distribution of each species
8. List of relevant publications by each author.