historically, piscicolidae has been divided into three subfamilies; pontobdellinae, characterised by two pairs of pulsatile vesicles on each urosome segment, pontobdellinae, characterised by a single pair of pulsatile vesicles on each urosome segment and platybdellinae, with no pulsatile vesicles. however, molecular phylogenetic analyses performed by williams and burreson in 2006, does not support these subdivisions. piscicolidae was confirmed as being monophyletic, but platybdellinae was shown to be polyphyletic with four distinct clades, piscicolinae was similarly polyphyletic, again with four distinct clades, and pontobdellinae was paraphyletic with respect to the genus "oxytonostoma".