Unravelling the Evolutions of the Rhizocephala: A Case Study for Molecular-Based Phylogeny in the Parasitic Crustacea

We use Rhizocephala to illustrate the problems inherent in estimating the phylogeny of parasitic crustaceans. The adult rhizocephalan parasite has such a reduced morphology that little else than the presence of a moulted cuticle relegates them to Arthropoda. Therefore, until the advent of molecular phylogenetic analysis relegation of the Rhizocephala to taxon relied exclusively on larval characters. The reduced adult morphology also entailed that very few characters were available for intrinsic rhizocephalan systematics and virtually none that could be compared with any outgroup. Thus, rhizocephalan taxonomy relied only on analysis within the group and, with few exceptions, was not based on any phylogenetic principles. The advent of DNA methods in phylogeny confirmed, with high confidence, rhizocephalans as cirripedes and nested them within the taxon as the sister group to Thoracica. This result was supported by SEM studies of the cypris larvae. Additional molecular phylogenetic studies yielded detailed insight into rhizocephalan relationships, culminating in the phylogeny presented here, which includes species from all but one family. Contrary to traditional hypotheses, the new phylogeny shows that infection of the host using a kentrogon stage represents the plesiomorphic condition, while rhizocephalans (Akentrogonida) without this stage are advanced. In this analysis, both Kentrogonida and the species-rich Sacculinidae are paraphyletic. In addition, recent hypotheses on family-level relationships in Akentrogonida, based on larval structure and the sexual system, were largely confirmed. This shows that when accurately analysed in a strict homology regime, morphological characters can be powerful partners to molecular data in elucidating rhizocephalan phylogeny.