Purpose: Derogenes ruber Lühe, 1900, the type-species of the genus Derogenes Lühe, 1900, is a poorly known derogenid digenean. The original description of this species was not illustrated and aspects of the morphology of the parasite from the type-host remain scarce. Available records of this species were brief and/or lacked illustrations and were based on morphology alone. Additionally, molecular data for Derogenes spp. are warranted to untangle species complexes as they provide a better assessment of interspecifc genetic divergence.

Methods: Derogenes ruber is redescribed based on newly collected specimens from the gall bladder of its type-host Chelidonichthys lastoviza (Bonnaterre, 1788) collected in the Western Mediterranean of the Algerian coast during 2017–2019 and molecular data are provided using a partial fragment of the nuclear 28S ribosomal RNA gene (28S rRNA), the internal transcribed spacer 2 (ITS2) and a fragment of the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene.

Results: We herein provide a detailed illustrated redescription and morphometric data of D. ruber from its type-host C. lastoviza. We report a new geographical record (of Algeria) for it. Derogenes ruber is also genetically characterised for the frst time. Species/lineages of Derogenes were recovered in fve strongly supported reciprocally monophyletic clades: (i) D. ruber from C. lastoviza of Algeria; (ii) D. lacustris from Galaxias maculatus (Jenyns) of Argentina; (iii) Lineage “D. varicus DV1” (D. varicus sensu stricto) from fsh hosts in the White and Barents seas and the North Sea; (iv) Lineage “D. varicus DV2” from mollusc hosts in the White Sea; and (v) Lineage “D. varicus DV3” from Eumicrotremus fedorovi Mandrytsa. in the Pacifc Ocean. Hence, comparison of the newly generated sequences with other available data for Derogenes species supports the distinction of D. ruber confrming its taxonomic status and helping assess interspecifc variation. Comparison of D. ruber with the closely related species Derogenes latus revealed overlaps in morphometric data and the validity of the latter species is questioned. Conclusion The combination of morphological and molecular data provided for D. ruber provides a frm foundation for further investigations of Derogenes spp. Although we do describe herein material of D. ruber from the type-host, given that the occurrence of a single Derogenes species in various hosts has been challenged by molecular data, and both D. lacustris and D. varicus sensu stricto had been genetically proven to occur in various hosts, D. ruber and D. latus may be indeed synonymous. Additional sequencing efort on Derogenes spp. will strengthen systematic comparative studies and evolutionary relationships within the Derogenidae in general.