Cutting the Gordian knot: a taxonomic revision of <i>Gyratrix hermaphroditus</i> Ehrenberg, 1831 (Platyhelminthes), with remarks on taxonomic decision-making

Abstract Taxonomic decision-making often requires balancing incomplete biological knowledge with the need for a usable classification. This is exemplified by the microturbellarian species complex Gyratrix hermaphroditus (Rhabdocoela: Polycystididae): this cosmopolitan ‘species’ has long been suspected to harbour substantial hidden diversity, but has never been formally split. Here, we integrate morphological, molecular, morphometric, and high-resolution SEM data to draw taxonomic boundaries within the complex. We also incorporate new sequences from specimens collected near the type locality into the existing molecular phylogeny. Combining these data, we here describe 14 species: G. hermaphroditus, G. tessensi sp. nov., G. timoshkini sp. nov., G. dannyi sp. nov., G. magnihamus sp. nov., G. plicatus sp. nov., G. bihamatus sp. nov., G. magnivexillum sp. nov., G. acutus sp. nov., G. karinae sp. nov., G. diezi sp. nov., G. victori sp. nov., G. longistilus sp. nov., and G. vinsenti sp. nov. Each species corresponds to a clear morphotype that can be readily recognized in nature, and is supported by molecular data. This study presents an extensive effort to resolve species’ diversity within a large species complex and illustrates how diverse datasets can be integrated into a robust taxonomy.