Cirripede cleavage patterns and the origin of the Rhizocephala (Crustacea: Thecostraca)

Abstract

Several aspects of phylogenetic relationships among barnacles (Cirripedia) are still unresolved. One of the contentious issues is the position of the parasitic Rhizocephala. In most molecular phylogenies Rhizocephala are resolved as sister group to a monophyletic Thoracica. However, since Rhizocephala are morphologically highly derived there is not a single morphological character supporting this view. Here we present data on the early cleavage patterns and the gastrulation of two rhizocephalan species. Based on our results and data from the literature we suggest that early cleavage and gastrulation indicate a monophyletic group comprising the thoracican Iblomorpha and the Rhizocephala. This renders thoracicans paraphyletic with respect to Rhizocephala. Based on this, we develop a new hypothesis for the origin of parasitism in the rhizocephalan stem lineage starting with parasite-like dwarf males of an iblomorph-like ancestor which already had the ability to penetrate the surface of a host animal – originally conspecifi c females, but later the decapod, probably a pagurid, host.