Molecular phylogenetic and detailed morphological analyses reveal a new species of Castrella Fuhrmann, 1900 (Platyhelminthes, Dalytyphloplanida) from Switzerland

Rhabdocoel microturbellarian flatworms of the family Dalyelliidae are key components of freshwater ecosystems and represent one of their most taxonomically diverse groups. Recent molecular phylogenetic studies revealed two distinct genetic lineages within Castrella truncata, a species with a wide Nearctic–Palearctic distribution. In this study, we integrated phylogenetic and morphological analyses to describe a Swiss lineage as a new species: Castrella schareri sp. nov. Phylogenetic analysis reveals that C. schareri sp. nov. is closely allied with C. alba, while C. truncata groups with C. pinguis. Morphologically, the new species is characterised by a distally unbranched stylet, featuring a shorter proximal portion and a higher number of shorter distal spines compared to C. truncata. This finding significantly enhances our knowledge of Castrella, marking the first new species described in the genus in over seventy years.