Phylogenetic position of Ibla (Cirripedia: Thoracica) based on 18S rDNA sequence analysis

18S rDNA sequencing was used to elucidate the phylogenetic relationships between Ibla cumingi from the Red Sea and other thecostracan crustaceans. Ibla is placed in a rather internal position within the Thoracica, among the Lepadomorpha, Scalpellomorpha, and the Balanomorpha. This contradicts the previously accepted phylogeny, placing the Iblomorpha at the base of the phylogenetic tree of the Thoracica. The contradiction between our finding and phylogeny based on anatomy and morphology is discussed. If we accept that rDNA sequences can be used as a molecular clock to date the evolution of cirripeds, our finding suggests that some of the peculiar features of Ibla that were regarded as primitive may have resulted from late, or secondary, evolution. Such reinterpretation is rather surprising, because the diversification of Ibla involves several important traits at the anatomical as well as the biochemical level.