Phylogenetic placement of the enigmatic worm-like Rhodopemorpha slugs as basal Heterobranchia

Rhodopemorphs are small, interstitial or psammobiotic heterobranch slugs, which have been troubling to place phylogenetically. Their small size and habit of living in or among sediment have led to a correlated reduction and simplification of morphology, and consequently to contradictory phylogenetic signal from anatomical features. When morphological data have previously been used to generate phylogenetic hypotheses, these were vulnerable to the effects of homoplasy. We collected multiple species of Rhodope, along with another rhodopemorph genus Helminthope, to produce DNA sequence data for the first time to test their monophyly. We sequenced mitochondrial and nuclear genes, and analysed the data under maximum likelihood and Bayesian inference. By analysing these data in a broader heterobranch dataset, we also examined the placement of Rhodopemorpha within Heterobranchia. Despite rhodopemorphs showing aspects of a euthyneurous (and pentaganglionate) condition, their placement in a molecular phylogeny occurs outside the taxon defined by this term (i.e. Euthyneura = Pentaganglionata). Instead, model-based inferences placed rhodopemorphs among basal heterobranch taxa, usually as a clade sister to a shelled group (Murchisonellidae) that was recently removed from the Pyramidelloidea. The Rhodopemorpha + Murchisonellidae clade is herein termed Allomorpha. Three-dimensional reconstruction methods have elucidated potential morphological homologies for Rhodopemorpha and comparisons with Murchisonellidae have also uncovered morphological support for this placement. Thus, we consider the phylogenetic placement of Rhodopemorpha solved, although relationships among lower Heterobranchia lineages remain challenging.