Mitogenomic and nuclear data redefine the phylogenetic position of Acrorbis, an enigmatic Neotropical planorbid lineage, with implications for Planorbidae systematics

Abstract

The freshwater snail genus Acrorbis comprises minute rheophilic gastropods endemic to waterfalls and splash zones of the Upper Paraná Atlantic Forest in South America. Despite its distinctive morphology and long-standing taxonomic instability, its phylogenetic position has never been explicitly tested using molecular data. Acrorbis is currently classified within Helisomatini (Planorbidae: Planorbinae), although previous anatomical interpretations suggested alternative affinities. Here, we assembled and characterised the first complete mitochondrial genome of Acrorbis petricola and integrated mitogenomic and nuclear datasets to reassess the evolutionary placement of this enigmatic lineage and explore implications for planorbid systematics. The mitochondrial genome of A. petricola was 13,617 bp long and contained the typical set of 37 genes, with strong AT bias, multiple short overlaps, seven intergenic spacers, and candidate regulatory regions associated with strand-switch boundaries. Comparative analyses of available hygrophilan mitogenomes identified 15 distinct mitochondrial gene orders, revealing substantial structural conservatism within some clades and lineage-specific rearrangements in others. Acrorbis exhibited a unique derived gene order not detected in any other sampled taxon. Phylogenetic analyses based on mitochondrial datasets, concatenated nuclear 18S rRNA plus mitochondrial cox1 sequences, nuclear 28S rRNA , and combined mitogenomic and nuclear rRNA datasets consistently rejected the current placement of Acrorbis within Helisomatini. Instead, all analyses recovered Acrorbis outside Helisomatini, variously as sister to Ancylinae, Bulinidae, the remaining Planorbidae sensu lato (excluding Burnupiidae), or within a Neotropical clade including Drepanotrema and Antillorbis , depending on dataset composition. Our study provides the first genomic evidence for reassessing the systematic position of Acrorbis and demonstrates that its current tribal assignment is unsupported. Phylogenetic analyses consistently indicate that Acrorbis represents a distinct evolutionary lineage within Planorbidae sensu lato , while its mitochondrial architecture documents a distinctive history of structural evolution. These findings are consistent with the historical recognition of Acrorbis as a distinct tribal lineage (Acrorbini) and support renewed evaluation of suprageneric classification within Planorbidae, particularly the status of historical family-group names associated with the genus. More broadly, this study underscores the value of integrating genomic structure and multilocus phylogenetics to resolve problematic freshwater gastropod classifications.