PHYLOGENETIC POSITION OF IBLA (CIRRIPEDIA, THORACICA) BASED ON 18S RDNA SEQUENCE-ANALYSIS

18S rDNA sequencing was used to elucidate the phylogenetic relationshi ps between Ibla cumingi from the Red Sea and other thecostracan crusta ceans. Ibla is placed in a rather internal position within the Thoraci ca, among the Lepadomorpha, Scalpellomorpha, and the Balanomorpha. Thi s contradicts the previously accepted phylogeny, placing the Iblomorph a at the base of the phylogenetic tree of the Thoracica. The contradic tion between our finding and phylogeny based on anatomy and morphology is discussed. If we accept that rDNA sequences can be used as a molec ular clock to date the evolution of cirripeds, our finding suggests th at some of the peculiar features of Ibla that were regarded as primiti ve may have resulted from late, or secondary, evolution. Such reinterp retation is rather surprising, because the diversification of Ibla inv olves several important traits at the anatomical as well as the bioche mical level.