Abstract
Rhabdocoels comprise a large group of flatworms that currently consists of two major subgroups: (1) Kalyptorhynchia with an anterior proboscis and (2) Dalytyphloplanida without a proboscis. Most genera of rhabdocoels can easily be classified into one of these two subgroups, except for the three monotypic representatives of the Mariplanellinae. Comparative morphological data and previous molecular phylogenetic analyses have so far been unable to determine the positions of these rarely encountered microturbellarians within the context of rhabdocoels. Here we describe a new species of Mariplanellinae, Mariplanella piscadera sp. nov. from Curaçao (Dutch Caribbean), and present an updated phylogeny inferred from 18S and 28S rDNA sequences, including data from M. piscadera sp. nov., M. frisia, and a selection of rhabdocoels, proseriates and other trepaxonematan flatworms. Our molecular phylogenetic trees reveal that Mariplanella belongs to a distinct higher-level group of rhabdocoels that forms the sister lineage to a clade uniting Kalyptorhynchia and Dalytyphloplanida. Therefore, we update the status of Mariplanellinae to Mariplanellida status novus, containing the family Mariplanellidae status novus, in order to better reflect our current understanding of rhabdocoel phylogeny and the establishment of now three major subgroups.