Myzostomida Are Not Annelids: Molecular and Morphological Support for a Clade of Animals with Anterior Sperm Flagella

Abstract

The myzostomes are animals with five pairs of parapodia, living as commensals or (endo)parasites mostly on crinoid and ophiuroid echinoderms. They are generally considered aberrant annelids, possibly phyllodocidan polychaetes. A phylogenetic analysis of 18S and 28S ribosomal DNA sequence data of Myzostoma glabrum, together with 60 morphological, developmental, ultrastructural, and life-history characters, is presented to show that myzostomes are a sister group of the Cycliophora, closely related to the rotifer-acanthocephalan clade (= Syndermata). Myzostomes and syndermates share predominantly the highly derived spermatozoa with anteriorly directed flagella (cycliophoran sperm is insufficiently known). The myzostome-cycliophoran-syndermate clade, accommodated within the Platyzoa (including Platyhelminthes s. str., Gastrotricha, Gnathostomulida, Syndermata, Cycliophora, and Myzostomida), is strongly supported by most analyses, regardless of alignment parameters, character combinations and weighting, species sampling, and tree-building methods. The new name Prosomastigozoa (“forward-flagellar animals”) is proposed for the group including three phyla (Cycliophora, Myzostomida, and Syndermata).