Systematics of the Hexacorallia (Cnidaria: Anthozoa)

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. (C) 2003 The Linnean Society of London.