The Phylogeny of Rhizocephalan Parasites of the Genus Heterosaccus using Molecular and Larval Data (Cirripedia: Rhizocephala; Sacculinidae)

Within parasitic barnacles of the family Sacculinidae, the genus Heterosaccus is the third largest, with 12 species infesting various brachyuran hosts throughout the world. As part of an effort to reconstruct rhizocephalan phylogeny we performed an analysis of four species of Heterosaccus and of selected sacculinid and non-sacculinid rhizocephalans. We used both molecular sequence data (16s rDNA and 18s rDNA) and morphological characters from an SEM analysis of the cypris larvae. Using Bayesian methods we obtained a highly supported tree in which the four species of Heterosaccus form a monophylum, whereas both the genus Sacculina and the family Sacculinidae are polyphyletic. The sistergroup to Heterosaccus is a clade consisting of the sacculinids Loxothylacus panopaei and the "classical" rhizocephalan Sacculina carcini. The molecular results found support in cypris morphology, where we identified two distinct synapomorphies for Heterosaccus, both present in male cyprids only: A large flap extending posteriorly from the third antennular segment, and the male-specific aesthetasc on the third segment being reduced to a rudiment or lacking completely. Female cyprids didn't show generic level apomorphies but resembled those of other sacculinid species. No morphological synapomorphies were identified between Heterosaccus, L. panopaei and S. carcini. While larval characters proved to be informative, we conclude that future studies on rhizocephalan taxonomy must rely primarily on molecular data, both to provide an overall phylogenetic framework and to assure an accurate identification of species for biogeographical and other biological purposes.