Abstract
The freshwater annelid genus Stylodrilus Claparède, 1862 is of great ecological significance due to its broad Holarctic distribution and utility in water quality assessment. While 38 nominal species are currently recognised, molecular analyses suggest the existence of cryptic diversity within the genus. This study primarily focuses on Stylodrilus brachystylus and S. absoloni , whose taxonomic status and phylogenetic relationships to other species were unclear. We aim to fill this gap by integrating phylogenetic and morphological analyses, using freshly collected specimens and museum material, including type collections. Phylogenetic reconstruction based on mitochondrial (COI) and nuclear (H3, ITS2) markers revealed two distinct lineages within S. brachystylus , suggesting the presence of an undescribed species of broad European distribution. Both lineages were part of a cryptic species complex closely related to S. parvus . Stylodrilus absoloni is morphologically distinct, and juvenile specimens from the type locality exhibit strong genetic ties to S. lemani . However, the monophyly of the entire genus Stylodrilus remains an open question. Our findings highlight significant hidden diversity within Stylodrilus and emphasise the necessity of integrative taxonomy for accurate species delineation. A more refined Stylodrilus taxonomy may have profound effects on the understanding of the exact roles of the various species in aquatic ecosystems.