Elucidation of the phylogenetic relationship of three thecostracans, Verruca, Paralepas, and Dendrogaster based on 18S rDNA sequence

The sequence of 18S rDNA was used to analyze the phylogenetic relationship of three thecostracans, Verruca spengleri, Paralepas palinuri, and Dendroga ster asterinae, with respect to other thecostracan crustaceans whose 18S rD NA sequences were reported previously. The ascothoracidan Dendrogaster is g rouped with the other ascothoracidan, Ulophysema. Verruca, a representative of the Verrucomorpha, is located among Ibla, Calantica, and the four balan omorphs analyzed. Our finding that Verruca clusters within the Balanomorpha does not agree with the suggestion that the Verrucomorpha and the Balanomo rpha evolved independently and that the order Sessilia is a biphyletic taxo n. Paralepas, which belongs to the Heterolepadomorpha (pedunculate cirriped s lacking shell plates) that were regarded as presenting plesiomorphic feat ures of the pedunculates, is placed internally within the Lepadomorpha. Thi s raises the possibility that the Heterolepadomorpha do not represent plesi omorphic features of cirripeds. Accordingly, Paralepas may be regarded as a true lepadomorph in which the loss of shell plates is apomorphic. Our stud y emphasizes the importance of molecular data in understanding thecostracan evolution.