Molecular and Morphological Evidence for a Monophyletic Clade of Asexually Reproducing Rhizocephala: Polyascus, New Genus (Cirripedia)

The Rhizocephala is a group of extremely reduced parasitic crustaceans, that exclusively parasitize other Crustacea. In the family Sacculinidae, the external sac-like part (externa) of the adult parasite contains the reproductive apparatus and is attached beneath the abdomen of the host crab. Hosts with more than one externa may occur and are in most cases believed to have arisen from multiple cyprid larvae. However, in three species of the genus Sacculina, multiple externae have been shown to originate by asexual reproduction from a single parasitic cypris larva. We present a phylogenetic analysis of ten species of Sacculina and outgroups based on partial sequences from the cytochrome oxidase 1 (CO1) and the entire 18s rDNA gene. A separate parsimony analysis from the 18s rDNA and CO1 genes resulted in two trees with almost identical topologies. Both genes strongly support a monophyletic, asexually reproducing clade and fail to support a monophyletic Sacculina genus. As a consequence we have established a new genus, Polyascus, to accommodate three members of this clade which also share a number of common morphological features.