Phylogenetic Study of Cirripedes and Selected Relatives (Thecostraca) Based on 18S rDNA Sequence Analysis

Sequence data from PCR-amplified 18S rDNA were used to infer phylogenetic relationships among the thecostracan crustacean taxa Ascothoracida (Ulophysema oeresundense), Acrothoracica (Berndtia purpurea, Trypetesa lampas), Rhizocephala (Loxothylacus texanus), and Thoracica (Calantica villosa, Lepas anatifera, Octolasmis lowei, Chthamalus fragilis, Chelonibia patula, Tetraclita stalactifera, Balanus eburneus). Sequence-divergence estimates ranged from 1.8% for 2 acrothoracican taxa to 24.3% for an ascothoracidan and a balanomorph thoracican. Parsimony, invariants, and neighbor-joining analyses all showed the Ascothoracida and Acrothoracica to be sister taxa and very early thecostracan lineages widely separated from the Rhizocephala, which in turn is the sister group to the Thoracica. A comparison of the secondary structure of the V3 variable domain in the 18S rRNA molecule for these taxa supports this result somewhat: the acrothoracican V3 structure is phenetically more similar to that of the ascothoracidan and branchiopods than to the other cirripedes. There is some disagreement between a morphology-based phylogeny and the current one based on molecular characters. It may result from molecular evidence suggesting an early origin and divergence of the Acrothoracica from the main cirripede lineage, as well as a substitution rate in the lineage leading to the Rhizocephala + Thoracica that is very much higher (10-12 times) than those in the other thecostracan lineages examined. Within the Thoracica, the sessile balanomorphs appear to be derived from a lineage related to scalpelloid lepadomorphs, but variation in the 18S molecule among balanomorphs is insufficient to permit inference of relationships within this group. We conclude that the Thecostraca, represented here by the subclasses Ascothoracida and Cirripedia (the latter including the Acrothoracica, Rhizocephala, and Thoracica), is a monophyletic taxon.