Abstract
The spermatozoa of six species belonging to the branchiobdellidan family Branchiobdellidae (i.e. Xironogiton victoriensis , Cirrodrilus kawamurai , Ankyrodrilus legaeus , Xironodrilus formosus , Branchiobdella kobayashii , Branchiobdella orientalis ) were studied and compared to the other sperma‐tozoa already described in the group. A parsimony analysis was performed on the spermatozoal data of the species examined, as well as on their somatic characters. The results of the two analyses were contrasted and a further parsimony analysis was run on the matrix comprising both sets of characters. The study of sperm ultrastructure confirmed the genera recognized with traditional somatic characters and the monophyly of the branchiobdellidans. Xironodrilus was proved to be the sister species of Ankyrodrilus and its inclusion into the family Branchiobdellidae was supported. Evolutionary hypotheses on intergeneric differences in the family consistent with its biogeography can be suggested by the cladograms: Xironogiton is an early offshoot of branchiobdellidan lineage migrating to North America and probably radiating only in recent times; Branchiobdella kobayashii has a spermatozoon completely different from that of the other species of the genus, thus suggesting a complex story for this widespread taxon with a disjunct distribution.