Arhynchobdellida are a group of specialized annelids characterized by the possession of trignathous (tripartite) muscular jaws used for feeding. Arhynchobdellid leeches are found globally in both freshwater and terrestrial environments and include some of the most widely recognized bloodfeeding species. Arhynchobdellida are remarkably diverse in life history strategies and morphology and were traditionally divided into two major groups: the carnivorous Erpobdelliformes and the diverse Hirudiniformes. This study included an expanded sampling of all major arhynchobdellid families in order to evaluate the phylogenetic relationships and the evolution of life history strategies for Hirudinida. Phylogenetic analyses included two nuclear (18S rDNA and 28S rDNA) and one mitochondrial (mt) cox1 gene data for forty-seven species and six clitellate species in the outgroup. The resulting hypotheses supported the monophyly of Erpobdelliformes and Hirudiniformes, yet conflicted with traditional classification schemes for Arhynchobdellida, particularly for Hirudiniformes. The phylogenetic hypothesis provides new insights towards our understanding of the evolution of bloodfeeding, terrestrialism and cocoon deposition in Hirudinida and a framework by which to re-evaluate traditional classification of Arhynchobdellida.

With respect to Hirudiniformes, the hypothesis suggested that Haemadipsidae was found in two clades—one associated with the IndoPacific and a second with the New Word and Europe. Haemadipsidae traditionally include species that are adapted to damp terrestrial environments and are ectoparasites of vertebrate blood. The phylogenetic and biogeographic relationships of the haemadipsids were assessed based on a broader taxonomic sampling from across their range and from the combined analyses of nuclear rDNA 18S and 28S and mt cox1 and cox3 data. The elevation of the subfamily Xerobdellinae to family for the non-IndoPacific land leeches was formally proposed for non-IndoPacific terrestrial leeches. IndoPacific leeches were monophyletic and found within two subclades clades roughly divided by geography and morphology. Three-jawed leeches were found to be paraphyletic, with the two-jawed leeches nested within the trignathous clade. Haemadipsidae is formally proposed for bloodfeeding terrestrial leeches from the IndoPacific. The phylogeny provides a framework for the revision of Haemadipsidae and provided new insights towards our understanding of the evolution and biogeography of this group.