Sperm types and their use for a phylogenetic analysis of aquatic clitellates

The spermatozoa of clitellates are filiform cells with the acrosome situate d in the anteriormost position, followed by the nucleus, the midpiece, and the tail. With respect to the basic plan of a 'modified' sperm type , the c litellate spermatozoon is characterized by the presence of an acrosome tube containing to a variable extent the (withdrawn) acrosome vesicle, by the m itochondria being interposed between the nucleus and the tail, and by pecul iar modifications of the central apparatus of the axoneme. The oligochaetes sensu stricto are characterized by the presence of a basal cylinder situat ed inside the basal body; the branchiobdellidans by an apical, conical inde ntation of the nucleus and by a helical marginal fiber coiled around the ta il; the acanthobdellids by a dense sheath and accessory fibers surrounding the axoneme; the euhirudineans by an anterior prolongation of the acrosome tube, the anterior acrosome. Other sperm characters are shared by various c ombinations of taxa. With the aim of using sperm morphology as a tool for p hylogenetic analysis, we identified 22 sperm characters in 21 species belon ging to 12 clitellate families: Capilloventridae, Enchytraeidae, Tubificida e (with representatives of three subfamilies), Naididae, Lumbriculidae, Bra nchiobdellidae, Bdellodrilidae, Cambarincolidae, Acanthobdellidae, Piscicol idae, Erpobdellidae, Hirudinidae. By the assumption of the monophyly of tho se Clitellata having atria or derivatives thereof associated with the male ducts, and using one enchytraeid and one capilloventrid as outgroups, we an alysed the data using the computer programme PAUP. Our results strongly sup port the monophyly of the hirudinean (i.e., acanthobdellid+euhirudinean) as semblage, as well as the close relationship between this group and the bran chiobdellidans. Although a link between the branchiobdellidan-hirudinean cl ade and the Lumbriculidae is not refuted, sperm do not provide enough evide nce for establishing the exact phylogenetic position of this clade within t he Oligochaeta.