Closing the mitochondrial circle on paraphyly of the Monogenea (Platyhelminthes) infers evolution in the diet of parasitic flatworms.

Relationships between the three classes of Neodermata (parasitic Platyhelminthes) are much debated and restrict our understanding of the evolution of parasitism and contingent adaptations. The historic view of a sister relationship between Cestoda and Monogenea (Cercomeromorphae; larvae bearing posterior hooks) has been dismissed and the weight of evidence against monogenean monophyly has mounted. We present the nucleotide sequence of the complete mitochondrial (mt) genome of Benedenia seriolae (Monogenea: Monopisthocotylea: Capsalidae), the first complete non-gyrodactylid monopisthocotylean mt genome to be reported. We also include nucleotide sequence data for some mt protein coding genes for a second capsalid, Neobenedenia sp. Analyses of the new mt genomes with all available platyhelminth mt genomes provide new phylogenetic hypotheses, which strongly influence perspectives on the evolution of diet in the Neodermata. Our analyses do not support monogenean monophyly but confirm that the Digenea and Cestoda are each monophyletic and sister groups. Epithelial feeding monopisthocotyleans on fish hosts are basal in the Neodermata and represent the first shift to parasitism from free-living ancestors. The next evolutionary step in parasitism was a dietary change from epithelium to blood. The common ancestor of Digenea+Cestoda was monogenean-like and most likely sanguinivorous. From this ancestral condition, adult digeneans and cestodes independently evolved dietary specialisations to suit their diverse microhabitats in their final vertebrate hosts. These improved perspectives on relationships fundamentally enhance our understanding of the evolution of parasitism in the Neodermata and in particular, the evolution of diet.