Abstract: We present a phylogenetic analysis of benthic ctenophores of the order Platyctenida, sampling all but one genus. Using complete mitochondrial genomes and nuclear ribosomal data and a reassessment of anatomy, our integrated analysis uncovers an unexpectedly close relationship between two unusual members of the Coeloplanidae: Coeloplana (Benthoplana) meteoris and Vallicula multiformis. These two species form a well-supported clade, deriving at or near the base of the tree of Platyctenida, distantly related to other Coeloplana, rendering Coeloplana and Coeloplanidae non-monophyletic. A unique mitochondrial gene order and a tentacle bulb with four extensions are newly identified synapomorphies of this lineage. We elevate the subgenus Benthoplana to the generic level, erect the new family Benthoplanidae for Benthoplana and Vallicula, and provide diagnoses for these taxa and their accepted species. We also show that planktonic Ctenoplana (Diploctena) neritica is the early life stage of Benthoplana meteoris, and suggest that the remaining Ctenoplanidae likely represent early life history stages of Coeloplanidae and perhaps other platyctenes. While both the nuclear ribosomal (18S and 28S) and mitochondrial protein-coding genes suggest a deep phylogenetic divergence between Benthoplanidae and Coeloplanidae, we detect conflicting phylogenetic signal between these markers, suggesting nuclear-mitochondrial discordance, leaving the placement of Tjalfiellidae and Lyroctenidae uncertain.

Key words: Benthoplana, Coeloplanidae, Ctenophora, Ctenoplanidae, Mitochondrial genome, Ribosomal RNA, Systematics