From marine sand to land and rivers: A combined phylogenomic strategy unveils the evolution of minute Acochlidimorpha slugs (Gastropoda: Heterobranchia)

Abstract

The repeated colonisation of non-marine environments in panpulmonate molluscs represents a major evolutionary transition, yet many lineages remain poorly understood. Among gastropods, Acochlidimorpha stands out for its ecological and morphological diversity, originating in marine interstitial habitats before independently invading freshwater and terrestrial ecosystems. Here, we present the most complete phylogeny of Acochlidimorpha to date based on a global taxon sampling collected over several decades. We integrate ultra-conserved element (UCE) data from 58 museum-preserved specimens—comprising over 1,700 nuclear loci—with all publicly available Sanger-sequencing markers, totalling more than 150 specimens. This pioneering approach sets a new benchmark for phylogenomic studies in minute molluscan lineages. Our results resolve long-standing systematic ambiguities, reinstating Pontohedylidae stat. rest. and Strubelliidae stat. rest. and establishing Helicohedylidae fam. nov. Within Acochlidioidea, Strubelliidae is recovered as the sister to Pseudunelidae + (Tantulidae + Acochlidiidae). At the species level, we clarify the position of key taxa, including Asperspina loricata and two unidentified species of Hedylopsis. Ancestral habitat reconstruction analyses reveal at least four independent transitions to freshwater and terrestrial environments, highlighting the anatomical innovations facilitating these shifts. Altogether, our study provides a robust evolutionary framework for Acochlidimorpha, offering new insights into the processes underlying terrestrialisation, morphological evolution, and ecological diversification in a lineage characterised by both extreme miniaturisation and evolutionary dynamism.